Plc Manual

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Manual No. CMN-**-I-12

PROGRAMMABLE LOGIC CONTROLLER (PLC) CONTENTS 1.0

Introduction

2.0

Functional Description of Hardware 2.1 Power supply 2.2 Input System 2.2.1 2.2.2 2.2.3

2.3

Output 2.3.1 2.3.2 2.3.3

2.4

3.0

Memory Storage Capacity Memory Map

Programmer Units

Design a System Based PLC 3.1 Project Execution 3.1.1

3.2 4.0

Registers Flag Registers Auxillary Relays Shift Registers Binary Counter Timers

Memory 2.5.1 2.5.2

2.6

Relay Outputs Solid State Relay Transistor Outputs

CPU 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6

2.5

Opto Isolated Digital Inputs High Speed Counter Inputs Analog Inputs 2.2.3.1 Digital to Analog Converter (DAC) 2.2.3.2 Analog to Digital Converter (ADC) 2.2.3.3 Multiplexer 2.2.3.4 Interfacing

System Analysis 3.1.1.1 Engineering Design 3.1.1.2 Software (Program) Development 3.1.1.3 Software / Hardware Integration 3.1.1.4 Systems Checkup and Startup

Ladder Diagram

Hardware & System Sizing and Selection 4.1 I/O Quantity 4.2 I/O Remoting Requirements

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4.3 4.4 4.5 4.6

Memory Quantity Redundancy Level Programmers Special Design Norms for PLC

5.0

PLC Installation 5.1 Safety Considerations 5.2 Implementation 5.3 Enclosure 5.4 Temperature Considerations 5.5 Noise 5.6 Hookup

6.0

Applications 6.1 PLC Peripherals 6.1.1 6.1.2 6.1.3

7.0

7.4

RS232 RS422, RS423, RS485

Local Area Network (LAN) 7.4.1 7.4.2

Response Time of Network Network Stations

DCS System Integration with PLCs 8.1 Man, Machine and Integration (MMI) 8.1.1

8.2

8.3

Sequencing

Integration with PLC 8.2.1 8.2.2 8.2.3

9.0

Operator Stations I/O Enhancement Programming and Documentation Tools

Communications 7.1 Introduction 7.2 Parallel Communication 7.3 Serial Communication 7.3.1 7.3.2

8.0

Manual No. CMN-**-I-12

Integration with Direct I/O Serial Linkages Links between Networks (TCP/IP)

DCS Integration with PLCs

RIL Installations

10.0 References

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1.0

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Introduction Programmable Logic Controllers (PLC), widely also called as Programmable Controllers (PC) is a major tool in today’s highly automated environment. A PLC is a modern way to perform industrial control functions (essentially Boolean Logic) that formerly required relays, solid state electronics or a microcomputer. The PLC has a number of inputs connected to it and its function is to activate outputs in accordance with a predetermined logic or program. Following figure illustrates the system operation : Power

Programming Panel

Scan all input and store the status in memory

Stored User Program

Solve all logic functions as per Program

Input Modules

Control Activate all Outputs

Output Modules

Power Unlike the hardwired system, the input is not physically connected to each line of the ladder logic, but the status as entered in the processors memory is used in solving the lines by digital electronic principles. Hence there is no limitations on the number of times the input or output status is used within the program for computation.

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The features and advantages of PLC are listed below : 1)

Solid State Stability

2)

Logic functions are easily programmed into the system which are alternatable again by program for modifications and future changes.

3)

Virtually off the shelf control system resulting in quicker project implementation.

4)

They are light weight, rugged and compact and they work in industrial environment.

5)

Ease of maintenance.

6)

Compared to minicomputers they are a dedicated piece of equipment and hence very reliable.

7)

Serial mode communication, with digital processors and can be used in the backup.

8)

Competitive in cost.

9)

optionally they can generate their own documentation or display the final entered program when called for.

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2.0

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Functional Description of Hardware Programming Device

Power Supply

CPU

Memory

Communication Bus I / O System Module

Output Devices

Input Devices

Solenoid, motor, starters, etc.

Switches, Push buttons etc.

Figure 1 : Block Diagram of a PLC

The block diagram of a PLC is shown in figure 1. The block consists of a Central Processing Unit (CPU), a main memory and connection circuitry for digital input / output devices. A communications bus (i.e. a group of parallel wires used for transmitting digital signals) forms a common link to allow each element to share information. The input image memory in the I/O system module is used to hold the on/off states of individual input ports. In the image memory, an ON state is stored as binary 1 and an OFF state is stored as binary 0.

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The CPU processes the binary data stored in the input image memory and the corresponding data held in the output image memory according to the users program which is stored in the main memory. The bit values held in the output image memory determine which output ports are energized. A binary 1 sets an output port on and a binary 0 sets an output port off. A special program called the operating systems controls the actions of the CPU and consequently the execution of the users program. The operating system is supplied by the PLC manufacturer and is permanently held in the memory. A PLC operating system is designed to scan image memory and the main memory which stores the ladder diagram program.

2.1

Power Supply : The power supply may be separate or integrally mounted. It always provides isolation necessary to protect solid state components from most high voltage line spikes. All PLC manufacturers provide the option to specify line voltage conditions. In addition, power supply is rated for heat dissipation requirements for plant floor operation. The power supply drives the I / O logic signals, the CPU, the memory unit and some peripheral devices. Power supplies fall into two categories : Linear and Switch mode. A linear power supply uses a simple regulator circuit to convert the mains supply to a constant DC voltage. A switch mode power supply uses a high frequency switching regulator to produce a series of pulses. Averaging the pulses provides a smooth DC voltage. The main advantage of switch mode power supply (SMPS) are : 1) 2) 3)

It is capable of providing a wide range of supply voltages. Switch action makes it highly efficient so that the amount of heat dissipated from the supply is small. It is compact and light weight.

Because of these advantages, SMPS are often used in PLC’s.

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2.2

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Input Systems : Inputs are defined as real - world signals, giving the controller real time status of process variables. These signals can be digital, low or high frequency, maintained or momentary. Typically they are presented to the programmable controller as a varying voltage, current or resistance value.

2.2.1

Opto Isolated Digital Inputs : A base unit input interface circuit will use an Opto isolator arrangement such as that shown in figure 2. An Opto isolator is a device which uses light to couple signals from one system to another; in this case the input device and the image memory circuit. It incorporates a light emitting diode (LED) and photo transmitter for this purpose. The device provides a very large degree of isolation between two circuits.

24 VDC

5V Status LED Internal Circuits

Inpu t Protection Diode

560 

Opto Isolator V 2.2 K

0V Figure 2 : Opto Isolated Input Interface

Figure 2 shows a typical Opto isolator input interface. When 24 VDC is applied to the input port, a current of approximately 10 mA flows through the Opto isolator LED causing it to emit light and so turn ON the photo transistor. If the supply is accidentally reversed, the protection diode protects the reverse voltage breakdown of the Opto isolator LED. When the photo transistor is turned ON current flows through the status LED which lights up. Thus the status LED tells the user the current logic state of the input point.

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Input interfaces can be divided into those which require the input device to source current and those which require the input device to sink current. The interface of figure 2 requires that the input device sources current from the power supply. Other types of interface require that the input device sinks current to the 0V terminal.

2.2.2

High Speed Counter Inputs :

A PLC is often required to read high speed pulses from an input device such as a shaft encoder to produce pulses to drive a stepper motor. PLC ports cannot be used to generate or read high speed pulses as the scan time WHICH depends on the program length, etc. is a limiting factor. Instead, use is made of interfaces which operate independently of the scan but are able to interpret it when some action is required.

2.2.3

Analog Inputs : Many of the transducers produce analog signals. Consequently PLC manufacturers provide ports for handling analog signals. To handle analog signals special interface devices based on analog to digital converters (ADC’s), digital to analog converters (DAC’s), multiplexers and demultiplexers are required. These are discussed below.

2.2.3.1 Digital to Analog Converter (DAC) :

Vref (MSB) B7 Binary Input

DAC

Vout Analog Output

B0 (LSB)

Figure 3 : Eight Bit DAC

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A Digital to Analog Converter (DAC) produces an analog output from a digital input (see figure 3). In all types of DAC, the analog voltage is produced from a reference voltage (Vref). Binary code is input to the DAC and determines what fraction of the Vref is presented at the output. The output from a DAC is not truly continuous but rather a series of discrete voltage levels. For example, the 8 bit DAC shown in figure 3 has an output given as : Vout = Vref (B7/2+B6/4+B5/8+B4/16+B3/32+B2/64+B1/128+B0/256) where bits B7 to B0 can take values 0 or 1 and are the binary inputs. B7 is the most significant bit (MSB), while B0 is the least significant bit (LSB). Consider an 8 bit DAC with a reference voltage Vref as 10 V. The binary input of 00000001 generates a smallest discrete output i.e. 10 / 256 volts. The next discrete output is 10 / 128 volts, generated from binary code 00000010. Clearly 256 discrete analog levels (referred to as quantization levels) can be produced from the binary input. The voltage resolution of an N bit DAC is calculated by dividing the maximum operating voltage by 2N - 1. The factor 2N - 1 represents the number of steps between quantization levels. An 8 bit DAC with a reference voltage of 10 V has a resolution of 10 / 255. The speed of a DAC is determined by how long it takes to settle to a stable value after a change in the input. This is specified as the setting time. The other main parameters of a DAC are linearity and accuracy. Linearity is the measure of the deviation from a straight line of output voltage plotted against binary input. Accuracy is the variation between the DAC’s actual output and the intended one.

2.2.3.2 Analog to Digital Converter (ADC) : Start Converter (SC) End of Converter (EOC)

Analog In

ADC

Binary Out

Figure 4 : Eight Bit ADC

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The analog to digital converter (ADC) produces an digital output from analog input (see figure 4). ADC’s incorporate start convert (SC) and end convert (EOC) connections. When the start convert signal is pulsed, the ADC converts the analog input at that time into an equivalent digital value. The ADC then produces an end of convert signal (EOC) to indicate that the conversion has finished. The main parameters of ADC’s are again resolution, accuracy, linearity and speed. Comments about resolution, accuracy and linearity have already been made in 2.2.3.1. Concerning operating speed, ADC’s are generally slower than DAC’s because the process involves comparing one signal with another. Successive approximation of the input value rather than ramping the DAC from a counter speeds up the conversion process. For high speed, so called ‘flash’ converters are used.

2.2.3.3 Multiplexer :

Input Channel s

0 1 2 3 4 5 6 7

Output

A B C C 0 0 0 0 1 1 1 1

B 0 0 1 1 0 0 1 1

A 0 1 0 1 0 1 0 1

Channel Selected 0 1 2 3 4 5 6 7

Figure 5 : Multiplexer

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Multiplexer allows several signal carrying channels to share a signal line. The block diagram of a multiplexer is illustrated in figure 5. This shows that each input channel may be connected to the output line when one of a bank of switches inside the multiplexer is turned on. In practice, the bank of switches shown in figure 5 is a bank of transistor switches controlled by lines A, B, and C. A binary code placed on the lines A, B, and C determines which of the channels of switched through the output. Demultiplexers are multiplexers which work in reverse.

2.2.3.4 Interfacing : The general rule when interfacing the analog signals is to match voltage levels and to ensure that the impedance of the sourcing circuit is less than or equal to that of its load circuit. Impedance matching is essential for optimum power transfer to the load circuit. To match the voltage levels it may be required to amplify the voltage level or perhaps convert a bipolar voltage into a unipolar voltage. A circuit for changing the impedance may be used for impedance matching.

2.3

Output : Common types of outputs are discrete and analog. Discrete outputs can be pilot lights, solenoid valves or annunciator windows; analog outputs can drive signals to variable speed drives or to I/P converter and thus to control valve. Generally I/O systems are modular in nature. A system can be arranged by the use of modules that contain multiples of I/O points. These modules can be composed of 1, 4, 8 or 16 points and plug into the existing bus structure. The bus structure is a high speed multiplexer that carries information back and forth between the I/O modules and the CPU. One of the most important function of the I/O is its ability to isolate real world signals from the low signal levels in the I/O bus. This is accomplished by the use of optical isolators.

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2.3.1

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Relay Outputs :

Manual No. CMN-**-I-12

+V Relay

Protection Diode

Output Port

270 

Input

Switching Transistor

Status LED

0V

Figure 6 : Relay Output Stage A traditional relay is a switch controlled by an electromagnet. Relays are used in PLC’s because they can handle large current and offer high degree of isolation between the PLC and the load circuits. A typical relay will be capable of switching a few amperes. However, the relays have the following disadvantages : a) b) c)

They are slow to operate When closed, their contacts can bounce before settling. Relay coils can generate large inductive currents when energized.

A typical electromagnetic relay based output circuit is shown on figure 6. A NPN transistor switches current through the relay coil to close its contacts. The transistor is controlled by the image memory circuit of the PLC. The diode is connected across the to protect it from its back emf. On a practical side, it is important not to exceed the maximum current that the output relay contacts can handle. For DC loads, the current rating is given in amperes. For AC loads, the maximum current could be given as a VA (volt amperes) power rating. For example, if the specifications of the output relays is given as 35 VA, then the maximum current the contacts can handle at 240 Vrms is calculated as I

=

(35/240)

=

146 x 10-3

=

146 mA

If the load current is likely to exceed the rated maximum current of the internal relay, then a external secondary heavy duty relay should be used. Prepared By : AAB Rev. : 00

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2.3.2

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Manual No. CMN-**-I-12

Solid State Relay :

L

24 V

Gate

Input

Output

Triac ZCC

N

0V

Zero Crossing Point

Output Zero Crossing Point

Off Delay

On Delay

Input

Figure 7 : Solid State Relay which switches at a Zero Crossing Point

A solid state relay performs the same functions as a traditional relay but no moving parts. It is basically a optically isolated triac. As shown in figure 7, a triac is a two back to back silicon diodes which are switched into conduction by a third electrode called the gate. As the solid state relay is optically isolated the gate may be thought of as being triggered by an LED.

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Solid state relays are often useful in switching mains and often incorporate a zero crossing circuit (ZCC). This monitors the mains cycle so that the relay can be turned on at a zero crossing point. Switching at the zero crossing point prevents high frequency noise being generated. However, on and off delays do occur as a result of waiting for the next zero crossing point (see figure 7).

2.3.3

Transistor Outputs : Transistor may be used for switching current through a load. The switching speed of a transistor is faster than that of an electromagnetic relay. Consequently, the use of transistor output ports can reduce response time. Switching capabilities of PLC transistor output ports are usually quoted in terms of the maximum voltage and current that can be used. For example 24 VDC at 0.5 A.

2.4

CPU : CPU performs the tasks necessary to fulfill the PLC functions like scanning, I/O bus traffic control, program execution, peripheral and external device communications, special functions or data handling execution and self diagnostic. The central processing unit of a PLC is built-up around a microprocessor which is an integrated circuit which performs the computing operations. The function of the CPU is to accept data in the form of groups of binary digits and perform arithmetic and logical operations on the data in accordance with instructions stored in the memory. The internal structure of CPU comprises input and output interfaces, a memory in the form of registers, and the control element called the arithmetic and logic unit (ALU). The input and output interfaces allow the CPU to read the data from the memory and write data into the memory via the communication bus. The ALU performs the arithmetic and logical operations on the data stored in the CPU registers.

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2.4.1

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Registers : Most CPU operations involve the use of register, which is a memory element used to store a group of bits on a temporary basis. CPU registers are located inside the microprocessor. So called data registers are located in the RAM and are used for storing flags, counter and timer constants and other types of data. Registers are having different storage capacities. A 4 bit register stores a nibble, which is 4 bits of data. An 8 bit register stores a byte which is 8 bits of data. A 16 bit register stores a word, which is 16 bits of data. 4 bit

8 bit

16 bit

Each location can store a binary 1 or 0 Figure 8 : Registers

2.4.2

Flag Registers : If a bit state (0 or 1) is used to indicate that some condition has occurred, than it is called a flag. A register which stores a group of flag bits is called a flag register. The CPU has an internal flag register which contains information about the result of the latest arithmetic and logical operations. The PLC image memory is effectively a flag register, as it contains the current status of inputs and outputs.

2.4.3

Auxillary Relays : Auxillary relays are single bit memory elements located in RAM that may be manipulated by the users program. They are called auxillary relays because they may be likened to imaginary internal relays. A battery backed auxillary relay is called a retentive holding relay and can be used for storing data during power failure. A number of auxillary relays may be grouped together to form a register.

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It is important to remember that because auxillary relays are bit values stored in the memory output, loads cannot be connected directly to them. However, output loads can be used to control output loads indirectly.

2.4.4

Shift Registers : Some registers are arranged so that bits stored in them can be moved one position to the left or to the right with the application of a shift command or pulse. Such registers are called shift registers and can be used for sequence control applications.

2.4.5

Binary Counter : The CPU can function as a binary counter since it is able to increment and decrement binary data stored in a register and compare binary data stored in two separate registers. Counters are used to count, for example, digital pulses generated from a switching device connected to an input port. An output is usually generated after a predetermined number of input pulses have been counted. The count value required is stored in a data register.

2.4.6

Timers : The CPU has a built in clock oscillator which controls the rate at which it operates. The CPU uses the clock signal to generate delay times. A delay time could be used, for example, to keep an output relay energized for a fixed period.

2.5

Memory : It is the storage place in which both application program and executive programs are stored. An executive program functions as the operating system of the PLC. It is the program that interprets, manages and executes the users application program. Memory is characterized by its volatility. A memory is volatile if it looses its data when the power to it is switched off and non-volatile otherwise. Common types of memory include semi-conductor memory and magnetic disk. The various types of semi-conductor memory are :

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1)

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RAM : Random Access Memory is a flexible type of read / write memory. All PLC’s have some amount of RAM, which is used to store ladder programs being developed by the user, program data which needs to be modified and image data. RAM is volatile. This means that RAM cannot be used to store data while the PLC is turned off unless the RAM is battery backed. A type of RAM called CMOS RAM (complementary metal oxide semiconductor RAM) is suitable for use with batteries because it consumes very little power and operates over a wide range of supply voltages.

2)

ROM : Read Only Memory is programmed during the manufacture using a mask. It is a non-volatile memory and provides a permanent storage for the operating system and fixed data.

3)

EPROM : Erasable Programmable Read Only Memory is a type of ROM which can be programmed by electrical pulses and erased by exposing the transparent quartz window found in the top of each device to ultraviolet light. EPROM is a non-volatile memory and provides permanent storage for ladder programs.

4)

EEPROM : Electrically Erasable Programmable Read Only Memory is similar to EPROM but is erased by using electrical pulses rather than by using ultraviolet light. It has the flexibility of battery backed CMOS RAM. However, writing data into a EPROM takes much longer than into a RAM.

2.5.1

Memory Storage Capacity : The storage capacity of a memory device is determined by the number of binary digits i.e. on / off states, it can hold. Clearly, the storage capacity of the user memory will determine the maximum program size. 1 K byte memory will hold 1024 program instructions and data if these are stored as groups of 8 bits.

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2.5.2

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Memory Map : Memory mapping is a term to describe the situation in which input / output ports are controlled by writing data into the image memory. A diagram which shows the allocation of memory addresses of RAM, ROM and I/O is called a memory map. Figure 9 illustrates the memory map of a typical PLC. In this image bits are stored in RAM above the users program and data for flags, counters and timers. With most PLC’s memory map is already configured by the manufacturer. This means that the program capacity, the number of input / output ports and the number of internal flags, counters and timers are fixed.

OPERATING SYSTEM

ROM

INPUT / OUTPUT IMAGE BITS

DATA

RAM

MEMORY ADDRESSES

0002

USER’S PROGRAM SPACE

MEMORY LOCATION

0001 BOTTOM OF MEMORY 0000

Figure 9 : Memory Map

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2.6

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Programmer Units : This unit provides an interface between the PLC and the user during program development, start up and trouble shooting. The instructions to be performed during each scan are coded and inserted into memory with the programmer. Programmers vary from small hand held units the size of a small calculator, to desktop standalone intelligent CRT based units. These units come complete with documentation, reproduction, I/O status and on-line and off-line programming ability. Many PLC manufacturers now offer controller models that can use personal computer as a programming tool. Under these circumstances, the manufacturer will sell a program for the personal computer that usually allows the computer to interface with a serial input module installed in the programmable controller. Programming units are the liaison between what the PLC understands (words) and what the engineer desires to occur during the control sequence. Some programmers have the ability to store programs on other media, including cassette tapes and magnetic disks. Another important feature is the automatic documentation of the existing program. This is accomplished by a printer attached with the programmer. With off-line programming, the user can write a program on the programming unit, then take the unit to the PLC in the field and load the memory with the new program, all without removing the PLC. Selection of these features depends on the user requirements and budget. Online programming requires the cautious modification of the program while the PLC is controlling the process or the machine.

2.7

Peripheral Devices : Peripheral devices are grouped into several categories : programming aids, operational aids, I/O enhancements and computer interface devices are most common. Programming aids provide documentation and program recording capabilities. Although some devices can program many models of different manufacturers PLC’s, most are dedicated to single supplier and specific models. The definite trend in programming aids is PC compatible software that allows the programmable is sold by the PLC manufacturer or a license and is often model specific. If the software also offers online programming and trouble-shooting characteristics it may infact be used on a single specific programmable controller. This isolation is achieved by means of a software or hardware keys that come with each copy of the software purchased.

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Operational aids include a variety of resources that range from colour graphics CRT to equipment or support programs that can give the operator specific access to process parameters. In this situation the operator is allowed to read and modify timer, counter and loop parameters but not have access to the program itself. Some aids facilitate the interaction between the programmable controller and the dumb terminals such as printers, to deliver process information in a desired format. Some devices have the ability to setup an entire panel and plug into the PLC through an external RS232C ports, thereby saving enormous panel and wiring costs. I/O enhancement group is a large category of PLC peripheral equipment. It includes all types of modules, from dry contact modules to intelligent I/O to remote I/O capabilities. Some I/O simulators used to develop and debug programs can be categorized in the I/O enhancement group. These specific devices are typically hardware modules that can be plugged into the PLC. The computer interface device group is a rapidly expanding section of programmable controller peripheral devices. These devices offer peer to peer communications (i.e. one programmable controller connected to another), as well as network interaction with various computer systems. Infact, this group of devices will certainly expand in number as communication standards become more openly accepted and more and more products are provided to facilitate such network interactions.

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3.0

Design a System Based PLC

3.1

Project Execution :

Manual No. CMN-**-I-12

The PLC project must take into account the important considerations of schedule and budget as it is true for any major undertaking. The PLC can facilitate the transition, however by simultaneously pursuing several activities, thereby condensing the overall project schedule.

3.1.1

System Analysis : The control system should be analyzed as a whole to determine plant control requirements. The PLC plays an integral part in these analyses and its capabilities should be thoroughly understood by the control engineer. Vital to the system analysis are process instrument diagrams (P & ID), the descriptive operational sequence and the logic diagram or electrical schematic. Part of this evaluation will be system sizing and selection. Once the appropriate PLC is selected and purchase orders are placed, two activities should begin immediately : engineering design and software development.

3.1.1.1 Engineering Design : The first step is the development of input - output (I/O) list. This detailed document will be used extensively and should be developed with great care. Once I/O numbers are assigned, it becomes very difficult to change all references to these numbers. The I/O list is followed by configuration drawing. The configuration drawing shows the arrangement of the I/O and support hardware. The point to point wiring diagram will be used by the panel shop and the installation contractor to make the I/O devices interconnect. Panel, or enclosure, deign should now be coordinated with the additional panel for instrumentation, such as light switches, meters and recorders. Once these steps are completed, panel fabrication and assembly can begin.

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Manual No. CMN-**-I-12

3.1.1.2 Software (Program) Development : The I/O list mentioned previously will be used to begin the program development. Basic control philosophy decisions need to be made at this point. Should valves fail to open or close? What fail safe provisions are necessary for analog control? These decisions should be documented and included with the process operational descriptions. Quite often this document is referred to as the software functional specification. Its purpose is to define as precisely as possible, the operation of controls. It has several other functions : 1)

It communicates the functional requirement of the control system to those wiring the PLC code.

2)

It records the thought process (regarding control) of the system designer to be used in the event of a personnel change. Such information can be invaluable.

3)

It provides a review document for personnel working in other disciplines (mechanical, process, etc.) to ensure that they understand the operations of the controls.

4)

It provides the guide for developing the operational description for the operator’s manual.

After the functional specifications have been reviewed and approved, a detailed operational sequence chart, timing diagram, logic diagram, flow chart or electrical schematic is developed from it. This schematic is translated or coded into the appropriate PLC language. The piping and instrumentation diagram is also cross referenced. In this way, future cross referencing of system drawings and PLC codes is facilitated. As the code is entered, a memory map or register index is kept by the programmer. This map is useful in organizing the program data in logical arrangements and will prove invaluable during start-up, when the programmer may need to located the available blocks of memory quickly for program revisions. Once the program is entered, a simulation is recommended and the program check out process is begun ‘on the bench’. This process uses the functional specification to prove that the software is compatible. A large percentage of the program can be proved in this manner. Program debugging can be completed before field installation.

3.1.1.3 Software / Hardware Integration : Prepared By : AAB Rev. : 00

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Manual No. CMN-**-I-12

Using the PLC and programming aids, the panel wiring can be ‘rung out’ (i.e. checked point by point) in through the PLC. Each I/O point should be activated separately to the terminal block or from the panel controls (buttons, lights, switches). In this manner, the electrical integrity of the panel from the terminal blocks inward is ensured. If any continuity problem exists hereafter, they will be located in the field wiring. Some organizations prefer to perform a simulated operation check out at this time. This is highly successful approach and can be implemented if the simulators and the I/O point arrangement are organized to simulate the process outside the panel.

3.1.1.4 System Checkup and Startup : After the electrical connections have been made and point to point wiring has been completed (mechanical completion), the system is ready for startup. The ability of the PLC to operate step by step through the startup becomes very useful at this stage. Experienced PLC personnel may provide temporary STOP, CONTINUE and STEP switches in the back of the panel in order to facilitate the startup procedures. These switches can be key locked, software locked, or disconnected for normal operation. They are also useful as future maintenance and trouble shooting tools to diagnose future problems as being either hardware or software based.

3.2

Ladder Logic : With a majority of PLCs, writing a program is equivalent to drawing a switching circuit. The switching circuit is drawn in a ladder diagram format. This format requires that : 1)

2)

Circuits are arranged as a series of horizontal lines containing inputs (refered to as contacts) and outputs (refered to as coils). Typical circuit lines are shown in figure 14 below. Inputs must precede outputs and are in the form of normally open and normally closed contacts. Ladder symbol for a normally open contact is | |. The symbol for normally closed contact is | | .

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3)

4) 5) 6)

Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

There must always be atleast one output on each line. An output is, for example, a PLC output relay. The ladder symbol for a PLC output is drawn either as two parenthesis close together, i.e. ( ), or as a circle. Circuits in the form of vertical lines are not used. Numerical assignments for inputs (contacts) and outputs (coils) form part of the ladder diagram. Other elements such timers, counters and shift registers can be implemented in ladder diagrams.

The term ladder is used because the lines of the completed diagram resemble the rungs of a ladder. The two vertical lines are called the bus lines and represent the power connections, in this case 24 V and 0 V. Each horizontal line represents a program line. The output on a program line is energized (turned on) when the input contact(s) to it are made (i.e., when the contacts connect the 24 V supply to the coil). A ladder diagram can be translated into a program consisting of instructions and data. Table 1 describes the Boolean instructions that are used by the PLC manufacturers. Ladder programs are entered into memory in an address instruction data format. An address is a number which activates a memory location. Instructions and data are entered into sequential memory locations usually starting from address zero.

24 V

Inputs

Inputs

Outputs

0V Program Line 1 Program Line 2 Program Line 3 Program Line 4

24V Bus Line

0 V Bus Line

Figure 1 : Ladder Format

Table 1 : Ladder Instructions Prepared By : AAB Rev. : 00

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INSTRUCTIONS LOAD LOAD NOT AND AND NOT OR OR NOT OUT

Manual No. CMN-**-I-12

DESCRIPTION Load logical state of start input Load logical state of start input and invert Logical AND operation Logical AND NOT operation Logical OR operation Logical OR NOT operation Output

It is always the ladder logic which determines how the outputs are energized. Consider figure 2, which shows two controllers both having a normally open switch connected to their input ports IN1. The controller shown in figure 2(a) is ladder programmed so that when the switch connected to IN1 closes the output connected to CR1 is energized (turned on). The controller shown in figure 2(b) is ladder programmed so that when the switch connected to CR1 is de-energized (turned off). The control action of figure 2(b) is opposite to that of figure 2(a) because NOT function is used in the ladder. In figure 2(a), the logical state of the input is loaded and that is used to control the output. In figure 2(b) the logical state of the input is read, its value is inverted and then it is used to control the output. N0 IN1

IN1 CR1

CR1

Controller

Ladder Program Load

2(a) N0 IN1

CR1

Controller

Load

2(b)

Ladder Program

Figure 2 : Ladder Control Action

4.0

Hardware & System Sizing and Selection Prepared By : AAB Rev. : 00

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Manual No. CMN-**-I-12

Hardware and system sizing of the PLC can be determined by an analysis of the following system characteristics :

4.1

a)

I/O quantity

b)

I/O remoting requirements

c)

Memory quantity

d)

Redundancy level

e)

Programming requirements

f)

Programmers

g)

Peripheral requirements

I/O quantity : In most PLC’s, plug in modules are used to convert the I/O signal level to one that is compatible with the bus architecture. These models can be composed of 1, 4, 8, or 16 points, depends on the manufacturers standard design. The I/O base (rack or housing) is used to hold the I/O module in place and to provide a termination point for the wiring. The bases may be mounted anywhere in the control enclosure, however, there are cable length requirements which must be met. The majority of the bases mount horizontally to allow proper module cooling. A terminal strip is built into the mounting base for field connections so that no wiring need be distributed in order to remove or replace a module. These bases typically hold various quantities of I/O anywhere from 1 to 128 I/O points. Whereas in most of the systems, the modules have the intelligence to communicate with CPU, some systems require the use of the serial interface modules.

4.2

I/O Remoting Requirements : Prepared By : AAB Rev. : 00

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Manual No. CMN-**-I-12

A unique feature of the PLC is the muliplexed nature of the I/O bus. This can be used to great advantage to reduce overall wiring cost. If I/O racks are centralized in logical clusters, plant wiring requirements can be greatly reduced. Remote I/O can be broken down into two distinct types : the integral type, which allows a limited transmission distance (upto 15,000 feet or 4500 meters) and transmitter / receiver type, which allows virtually unlimited transmission capability. It is important to remember the major weaknesses of remote I/O systems. If the bus is cut or interrupted, the effects of I/O failure will be relatively unpredictable. One must consider the effect of system failure on each step in the sequence. For this reason, duplication of smaller CPU’s at each remote location is often considered preferable to a large central CPU. This is actually an extension of distributed control within the network of the PLC itself. This approach can be very cost effective, since requirement for the central unit size can be reduced.

4.3

Memory Quantity : The type and quantity of PLC memory used depends on the controllers size and the company that manufactured it. Most small PLC’s come with a fixed quantity of RAM. Although it is 2K or 4K of memory, the actual number of memory locations is not as important as the average size application program the PLC can be expected to handle. Size refers to the number of I/O points that are to be controlled and the average number of logic, timer, counter and math operations that are to be performed. Some manufacturers may provide an extra expense option of PROM or ROM memory with their small PLC’s. Midsize and large PLC’s provide users an option for almost any type of memory desired. Total memory, as stated in the manufacturers literature, does not necessarily mean that the entire content is available to the user. Some manufacturers reserve large blocks for the system executive. A system with 4K of 16 bit words of user memory may comfortably accommodate a program, whereas, another system with 8K of 8 bit words may have too small a memory for the same program.

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Manual No. CMN-**-I-12

Special programming language features are an important aspect of memory sizing, especially in process control. For example, one manufacturer requires 33 words of user available memory, whereas another may need in excess of 1000 words. Obviously the memory sizing for a loop control program would vary in these two systems. The closer the language is to machine code (binary based), the more user memory is required to perform the more complex functions. The best way to determine program memory prerequisites is to write a representative sample program reflecting some actual project requirements and to request information about user memory size from various manufacturers. If the manufacturers recommendations are followed, the user can be reasonably assured that the memory will not be undersized. The final area of caution about memory size concerns the consideration if data storage. Data tables, scratch pads and historical data retrieval requirements can inflate the size of the PLC memory. It should be remembered that the primary task of the PLC is control of the process. If the data requirements are large, connections to auxillary devices such as mini and micro computers should be given serious consideration. It is not good engineering practice to degrade control capabilities by burdening the PLC with excessive data acquisition functions.

4.4

Redundancy Level : The availability of the PLC system depends upon the safety aspects involved in the plant process. The Risk graph R (according to standard DIN V 19250), a process unit can be classified into various safety classes like class 1, 2, 3, 4, 5 and 6. Degree of risk factor increases from class 1 to class 6. The graph gives a better idea. Hence the availability level of the system, depends upon the safety classes as mentioned below : i)

Upto class 4 : Normal availability is required. One CPU is connected to single (non-redundant) I/O modules via an I/O bus.

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Manual No. CMN-**-I-12

ii)

Upto class 5 : A redundant CPU modules are connected to nonredundant I/O modules and via a common I/O bus. In turn the CPU, the same program is processed simultaneously via a fast data link the processor exchange the state of the inputs and outputs during a cycle and thus check each other. The defective module can be easily exchanged during operation. If the safe PLC is used in requirement class 5 - for which the standard requires a redundant central unit single channel operation in the case described is permitted for upto 72 hours.

iii)

Upto class 6 : In this case the redundancy level is extended to I/O points, I/O buses and CPU level. The failure of one CPU, usually leads to complete shut down of the system. If according organizational steps are taken, a single channel operation upto one hour is possible before the plant automatically shuts down.

Damage

Duration of Stay

Hazard Relative Low Prevention High

Very Low

Injury

1

-

-

2

1

1

3

2

1

4

3

2

5

4

3

6

5

4

Casualties

7

6

5

Catastrophe

8

7

6

Possible Value Not Possible 1 Casualty Possible Frequent Not Possible Value Several Frequent

Figure 1 : Risk Graph

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4.5

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Manual No. CMN-**-I-12

Programmers : Three basic programming tools generally provided by manufacturers are : -

Hand held programmers

-

CRT Programmers

-

CRT programmer simulators (that run on PC)

The hand held programmers enables the operator to enter a program one contact at a time. These units are widely used because they are rugged, portable and easy to operate. They are very cost effective and give an engineer the capability to enter a program and to diagnose trouble in logic and field devices. The CRT programmer provides the engineer with the visual picture of the program in the PLC. Ladder diagrams are drawn on the screen, just as they could be drawn on paper. Design and trouble shooting is reduced with the use of the CRT. With menu driven software, programmer training time is decreased. The CRT is designed for desk top or factory floor orientation. These units can be ordered with memory storage capabilities. Some CRT programmers provide complete documentation capability, including ladder diagrams, cross reference listing, and I/O listing. The CRT programmer also includes an external RS232C port for connection to a printer. The CRT programmer simulator operates on a personal computer. Cost is an important factor to be considered. The second factor is the program version because of which the simulator may not as versatile as the CRT programmer. This versatility mismatch is in the functions it provides and the number of programmable controllers that can use it. Since the CRT was initially the primary programming tool sold by the PLC manufacturer and the CRT programmer simulator may have been developed by a software house licensed by the PLC manufacturer, there is no guarantee that the program will have all of the functions or operate the same way that the CRT programmer does.

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4.6

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Manual No. CMN-**-I-12

Special Design Norms for PLC : a)

The PLC program should be protected from unauthorized changes by the use of security measures such as passwords or key lock switches.

b)

The system should incorporate comprehensive self diagnostic so that all permanent and transient faults are identified. located. alarmed and reported. All diagnostics should be performed automatically on-line without disturbing the process or reducing the reliability of PLC.

c)

PLC on-line diagnostics should do the following : Test all the spare board in the system. Test board ID and status at a minimum frequency of once per minute. Check the I/O board configuration and set the main chassis alarm if boards are missing or faulted. Check I/O boards for faults, including fuse failures where applicable and if detected, turn on fault LEDs on the board. Perform diagnostics on the communication processor and cables which handle I/O board communications.

d)

PLC must perform diagnostics on its main processor as follows : Diagnostics on the processor and the floating point unit are performed continuously in the back ground. Random Access Memory (RAM) diagnostics are also performed continuously. The micro processors on the main processor board are checked for proper response very minute. The control program checksum is verified. Universal Asynchronous Receiver Transmitter (VART) diagnostics are run continuously. The checksum for all program read only memories (ROMs) on the main processor are checked continuously. Redundant process and programs are verified as good and current. The PLC should perform extensive and power-up diagnostics on the main processor.

e)

The processor should be modular and electrically isolated from associated I/O components. In the event of power loss, the processor should retain its memory for minimum six months.

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Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

A read time clock with a minimum 10 ms resolution should be provided for time tagging events, rate calculations and other time dependent functions. The processor should be capable of scanning and updating the I/O and of executing user defined discrete logic a minimum of ten times per second (100 ms) and analog functions a minimum of four times per second (250 ms). The processor should be able to execute commands using the following functions and parameters : Math functionality using both integers and real numbers. Logic including transition inputs and latching outputs. Time delays, counters and timers. Arithmetic, algebraic and trigonometric functions. PID and process control functions. If-then-else statement programming. Median select and mediation deviation function for analog input voltage. k)

The information to be transferred to from the DCS via PLC interface should include, but not limited to, System alarms and status Discrete I/O status. Analog I/O status. The speed of transmission shall not increase 4 seconds.

l)

Power supplies should be redundant for critical PLC applications with each capable of supplying complete system power. The system should accept power from two different power sources. System power supplies should have over temperature protection, integral fuse protection and status LEDs to indicate power supply faults. In addition, each power supply should have an alarm contact to indicate presence of a fault.

m)

At least 20% spare capacity should be available within each system. This includes marshaling cabinets, terminations, monitor switches. User program memory should have at least 40% spare capacity.

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5.0

Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

PLC Installation Installation of programmable controller systems is not a difficult or mysterious procedure, but the following general rules will save time and trouble for the systems designer or installer. The basic principles of PLC installations are the same as those for installation of relay or other control systems. Safety rules and practices governing proper use of electrical control equipment in general should be observed. These include correct grounding techniques, placement of disconnect devices, proper selection of wire gauge, fusing, and logical layout of the device. PLCs can often be retrofitted into existing hardwired relay enclosures because they are designed to withstand the typical plant environment. PLC vendors provide installation manuals upon request.

5.1

Safety Considerations : Perhaps the most important safety feature, which is often neglected in PLC safety design, is emergency stop and master control relays. This feature must be included whenever a hardwired devices is used in order to ensure operator protection against the unwanted application of power. Emergency stop functions should be completely hardwired. In no way should any software functions be relied upon to shut off the process or the machine. Disconnect switches and master relays should be hardwired to cut off power to the output supply of the PLC. This is necessary because most PLC manufacturers use triacs for their output switching devices, and triacs are just as likely to fail on as off.

5.2

Implementation : Planning ahead is every bit as important in designing a complete PLC system as in laying out a relay logic panel. Care in counting I/O points in the beginning and leaving a safety factor will save headache in the panel fabrication stage. Panels should always have plenty of expansion room left over, since I/O is invariably added as the job progresses and the operators see the advantages of the PLCs. The designer should refer to the layout considerations provided by the manufacturer. Extra space should be left to provide access to the boards and connectors of the PLC. The diagnostic and status indictors should all be visible. The designer should leave room between I/O racks for wire ways and large hands.

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Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

One good technique for ensuring efficient panel layouts is to involve maintenance personnel in the design procedure. This not only optimizes the layout but also introduces the staff to the hardware (figure 1). In general, the best defense against creating a tangled mess when designing a PLC system is to follow proper documentation techniques. A little more time spent documenting panel layout, I/O counts, and wiring diagrams results in a lot less time spent starting up the system. PLCs can handle large amounts of I/O points with varying electrical characteristics, so things can get pretty confusing in a hurry. Cable requirements between hardware boxes vary from one type of PLC to another, so this is an important consideration in panel layout.

Figure 1 : Typical Enclosure Layout

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5.3

Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

Enclosure : Enclosures should nearly always be provided for the PLCs themselves. This protects the electronics from moisture, oil, dust particles, and unwanted tampering. Most manufacturers recommend a NEMA 12 enclosure for the standard industrial environment. This type of environment is readily available in a variety of sizes and, infact, may be already included with a new system. Programmable controllers are designed to be located close to the machine or process under control. This keeps the wiring runs short and aids the troubleshooting procedure. At times, however, mounting the PLC directly on the machine or too close to the process is not advisable, such as in case of vibration inherent in the machine, electrical noise interference, or excessive heat problems. In these situations, the PLC must either be moved away or successfully protected against these environmental conditions.

5.4

Temperature Considerations : Installing any solid state device requires paying attention to ambient temperatures, radiant heat bombardment and the heat generated by the device itself. PLCs are typically designed for operation over a broad range of temperatures, usually from 0 to 60°C. When analyzing the proposed PLC environment, however, one should remember that enclosure temperatures usually run a few degrees higher than ambient temperatures. Radiant heat on an enclosure from surrounding tanks can raise the internal temperature beyond that specified by the manufacturer. Heat generated by the PLC is a key issue when the device is placed in ambient temperatures close to the extremes mentioned in the specifications. The temperature rise caused by the power consumption of the PLC itself is not hard to estimate. In addition, most manufacturers will provide a notation of power consumption of the triacs driving field loads. When designing the hardware layout within the panel, one should adhere to the manufacturer’s suggestions regarding ways to minimize heating problems. Most PLCs use convection over fins to take heat away from particular areas within the hardware. Care must be taken to ensure that no obstruction to air flow over these fins is introduced by placement of the PLC in the enclosure. Wire ways are typically provided with holes to allow air to pass through. Generally, one can avoid problems with PLC enclosures by simply leaving plenty of air space around the heat producers.

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Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

Should all of these factors combine to cause a temperature problem, the panel can be vented, air conditioned, or moved to another location. Usually, simply blowing filtered air through the enclosure will resolve minor difficulties. If air conditioning is required, small units that are designed for cooling electronic enclosures are readily available.

5.5

Noise : Noise or unwanted electrical signals can generate problems for all solid state circuits, particularly microprocessors. Each PLC manufacturer suggests methods for designing a noise immune system. These guidelines should be strictly followed in design and installation phases, since noise problems can be very difficult to isolate after the system is up and running. I/O systems are isolated from the field, but voltage spikes can still appear within the low voltage environment of the PLC if proper grounding practices are not followed. A well grounded enclosure can provide a barrier to noise bombardment from outside. Metal-to-metal contact between the PLC and the panel is a must, as is a good connection from the panel to the ground. Noise producers within the panel should be noted during the panel design phase, and the PLC must not be located too close to these devices. Wiring within the panel should also be diverted around noise producers so as not to pickup any stray signals. Often it is necessary to keep AC and DC wiring bundles apart, particularly when high voltage AC is used at the same time that low level analog signals are present. Line voltage variations can cause hard-to-trace problems in the operation of any computer based system. PLCs are no exception, even though they are designed to operate over a much larger variation in supply voltage. Large spikes or brownout conditions can cause errors in program execution. Most manufacturers protect against this, enabling the controller to come up running after a brownout, but these measures may not be acceptable in all applications. The designer may wish to add an isolation transformer to a proposed PLC system, sized for twice the anticipated load. This is cheap insurance, and PLC manufacturers will help determine the required load.

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5.6

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Manual No. CMN-**-I-12

Hookup : PLC can be very neat and orderly if all the terminals are arranged in a logical fashion. The actual result is a direct function of the time spent during the design process. Interposing terminal blocks between the PLC I/O structure and the field is suggested, since the terminations provided by PLC manufacturers are shrinking in the race to provide higher density I/O. This also gives the panel designer the ability to place the field termination points where they are easily accessed. Wiring ducts keep the panel neat and protect the wire from mishap. Many noise problems can be averted by following good wiring practices. Low voltage signal wiring should be kept away from noise sources. Analog signals should be shielded, with the shield terminated at an isolated ground in the panel only (to prevent shield grounding loops). Again, these analog signals should be separated from power wiring.

L1 Input L1

R Common

N

Figure 2 : Dummy Load for Leakage

L1

Common R

C Output (Triac)

N

Figure 3 : Mechanical Contact in Series, RC suppresser

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L1

Manual No. CMN-**-I-12

Common MOV Output (Triac)

N

Figure 4 : Mechanical Contact in Parallel, MOV suppresser

Triac outputs require some special attention that will be new to relay users. Triacs used for AC loads typically leak a small amount of current. In the case of triacs outputs from a PLC, this leakage may be enough to keep panel lamps glowing or small relays energized. When a triac is used to switch the input on a PLC, the leakage may be enough to make the PLCs ‘think’ the input is on. A dummy load (shown in figure 2) can be used to drain this leakage when the input should be off. Whenever a mechanical contact is used in series with a load energized by a triac (as shown in figure 3), a resistance-capacitance (RC) network should be used as shown to protect the triac from inductive kickback. A varistor should be provided in parallel with a load whenever the load can be ‘hot-wired’ around the triac (figure 4). The user should check with the PLC manufacturer for the suggested RC and MOV (metal oxide varistor) types for the particular application. Triacs cannot directly drive large motor starters and similar devices. PLC manufacturers provide surge specifications for the various I/O cards. Sometimes an interposing relay or dry contacts will be required for large loads.

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6.0

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Manual No. CMN-**-I-12

Applications PLC suppliers can supply numerous application notes for the products that they offer. Most major PLC vendors also publish detailed articles about applications in technical journals and prepare papers for engineering societies and industrial symposia on control, automation, and so forth. Each manufacturers software package usually has its own application programming techniques. Vendors are also valuable source of ‘how-to’ information, providing training courses in their local office or at the factory and actual hands-on experience to help users gain familiarity with the PLC. Most vendors offer an applications or programming manual that gives insight on how to use available programming features. Of course, familiarity with one brand of PLC will help the engineer learn to use another brand quickly.

6.1

PLC Peripherals : The popularity of the programmable controllers has led to the creation of a strong third party peripheral manufacturing industry. These companies are always developing new products that assist the PLC user with interfacing a particular application to a PLC. There are three categories of products : Operator stations, I/O enhancements, and programming and documentation tools. The operator stations facilitate operator interface with the PLC controlled process to monitor process variables, to alter program parameters, to conduct online program alterations, and to conduct troubleshooting procedures. I/O enhancements include all capabilities not ordinarily supplied with PLCs or those items that a particular manufacturer may not choose to support. Programming and documentation tools include products supplied by the manufacturers or made available by third party vendors.

6.1.1

Operator Stations : Operator stations include those provided by manufacturers and intended to be used with their particular PLC and those offered by third party for use with either a particular brand or anyone’s PLC. These stations may include devices such as timer / counter access modules (TCAMs), loop access modules (LAMs), data terminals, colour graphics consoles, computers, printers, and manual backup stations.

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Manual No. CMN-**-I-12

Most PLC manufacturers provide an operator interface unit (OIU) designed specifically for their PLC. These are either the part of the standard system or offered as an option. They are usually mounted directly on the PLC but may be designed to be panel mounted and cabled back to the controller. Functions include access to read write register data, simple programming, and diagnostics. Some specialized devices, such as TCAMs, LAMs, and OIUs, provide operator interaction with PLC internal registers and loop tables. This gives the systems designer the ability to provide real time changing of variables, loop tuning and inspection, manual control of analog outputs, and the ability to provide batch-or-menu-type information at low cost. Communications with the PLC are multi-dropped over an RS422 or a similar differential line. Unauthorized data entry is prevented with software locks, key-lock protection, or both. Some PLCs can support communications directly with dumb data terminals. Operators enter data by issuing special control characters to the PLC communications port. Data terminals can be provided in industrial versions intended for the plant floor or in office machines for entry of data by a supervisor. This operator interface approach is not very user friendly and can be intimidating. Colour graphics consoles offer process graphics and communications facilities to many brands of PLCs simultaneously. These systems range from those that can simply be purchased and put online with a minimum of engineering effort to those that can require some programming. The basic differences are in flexibility. Those that do not require programming may not be able to provide the custom menus and graphics that are required. The ease of communications with different types of PLCs also varies according to manufacturer. Finally the method of generating the graphics pages differs greatly. Most colour graphics consoles offer multiple graphics pages that are animated by reading data tables in the PLCs. Operators enter the data by means of standard keyboards, user configurable industrial keyboards, light pens, touch screens, and the like. Different graphics pages may be selected with pre-formatted or custom menus programmed by the user or the system house. Development stations are often required to give the final user the ability to change graphics menus or key commands after the initial project is completed.

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Manual No. CMN-**-I-12

Computer systems can be made to perform man-machine interface functions. Indeed, the colour graphics consoles described in the previous paragraph are simply computers with standard graphics and communications software packages. Most PLC manufacturers provide board-level additions or modules that give the PLC the ability to converse via the RS232 protocol to most any computer. Of course, both the communications software and the particular applications software must be generated to provide an interface. Many vendors and systems houses are providing communications packages for various PLCs to run on microcomputers and personal computers. These small systems offer low cost operator interfaces to PLCs, providing data handling capabilities and the ability to be networked into a true distributed architecture. In this way, PLC purchasers can be assured that their investment will be protected from factory automation. Microcomputers that have the ability to multi-task and access large amounts of both RAM and nonvolatile memory, have proper software support, and are able to be networked will provide a good investment in terms of operator interface functions as well as total system capability. Printers have always been an important part of the PLC system both as a development tool and for handling some of the operator interface functions. Many PLCs are able to provide communications directly to dumb printers. A stand alone PLC system then can often provide performance reports, alarm logging, and the like without ever involving a computer. This feature is usually somewhat limited, since PLCs were designed primarily to control the process machine. Large amounts of data, sophisticated print logs, and multiple alarms are not really within the realm of a standalone PLC system. This type of data manipulation is too cumbersome and requires too much memory for most PLCs. Manual control stations are important as backups in case of failure of the PLC controlling PID loops. Loop access modules provide manual control capabilities but still rely on the integrity of the PLC, so they are not truly manual in the hardwired sense. A manual control station is an important part of the distributed control system because it gives true manual control of the loops locally or in the control room, even when the local controllers are down.

6.1.2

I/O Enhancements : PLC manufacturers are providing more and more types of input and output capabilities for their products. There are, however, many third party peripherals that aid the PLC in interfacing to the field devices. New I/O capabilities that are being offered include faster response, new analog capabilities, intelligence, high speed pulse counters, dry contact, and specialty modules.

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Fast response I/O is currently offered in both discrete and analog versions. Discrete rapid response modules are facilitated by the PLC logic, but the output does not rely on ladder logic scan times to get updated. For analog modules provide quicker analog-to-digital (A/D) and digital-to-analog (D/A) conversions. This gives PLCs the ability to control faster PID loops and to make analog measurements of assembly line parts (weight, for example). Analog I/O capabilities for PLCs are being expanded from the conventional 4 to 20 mA, 0 to 5 V, o to 10 V versions to include direct thermocouple and RTD inputs. These modules typically accept eight to ten points each, and different types of T/Cs and RTDs are accommodated. Intelligent I/O modules include all modules that are able to perform processing functions. Because the tasks performed by the PLC are further distributed, greater speed and reliability for the overall system can be realized. Intelligent I/O modules give the PLC multiple additional capabilities, which may include memory storage and retrieval, computing tasks, and communications. Memory modules provide additional room to store data points, alarm messages, lookup tables and the like. This approach leaves the main operating memory free for control tasks. Computing modules give PLCs the ability to perform true computer functions using a language like BASIC. Again, the real time tasks are left in the main memory, but tasks such as set-point calculation, formation of data, and some operator interface tasks may be placed in the computer module. Communications modules can provide the PLC with a range of capabilities, from simple ASCII output strings to communication networking. The storage of ASCII messages for a printer or a display can be contained outside the main memory of the PLC, and the data can be output when required. Full system communication networking capabilities are provided with network modules, giving the designer the ability to multi-drop PLCs off a single operator interface device or a supervisory computer. High speed pulse counter modules provide the ability to interface with turbine meters, stepper motors, and optical encoders. High speed pulses cannot normally be interfaced to PLC inputs because of the scan time of the ladder logic. These modules provide a interface that does not rely on the scan time, so that the PLC is able to monitor pulses that indicate position or flow. Dry contact modules are offered by both manufacturers and third party vendors. These modules solve the problems normally associated with triacs, low power, and uncertainty of failure state.

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Specialty modules are designed to solve a single interface problem. X/Y positioner modules can be included in this category, as well as servo axis controllers, stepper motor outputs, and even maintenance access modules. These modules are a further extension of the distributed technology. Clock modules that fit into the I/O bus may be considered to be part of this group. These modules provide real time and day / date functions upon interrogation from the PLC. Most are backed up by a battery to ensure time keeping during power outages.

6.1.3

Programming and Documentation Tools : Both PLC and after market parties offer programming and documentation tools for the system designer or user. These tools include programmers, CRT documentors, and complete microcomputer based systems. Programmers are typically provided by the PLC manufacturer and are designated to program a specific machine or family of machines. Some third parties are offering universal programmers and documentors. These microcomputer devices vary greatly in price and capabilities but offer on and off-line programming to many different types of PLCs, real-time status, and some very sophisticated annotation. Communications to different PLCs are usually supported with different software packages. Each vendor’s product offers different types and amounts of ladder and contact comments. Again, many types of cross references are available to be printed out. Often other PLC design documentation problems may be solved, such as the generation of panel configuration drawings, point-to-point wiring diagrams, and I/O layout. (One system even prints out the wire labels.) Some of these devices offer still other computer services - word processing, BASIC programming, and even computer-aided design (CAD) facilities.

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7.0

Communications

7.1

Introduction :

Manual No. CMN-**-I-12

This section is about getting two or more pieces of equipment to transfer information. The information transfer may involve a point to point link such as a computer to PLC or a network of various types of devices. All communication interfaces are either parallel or serial.

7.2

Parallel Communications : Parallel communications interface use a parallel bus (usually 8 bits wide) to transmit data. They allow data to be transmitted over short distances at high speed. Two common standard parallel communication interfaces are the Centronics and IEEE-488. The Centronics interface is used for connecting printers. The IEEE-488 is mainly used for connecting laboratory instruments to computers.

7.3

Serial Communications : A serial interface transmits and received data one bit at a time. This means that a data byte has to be separated into component bits for transmission and reassembled back again when received. Serial communications interfaces are used for transmitting data over long distances.

7.3.1

RS232 : The most common standard serial communications interface is the RS232 which is also called V24 and EIA. The usual RS232 connector is the 25 pin D connector as shown in figure 1. A minimum cabling configuration will use pins 2, 3, and 7. The connecting line to pins 2 and 3 normally have to be crossed over so that each device transmits data to receive pin. A logic 1 is represented by a -12 V and logic 0 by +12 V. The transmission distance is about 15m. With RS232 the user may set several options within the communications process. Both communicating devices must agree on these options, which are as follows :

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1)

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Baud Rate : This is the operation speed of the serial interface. It is approximately the number of its bits transmitted or received per second. Standard baud rates are : 75, 110, 150, 300, 600, 1200, 2400, 4800 and 9600 baud.

2)

Number of Bits : This is the length of the data to be communicated. If data bytes are to be transferred then the number of bits is 8. ASCII codes use 7 bits. Teletype equipment uses 5 bits.

3)

Parity : Parity is an optional bit added to the data and provides a way of checking whether data has been corrupted. Even parity is when a logic 1 is added to the data to make the number of logic 1s an even number. Odd parity is when logic 1 is added to the data to make the number of logic 1s an odd number. Space parity is when the parity bit is fixed at logic 0. Mark parity is when the parity bit is fixed at logic 1.

4)

Stop Bits : Bits added to the end of the data are called stop bits. One or two stop bits may be chosen.

5)

Duplex : Full duplex requests that a communicating device echo back what it receives. Half duplex tells the communicating device not to echo back it what it receives.

6)

Flow Control : The simplest way of controlling the flow of data between two pieces of equipment is to set the baud rate to that of the slowest link. Alternatively, software and hardware methods may be used. The hardware method requires that the flow control lines 4, 5, 6, 20 of the two RS232 connectors are wired straight through and crossed 4 to 5, 5 to 4, 6 to 20 and 20 to 6. The software methods called XON-XOFF and ETX-ACK use control characters to regulate the flow of data and so do not require pins 4, 5, 6, 20 to be connected.

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Figure 1 : RS232 D Connector

Figure 2 :

RS232 Data line at 600 baud. The data format is 7 bits with even parity

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A typical RS232 signal is illustrated in figure 2. The data is transmitted at 600 baud. This means that each bit is present for 1.66 ms which equates to 1/600. The first bit is the start bit. The next seven bits represent the data. The least significant bit (LSB) appears first and the most significant bit (MSB) last. The following bit is the parity bit. Even parity is used as logic 1 is added to the end of the data to make the number of logic 1 s an even number. The last bit is the stop bit. Data transmitted via an RS232 interface is coded into ASCII codes. ASCII stands for American Standard Code for Information Interchange and is a way of coding characters into a 7 bit form. Figure 16 represents the ASCII code for the letter C. A computer which is used as a programming terminal is normally connected to a PLC via an RS232 link. If programming is done using the computer while the PLC is running and controlling outputs it is termed on-line programming. If programming is done using the computer with the PLC not controlling outputs it is termed of-line programming.

7.3.2

RS422, RS423, RS485 : Standards such as RS422 and RS423 are similar to RS232 although voltage levels for the states 1 and 0 differ. An RS485 port is setup in a similar way to an RS232 (i.e. baud rate, data bits, stop bits and parity must be agreed). Standards such as RS423 have been developed to improve the speed of data transfer.

7.4

Local Area Networks (LANs) : A local area network (LAN) allows a set of PLCs and other devices to be connected together so that they can exchange information. The term local is used because the hardwired link has a limited range. Usually the range is large enough to service a medium sized factory (500 to 1000 m). Networks which are used for long distance communications are called wide area networks or WANS. A network consists of a number of active points (e.g. PLCs) which are called nodes. There are various ways in which nodes can be arranged, depending upon whether the network uses a series of point to point links, a central cable with spurs, or links which makeup a ring. Signals transmitted may be any of the following :

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Category A

1) 2)

3)

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Manual No. CMN-**-I-12

Baseband : Baseband systems simply send a digital signal along the connecting cable. Noise immunity can be poor. Single channel modulated : The signal is superimposed on top of a high-frequency sine wave so that it carries information is called modulation. Broadband : Broadband systems modulate a carrier signal so that information is carried in separate frequency bands called channels.

The layout of a typical PLC network is illustrated in figure 3. It consists of a central network cable with the PLCs connected to it by spurs. The network cable may be a fiber optic, coaxial cable or a twisted wire pair. A fiber optic link is preferred because it is not affected by electromagnetic interference and other types of noise generated in a factory. Repeaters are used throughout the network to boost signal levels. All networks use a protocol which allows nodes (i.e. PLCs) to communicate without cross talk. Many PLC manufacturers have developed their own network protocol for their own equipment. These are called proprietary networks. Examples of proprietary networks are listed in figure 3. Generally the user of a PLC network system does not need to be concerned with the technical details of the network interface or protocol. In figure 3, the computer is linked to the network via a communications converter. This converts an RS232 signal into a network signal.

Figure 3 : PLC Network

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7.4.1

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Response Time of Network : The response time of a network (also called the access time) is the time taken for two nodes to communicate. It effectively gives the measure of how fast data can be transferred from one PLC to another. The response time increases as the number of active nodes or PLCs on the network increases. Typically the response time is about 10 ms. It is essential to know a precise value for the response time if two or more PLCs are to work in unison on a time critical control application. If the network operates too slowly the control action will fail. Emergency stop signals should never be sent on a network but should be hardwired.

7.4.2

Network Standards : Most PLC networks use a data protocol system developed by PLC manufacturer. This means that two PLCs from different manufacturers cannot be networked together. Various standards have been suggested which, if adopted by different manufacturers, would overcome this problem. Two standards are now accepted by IEEE (Institute of Electrical and Electronics Engineers). These are as follows : 1)

IEEE 802.3 : This is the Ethernet standard developed by the Xerox Corporation. It is a baseband system which uses signal coax for the connecting cable. It uses a protocol called CSMA/CD (Carrier Sense Multiple Access with Collision Detection).

2)

IEEE 802.4 : This standard has been developed by a number of companies and used as a Manufacturing Automation Protocol (MAP). It is a broadband system which uses coaxial cabling. It uses a protocol called token passing. It is a high performance LAN which allows the users to predict the response time of the network.

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8.0

Reliance Industries Limited Patalganga Training System

Manual No. CMN-**-I-12

DCS - System Integration with PLCs The first step in integrating DCSs with PLCs is to define what is meant by ‘integration’ for the particular project. In some cases integration refers to a vertical communication for information exchange only. In other cases, integration includes horizontal linkages between the DCS and PLC or computer sharing control responsibility as peers in the system. Some might regard the exchange of data on a few variable as integration. Others would argue that integration is only achieved when common variable names, addressing, and functions exist, all sharing a unified operator interface, with data and information in the system universally accessible throughout. The truth is that integration, like beauty, is in the eye of the beholder. What suffices as integration for one user may not even approach it for another. Therefore the contents of this section are more like a menu of system integration tools and techniques as opposed to a rigid standard. In many cases a collection of integration tools can be selected and combined. Many of the alternatives are combinational rather than either/or options.

8.1

Man Machine Interface (MMI) : One of the key reasons to integrate DCS with PLCs is to obtain a superior man-machine interface (MMI). PLCs generally do not have an embedded MMI. The PLC systems have a processing capability and excellent input / output (I/O) systems for digital information. However, the typical MMI for a PLC based system is a bench board with numerous push-button switches and indicator lights. The DCS, on the other hand has a excellent graphics display system in the form of an operators station. To take advantage of this robust and user friendly interface it is necessary to have the PLC and DCS share information. The PLC must provide the status of controlled devices to the DCS, and the DCS should provide the PLC with control signals which will start or stop a particular motors or a group of motors, or open and close valves. Integration can inform the operator when a requested action is inhibited and can advise the operator what is preventing the action from occurring. For example, if a motor start is requested, but it is inhibited because a limit switch on a safety device is not actuated, rather than just not starting the motor, a well integrated system might describe the nature of the problem to the operator by changing the running light colour from red (stopped) to yellow, instead of to green (the colour associated with running). If the systems are very well integrated, the operator may be able to use the DCS MMI to query the PLC to determine which limit switches are preventing the requested action from taking place.

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8.1.1

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Sequencing : One of the most common reasons for providing a hybrid DCS / PLC system is to take advantage of the PLCs superior sequencing / interlocking capability, as well as the PLCs low cost and highly reliable digital I/O. PLC have been optimized for sequential control. The relay ladder style of programming is well suited for the task of interlocking a number of devices. For this reason, many users elect to use the DCS for analog loop control, MMI, and basic storage, while using the PLC for sequential digital control. This presents the need for integration between the devices. Often pumps or stirrers are not started until certain temperatures or levels are reached. These heating, filling, or other continuous throttling operations are monitored and controlled by the DCS as continuous control loops. The information exchanged between the PLC and the DCS is generally digital in nature, indicating on/off or open/closed status.

8.2

Integration with PLC : Beyond the control requirements there often exists a need to just move information from a PLC to a DCS because the DCS has better storage mechanism for production data. The history modules of many DCS systems are merely ruggedized fixed magnetic disks. Rather than storing production related information in the valuable PLC RAM, the information is passed to the DCS for storage and totalization. The typical types of information exchanged include actual running times for equipment, cycles or actuations for linear devices such as some valve positioners. Another category of information is the PLC program itself. Sometimes a result of a product change, a new operating program must be down loaded into the PLC, because different sequencing is required for making the new product. Rather than having a maintenance person load a new program into the PLC, the DCS can store a number of programs, and when a product change is initiated, the DCS can download the program into the PLC. This takes advantage of the magnetic storage media in the DCS and its capability to synchronize the downloading step with the analog changes being implemented in the process.

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8.2.1

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Integration with Direct I/O : The first method of integrating PLCs with DCS was to provide direct I/O connections to each of the systems. Figure 1 illustrates such a PLC-to-DCS connection. In essence, this configuration creates a high speed parallel connection which operates at the I/O scan speed of the connected devices. While the response is certainly in real time, this method of connection has fallen into disfavour as high speed network based interfaces have become available. Nevertheless, it still is used, particularly where the amount of information transferred is small. It should be obvious that such a wiring scheme, using one or two wires per digital I/O point, is very cable intensive. In addition, it is usually desirable to include a circuit from each side which serves as a watchdog. This watchdog line is a pulse generator which turns on and off on a fixed time basis. Each side (DCS and PLC) monitors the line from the other device. If a circuit fails to oscillate at the proper frequency, the ‘listener’ assumes that the other device has failed, alerts the operator, and follows the preprogrammed failure routine. System integrator like to include connectors like those illustrated in figure 1, because they ease the temporary assembly for testing and troubleshooting, as well as subsequent disassembly for shipment and re-assembly at the final destination. While such connections greatly improve these aspects of the integration effort, it should be noted that they also introduce additional connections which are potential failure points. In process industries with free chloride or sulphide ions in the atmosphere, these connectors must be of a high quality and gastight construction, or problems which arise may be extremely difficult to diagnose.

Figure 1 :

A direct connection between PLC and DCS systems using high speed parallel I/O wiring

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Finally, because these types of connections uses so many individual wires, accurate and detailed documentation is essential. Every wire must be labeled as to its origin and destination, and all DCS loops sheets and PLC logic diagrams should be cross-referenced to facilitate troubleshooting and maintenance. Poor documentation has led to more downtime with this style of connection than have actual failures of the wires and associated connectors.

8.2.2

Serial Linkages : Serial data links are far more common between DCS and PLCs, especially where very high speed communication is not essential to proper performance. Where the PLC serves only to provide information to the DCS for purposes of archival storage or where minimal MMI related data is transferred, serial links are the appropriate choice. A serial link transfers the data as a string of pulses. Figure 2 shows how a string of pulses can represent, in this case, eight separate digital states. Additional bits are required for synchronization and error detection. Typical transmission speeds are from 1200 bits per second to 19,200 bits per second. With a 70% data-to-overhead ratio, 840 data bits per second to 13,640 per second.

Figure 2 : Serial links transfer the data as a string of pulses

The major limitation of serial data communication is that the information transmitted has no specific meaning. Unlike in the parallel wiring scheme (figure 1), there is no physical attribute to tie the information to a particular meaning. A protocol must be agreed to by both parties (PLC and DCS) defining the meaning of information transmitted across the serial link. Usually a message is prefaced with identifies specifying the block of data to follow, and then the actual data. This drops the efficiency of this transfer mechanism even further. Also, the agreement on protocols requires the cooperation of the vendors, which is not always forthcoming as the suppliers are in competition for an overlapping market.

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Manual No. CMN-**-I-12

Finally, serial data communication is much more difficult to troubleshoot. There are multiple coding schemes, data rates, and even a variety of error correction schemes. If all are not perfectly matched, communication cannot take place. As with the parallel wiring discussed previously, good documentation is essential. Because there is generally a single cable with four to twelve wires between the devices, the physical labeling is less important. The meaning of the data communicated; the orders, variable names, addresses; and other pertinent information must be thoroughly documented, however. Because there are so many variations, the buyer must be careful to specify in advance what standards apply in terms of bit rate, type of parity, and number of bits per character. While serial communication represents a significant savings in both wire and I/O module hardware at both sides, it is a more difficult interface to troubleshoot and maintain. The classic tools of the electrician, such as a voltmeter or oscilloscope, are useless for troubleshooting serial links. A special purpose data analyzer is required. In addition, because the data transfer usually operates outside of the control blocks programs, more specialized programming skills are required to use the various data analyzers.

8.2.3

Links Between Networks (TCP/IP) : Today the most common integration tool for tying PLCs and DCS systems is a network-to-network link. Many PLC manufacturers support either proprietary or standard minicomputer network protocols such as MAP, TCP/IP (both standards), IBM’s SNA, and Digital Equipment Corporation’s DECNet. Like X.25, SNA was originally designed to support terminal to mainframe traffic and is not the ideal peer-to-peer networking protocol. MAP, while very functional, has still seen limited acceptance in North America outside of the automotive and aerospace industries. TCP/IP is a far more common open standard than the OSI-based MAP, at least for the present. TCP/IP is shorthand for a stack of protocols using either Ethernet or Token Ring networks, with a collection of upper-layer services. The TCP/IP suite of protocols serves as the mechanism for the research Internet, which grew out of the old ARPANet, now replaced by a dedicated Defense Data Network. The following discussion regarding TCP/IP also holds true for using MAP as the integration technology. TCP/IP was chosen as the example because of its popularity. Figure 3 shows how a process management system (PMS) has been tied together, and with its various components using TCP/IP to communicate. Note that the DCS or PLC devices can tie directly to the TCP/IP network if a TCP/IP interface is available, as is usually the case for the DCS, or through a simple gateway device such as a personal computer, in the case of most PLCs.

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Digital’s DECNet can also be used. Allen Bradley, GE-Fanuc, Siemens, and AEG Modicon, among others, are all PLC providers with DECNet interfaces. Rosemount, Bailey, Honeywell, Fisher, and most every DCS vendor also has a DECNet interface. DECNet is the most popular minicomputer based proprietary network in the marketplace today. In both the proprietary and the open standards based networks, the prime advantages over both serial and X.25 connections are speed and further formatting. The typical network speed is 10 million bits per second, thousands of times faster than serial communications. This high data rate can support true control interaction over the network, instead of just simple information exchange. Also, most LAN technologies support basic messaging services and file transfer capabilities and have generally well defined protocols. Because these networks exist outside the realm of the control systems, troubleshooting tools and support staff are often already available.

Figure 3 :

A Process Management System (PMS) with its components being tied together by TCI/IP protocols using Ethernet or Token Ring networks

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8.3

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DCS Integration with PLCs : The advantages of tying PLCs to DCS systems are numerous. Improved MMIs for the PLC often result in improved performance and higher productivity. Where advanced interfaces are provided, which can explain why actions may be inhibited, the resulting productivity improvements through reduced downtime can be substantial. Also, the use of a computer or DCS to store alternate programs corresponding to different products or product grades can greatly simplify PLC logic and can often reduce the size of the PLC needed to perform a task. Feeding production related information to the DCS also frees the PLC from performing tasks which it cannot perform in an optimized manner. This also reduces the initial capital investment and simplifies the programming of the PLC. By integrating the DCS with the PLC, the best features of each device can be used. The PLC can be used for sequential logic and to provide inexpensive and robust I/O. The DCS can be used to provide a better MMI, the ability to handle analog I/O, PID control, midterm storage.

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9.0

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Manual No. CMN-**-I-12

RIL Installations Brief details of PLCs across the Complex - Patalganga (as on 25/12/97)

Plant PX PX PX

Model 530 5TI Sequencer MUR40B

Qty. 3 2 1

PX PX

Make Texas Texas Crompton Greaves Modicon Allen Bradley

984 S5

1 1

PTA

Allen Bradley

SLC500

2

PTA PTA PTA PTA

Omron Omron Siemens Siemens

Sysmac S6 Sysmac C200H S5115U S595U

2 2 1 1

ENC ENC ENC ENC ENC

Modicon Allen Bradley Allen Bradley Siemens Mitsubishi

1 2 2 1 3/2

ENC

3

LAB

Crompton Greaves General Electric

984-685 5/10 5/40 S5110U FXOS30MR / F140MRES R64

Crystallizer Chiller Compressor of Crystallizer Off-gas Drier (New) Blow Egg (Area 75) Off-gas Drier (Old) Polden Sulzer Oxygen measurement in Silos (for switching) HRSG1 Soot Blower for HRSG BMS for HRSG Load Shedding Logic Air Instrument Dryer + Vitox (PSA) DM Plant

1

Back end BMS

LAB PSF PSF PSF PSF PSF

LT - Yaskawa Allen Bradley Allen Bradley Allen Bradley Allen Bradley Symax

Series Six Programmable Controller U-84 System 2 / 20 SLC500 5 / 80 Microtrol 500

1 2 3 1 1 4

Front end BMS Chipper TOW Pack / Chipper Spinning TOW Pack Baler

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Application CCR PAREX CCR Air Dryer System

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Plant PSF PSF PSF PSF PSF PSF PSF PSF FDY UTI UTI UTI UTI UTI

Make Siemens Siemens Siemens Siemens Siemens Siemens Siemens Siemens Mitsubishi (Melsec series) Omron Allen Bradley Crompton Greaves Allen Bradley Allen Bradley

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Model S5135U S5135U S5135U S595U S590U S5110U S5100U S595U A2USCPU-S1

Qty. 1 1 1 1 1 1 1 1 5

CQM1 Microtrol 4 R 64

2 2 1

SLC 500 PLC 5

Reviewed By : RB Date : 25/12/97

Manual No. CMN-**-I-12

Application Draw M/c Draw M/c Conveyor / Baler Conveyor / Baler Conveyor / Baler Siemens Inverter PFY-II TOW Baler Extruder (winder monitoring panel) N2 adsorption changeover New Air Dryer Cooling Tower Fans Thermax & Bertram Communication between AB Station and SLC 500

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Manual No. CMN-**-I-12

10.0 References 1)

Instrument Engineers Handbook (Process Control) - B. G. Liptak

2)

API - 550 Manual.

3)

TUV Standard on Safety Interlock.

4)

PLC and their engineering applications - Allan J. Crispin (McGraw Hill Book Company)

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