PLC Structure and Hardware

INDUSTRIAL AUTOMATION AND NETWORKING SECTION Programmable Logic Controller_1

== PLC STRUCTURE/HARDWARE ==

1.

BASIC COMPONENTS

Basic components of a PLC:


Power supply




Central Processing Unit (CPU)




Inputs Modules




Outputs Modules

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INDUSTRIAL AUTOMATION AND NETWORKING SECTION Programmable Logic Controller_1

The general structure of programmable logic controller is as follow:

I/O Bus

Program Memory

Power Processor

Supply

Inputs and

Data Memory

Outputs

Central Processing Unit External Power Supply

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I

O

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1.1 - Power Supply Module

There are several types of power supply module:


240V ac




110V ac




24V dc

Function: Provides all the voltage level needed to operate the PLC. Converts 110V ac or 240V ac into the dc voltage required by the CPU, memory, I/O interface.

Example of a power supply module

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Power supply wiring/installation guide:

1.2 - Central Processing Unit (CPU)

Consist of:


Processor




Memory

Function: Receives information from input interface, process according to the program stored, update the output information.

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Example of a CPU module

Memory

Memory location is an address in RAM or ROM where a group of bits (binary digits) can be stored.

Grouping of bits depending on PLCs: 8 bits per word 16 bits per word 32 bits per word

LSB : Least Significant Bit - Digit that represents the smallest value MSB : Most Significant Bit - Digit that represents the largest value

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Example: The memory size of PLC is 884 words. How many bits of information can be stored? Solution: 8 bits PLC : 884 x 8 = 7 072 bits 16 bits PLC : 884 x 16 = 14 144 bits 32 bits PLC : 884 x 32 = 28 288 bits

Types of memory:





Volatile 

information lose when power lost



easy to alter



suitable when supported by battery backup

Non-volatile 

no information lose when power lost

RAM (Random Access Memory) 

Volatile memory



Read and write easily



PLCs usually has CMOS-RAM with battery backup

ROM (Read Only Memory) 

Non-volatile memory



Read only



Information stored by manufacturer for internal use of the PLC

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EPROM (Erasable Programmable Read Only Memory) 

Non-volatile memory



Can be reprogrammed after being entirely erased with the use of an UV light

source

EEPROM (Electrically Erasable Programmable Read Only Memory) 

Non-volatile memory



Same programming flexibility as RAM. Provides permanent storage of the

program but can be easily changed using standard programming devices.

Memory Organisation or Memory Map: Memory are divided into 3 areas:


Data table area Stores information for user program:  Input table  Output table  Internal relay  Function blocks current and pre-set value (timer / counter) If signal is ON/OFF: information is stored in unique bit location (bit address) If signal is number or code: information is stored in unique register or word location

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User program area Stores the programmed ladder logic/sequential function chart. Most instructions require one word of memory & occupy the largest area of memory




System area

Memory Areas Used With CQM1H

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Programming a PLC

 Using a PC



Point-to-point connection



Connection through network

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 Using a hand-held programming terminal

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1.3 – Inputs Module

Function: To receive and convert field signals from pushbuttons, sensors or switches into a form that can be used by the CPU.

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1.4 – Outputs Module

Function: Takes signal from the CPU and translates them into forms that are appropriate to produce control actions by external devices such as indicator lights, solenoids or motor starters.

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1.5 – Inputs / Outputs Signal Processing

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2.

TYPES OF PLC

There are two types of PLC:







Compact




Modular

Compact PLC

The compact/ micro has the power supply and I/O on-board. It can be used on smaller, stand-alone applications such as elevators, car washes, or mixing machines. It can also be used on more complex industrial applications such as bottling and packaging machines.

Siemens

Allen Bradley MicroLog

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Modular PLC

The modular PLCs are used in more complex applications that support a greater number of I/O points. They are are expandable. The power supply and I/O consist of separate modules connected to the CPU. Choosing of modular PLCs depends on the complexity of the task and possible future expansion.

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