Seminar Report on Dcs

CHAPTER – 1 INTRODUCTION TO DCS It is now a standard practice to build large, complex, systems based multimicroprocessor distributed control. Distribution of control requires high speed reliable communication matched to the computer and control requirements amongst various processors.

1.1 WHAT IS DCS? A distributed control system (DCS) is a computerized control system, in which controller element are not centrally located but distributed throughout the system. That means it is a type of automated control system that is distributed throughout a machine to provide instructions to

different parts of

the

machine.

Instead

of

having

a

centrally located device controlling all machines, each section of a machine has its own computer that controls the operation. For instance, there may be one machine with a section that controls dry elements of cake frosting and another section controlling the liquid elements, but each section is individually managed by a DCS. A DCS is commonly used in manufacturing equipment and utilizes input and output protocols to control the machine. Distributed control concept is a cost effective approach for implementing large systems and networks based on standard low cost processing elements (microprocessor) and components. At one end of the application we have the simple industrial control and at the other end the high performance control for telecommunication switching. Most applications will fall between these two extremes. Industrial control systems are generally based on a number of loosely coupled processing elements, each of which handles number of relatively slow signals. A wide range of signals are required to be processed which include simple onoff conditions, dc and ac voltages, frequency components, etc from various transducers. Signal conditioners are required to bring all input signals to a standard level and impedance range. In certain applications, the signals, although falling in the slow category will nevertheless require highly complex processing as; in the case of speech recognition. Each channel will need a powerful digital signal processor to achieve even rudimentary level of processing. Such applications are likely to be more common as digital signal processors of

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highest capability become available at reasonable cost. More and more systems involving on-line speech and pattern recognitions as well as certain degree of artificial intelligence are needed. An industrial security network is a good example of such applications. Communications switching requires one of the most complex distributed control systems. Apart from the need to process a very large number of random external events, the requirements of operability, reliability and maintainability are extremely stringent. A down time in the range of half hour in about twenty years of operation is envisaged. Further, the system is required to be modified, expanded or repaired in hot condition. These requirements put additional strain on hardware and even more on the software design. The call processing i.e. finding a free channel and establishing a connection is only a small portion (about10%) of the total task. Considerable effort is required to make the system maintainable. This requires starting from the basic architecture, good partitions that can be efficiently diagnosed on-line, replace a faulty module "automatically" and provide fast system recovery after repair. It is common to build a system handling 100,000 subscribers to be built from basic modules of 100 lines. In order to be able to manage efficiently the total control, apart from the inherently powerful processing elements, a good message switch is required at every module. The number of messages to be Processed at each module rise very fast as the total number of modules increases, leading to an undesirable overhead. A good balanced architecture, both at hardware and software levels is very crucial for achieving satisfactory performance. The need to handle image information and wide band services will result in new architectures that can cope up with problems posed by high speed communication and signaling requirements.

1.2 SYSTEM CONSIDERATIONS In order to be able to meet the required functions, the system has to meet a number of specifications and features. The most important of these are given below:

Architecture:The system is distributed over a wide range of area. The distance between the central control unit and a remote control unit could be several kilometers. Hence a reliable data transfer mechanism is also needed. Problems of feeding power will also be critical (common 2

conductors for both signals/data as well as the power would be required). Duplication of hardware at the optimum level would be required to meet the system availability objective.

Configuration:In many control applications, parts or all of the system must operate continuously. Also, although some control systems are relatively static, changes to their components or to the relationships between them will occur over timescales of weeks to years. The changes are usually in connection with reconfiguration or adjustment of the control functions. This has following implications:

Fault avoidance:The system must be built so as to minimize the number of faults and hence the probability of errors which might impair its operational capacity.

Fault tolerance:The system must include sufficient error recovery capability to ensure that residual faults do not seriously impair operation. It must be possible to isolate individual hardware or software components which fail, without bringing the entire system to a halt.

Modifiability:It must be possible to replace a component of the system (to reflect changes in equipment or to repair a fault) without halting the entire system.

Extensibility:It must be possible to extend the system by incremental addition of both hardware and software. This may require those parts of the system which are directly affected to be halted during this operation, but the rest of the system must continue running.

Modularity:It is well known that none of the above requirements remains without modularity. This is the ability to construct a system from a set of self-contained units with well define 3

interfaces. At the hardware level, suck units are readily available i.e, microcomputers, stations and communication lines. Similar modularity must exist in the application software.

Communication:A wide range of communication requirement exists. Smaller, compact systems can interconnected via multi-conductor which also provides high speed bandwidth capability. For communication over longer distances, series communication will be needed with appropriate coding d-c removal, etc. The speed will depend upon its application, with higher end application requiring large number of message exchange. This can best be done by having a general purpose "message switching chip" at each processor node. This approach will permit handling all type of communication in a uniform way.

Other Considerations:

System is capable of handling overloads and under adverse condition the degradation is graceful, fast system recovery.



Special graphics oriented operation and maintenance tools make the network very user friendly.



Use of CMOS components results in a rugged system that does not require any special air-conditioning.



Hardware/software partitions take into consideration the packaging and manufacturing aspects.

Fault Diagnosis and Tolerance:This is perhaps the single most important aspect of distributed control systems. There have been significant advances in the last 25 years but except for single faults in combinational logic formal tools are not available for handling faults in sequential circuits. Handling intermittent faults, multiple faults are even more difficult. Present techniques are therefore based on intuitive reasoning, thumb rules and past experience. Electronic Switching Systems [ESS] have the most elaborate fault diagnosis and tolerance techniques. These could be modified, simplified and adapted for less critical systems. A three level approach could be adopted. At the first level on1y passive techniques are to be used. Each 4

module is expected to have a set of test points that can be scanned at fixed periods to check the validity. Parity, CRC checks, pulse/clock absence detectors, voltage Level detectors are used for this purpose. Micro diagnostics can be employed for units based on micro program control. Obviously such checks are possible in rather simple modules but they are very crucial for the overall system fault detection strategy. The next level is based on interactive operations. A module under test is subjected to certain test signals during its idle conditions and the response is checked, Arithmetic logic units, signal converters and preprocessors can be checked by this method. A loop back facility helps these tests. The third level is based on overall system testing using periodically simulated signal conditions for checking the overall performance. A combination of all the three would provide an acceptable solution. Also important is the system recovery aspect. After repair/fault removal it is necessary to bring the system to a fully operational level in shortest possible time. In duplicated system the repaired module is to be brought in synchronism with its duplicate. Hardware as well as software aids are required for this purpose. For the overall plan it is to be noted that the additional hardware may not be more than about 10%. Otherwise the unreliability of diagnostic hardware itself would cause some problems.

Redundancy Plan:Depending upon the criticality of the application redundant hardware is introduced into the system. This will "take over" the functions of a unit that becomes faulty during the operation. The various schemes include: full duplication in a hot standby mode, cold standby, N+1 redundancy, triple modular redundancy with majority logic and duplicated doublets are used. The selection depends upon the amount of loss of data that is permitted, speed of fault detection/diagnosis, and time available for repair/ replacement. As the cost of hardware goes down, it is reasonably economic to duplicate only the common/control hardware that is the hardware that affects a majority of the signals and processing. Several interesting schemes with simple asynchronous operations are available for this purpose.

1.3 SOFTWARE CONSIDERATIONS As is true with any real time system the software is the most crucial factor. The issue of real time control at the individual module level and at the total system level includes 5

management of hardware resources and the execution of the application processes. Features includes homogeneity if ability to allocate task to any mode based solely on deadlines and availability, scalability i.e. ability to tailor computing needs without hardware redesign and survivability i.e. ability to provide alternate paths for communication and control. Physical processes can be added without affecting the application software. Broadly the O.S. takes care of interrupt handling, hardware configuration management and fault diagnosis and recovery. The executive performs scheduling, allocation of storage space, management of nonresident software modules, and dispatch of tasks with priority, etc. The application software takes care of user problems and isolating the hardware details.

Other considerations include:There is no need for software components to migrate dynamically. In general a software component is tied to the processor to which the controlled devices are attached. Hence, only computation bound software could migrate between processors. The only justification for migration is to share processing power, assuming that it were a scarce resource. However, the falling cost of microprocessors means that very few control applications will be limited by processing power. Since the degree of parallelism inherent in a function is known when the function is programmed there seems no requirement for the dynamic spawning of software components (processes or tasks). There are, however, some exceptions to this e.g. telephone exchange control may require a task to be created on each call arrival.

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CHAPTER-2 EVOLUTIONS OF DISTRIBUTED CONTROL SYSTEM Control system can range from simple local control to highly redundant distributed control. DCS systems, by definition, apply to facilities that are large enough that a distributed control system is necessary.

2.1 LOCAL CONTROL SYSTEM Initially control was performed local to the equipment, in which sensors, controller, and controlled equipment are within close proximity and the scope of each controller is limited to a specific system or subsystem. Local controllers are typically capable of accepting inputs from a supervisory controller to initiate or terminate locally-controlled automatic sequences, or to adjust control set points, but the control action itself is determined in the local controller. Required operator interfaces and displays are also local. This provides a significant advantage for an operator troubleshooting a problem with the system, but requires the operator to move around the facility to monitor systems or respond to system contingencies. Examples of local control are the packaged control panels furnished with chillers or skid-mounted pump packages. The advantage of this system was low wiring cost and the disadvantages were not much control, monitoring, alarming & history.

Fig: 2.1 Local Control Systems

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2.2 CENTRALIZED CONTROL SYSTEM It was introduced in 1960’s to replace local control by using computers, in which all sensors, actuators, and other equipment within the facility are connected to a single controller or group of controllers located in a common control room. Locating all controls, operator interfaces and indicators in a single control room improves operator knowledge of system conditions and speeds response to contingencies. This type of system architecture was common for power plants and other facilities using single-loop controllers or early digital controls in the past, but it has now been largely supplanted by distributed control because of the high cost associated with routing and installing all control system wiring to a central location. Centralized control systems should only be considered for small industries and if used, must have fully redundant processors. Where redundancy is provided in a centralized control system segregated wiring pathways must be provided to assure that control signals to and from equipment or systems that are redundant are not subject to common failure from electrical fault, physical or environmental threats .

Fig: 2.2 Centralized Control Systems 8

2.3 DISTRIBUTED CONTROL SYSTEM It was introduced in the mid of 1970’s. First DCS was developed by Honeywell, U.S. in 1975. In a distributed control system, controllers are provided locally to systems or groups of equipment, but networked to one or more operator stations in a central location through a digital communication circuit. Control action for each system or subsystem takes place in the local controller, but the central operator station has complete visibility of the status of all systems and the input and output data in each controller, as well as the ability to intervene in the control logic of the local controllers if necessary.

Fig: 2.3 Distributed Control Systems 9

CHAPTER-3 WORKING OF DCS 3.1 WHAT DOES DCS CONSISTS OF? When we take about distributed control system it means we are talking the control system of large industries, in which control of all the processes at a time are not so easy. So we require a large control system such as DCS which can be made by assembling various components and these components are as follows: a) Field Control Station b) Operator Station c) Engineering Station

Fig: 3.1 Components of Distributed Control System

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3.1.1 FIELD CONTROL STATION (FCS) It consists of input/output modules, CPU (central processing unit) and field instruments like transmitters and control valves are wired to the FCS. All the instruments and interlocks created by software reside in the memory of the FCS. All input signals coming from the field are first given to this component of DCS. It is used to control the process.

3.1.2 OPERATOR STATION (OS) This component of DCS is used to monitor the process and to operate various instruments. This is similar to a computer monitor, which is used to provide feedback information.

3.1.3 ENGINEERING STATION (ES) Engineering Station is used to do the engineering builder for all the stations like HIS (human interface station), FCS (field control station), CGW (communication gateway) and BCV (bus converter). It is also used to control anything required to be monitor on operator station.

ES is a PC loaded with Engineering software. The HIS can be loaded with

engineering software so that it can be used as HIS as well as ES.

3.2 HOW DOES IT WORK? Basically, distributed control system receives input signal from field this signal is then converted into digital signal via the input module. Again this signal is given to the DCS CPU which will instruct the controller to take action. All this functioning is monitored in the operator station. Now, the signal which required being control is in the form of analog signal which is given as an input to FCS. Here this signal is being converted into digital form and given to the CPU. The signal coming from field is known as process or measured value, it can be either voltage or current. If it is in the form of analog signal then the value of current would be 4-20mA and that of voltage would be 1-5V DC, if it is digital signal then it would either be logic0 or logic1.

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The CPU will just compare the process value (PV) with the value sited by the operator i.e. set point value (SV) and if there is an error, the CPU will generates an output known as manipulated value (MV) and send it to the field via the output module to control the process by controlling the valve. In distributed control system it is not necessary that the monitoring should also be distributed it can be centralized but the controllers are distributed throughout the system. Communication bus is used to connect all the element of DCS with each other. It is also known as data bus which is responsible for data transmission from one place to other. The data bus is a bi-directional data bus which can exchange information in both directions.

Fig: 3.2 Working of Distributed Control System

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To better understand it let us take an example of simple level control mechanism. The water in the tank is required to be control the level transmitter will measure the level of water and sends a signal known as process value (PV) to DCS system. Here this process value is compared with the set point value (SV) by the help of CPU. If there is a difference between these two values then based on the result an output will be generated known as manipulated value (MV), which will instruct the control valve or actuation device to open or close until the desired level of water will reached.

Fig: 3.3 Simple Level Control Mechanisms

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CHAPTER-4 ARCHITECTURE OF DISTRIBUTED CONTROL SYSTEM

4.1 SCHEMATIC OF DCS A schematic of the DCS network is shown in figure 4.1. Basically, various parts of the plant processes and several parts of the DCS network elements are connected to each other’s via the data highway (field bus).

Fig: 4.1 Schematic of DCS network 14

The block in the bottom represents the plant processes, there are number of processes that occur in the production line of industry which required to be controlled. These processes are known as plant processes. Now the process which requires to control is first given to this block as an input which is in the form of physical signal, here it is converted into electrical signal by the help of field instruments and then transmitted to the upper level i.e. automation system. The automation system consists of various cabinets in which different modules are placed and a local screen display which the monitored the functioning of the cabinets. In this level the electrical signal coming from plant processes is first given to the input module which can convert this signal into digital signal by the help of analog to digital converter and then send it to the CPU module which will compare this value the value sited by the operator i.e. set point value and based on the result decides what control action is to be taken. The uppermost level of the DCS network is operating and monitoring system which has the function to monitor the processes and to operate various instruments.

4.1.1 PLANT PROCESSES It consists of following elements: a)

Transmitter

b)

Terminal box/Junction box

Transmitter:A “transmitter” is a type of transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function of only the measured variable. The sensor will sense the signal coming from the field and generates a signal known as process value this signal is again given to the transducer which will convert it to an electrical signal and then given to the transmitter for further transmission.

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Terminal box/Junction box:It is just a terminal for connecting many cables. Each terminal box conditions one space within the building. This may be an office, meeting room or part of an open plan office.

4.1.2 AUTOMATION SYSTEM It consists of following elements:a)

Local screen display

b)

Cabinets

Local screen display:It is generally used to monitor the functioning of processors placed in the cabinet. This device usually provides analog display stations, analog trend recorder, and sometime video display for readout.

Cabinet:It is simply a place in which various processors, I/O module, CPU module, Bus coupler, etc are placed. Figure 4.2 shows the picture of cabinet into which I/O slots are provided to carry different modules required for different processes, for example if it is required to control the temperature one processor is used and if it is required to control the pressure another processor is used. CPU module will just compare the process or measured value with the set point value and based on the result it will generates an output known as manipulated value via the output module to the field. Cables are used to create a link between I/O slots and CPU module. These are generally made up of coaxial cable type in which a single copper conductor is insulated and then covered with a concentric braided copper shield which also serves as a current carrying conductor. Bus couplers are simply used to multiplex or demultiplex the buses necessary for the operation. Power supply unit is used to provide external supply of ±24DC to the processor 16

and all auxiliary voltages are generated internally to the processor. Bottom unit is simply used to provide a platform to the cabinet.

Fig: 4.2 Cabinets

4.1.3 OPERATING AND MONITORING SYSTEM Operating and monitoring system operates on 230V AC supply. It is generally used to monitor the operations in the plant. It consists of following elements:a)

Human machine interface

b)

Graphic display

c)

Historical data storage and retrieval

d)

Data highway

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Human machine interface (HMI):Human-machine interface provides interface between user and terminal system that consists of a physical section and a logical section dealing with functional operation states. Operator interfaces, or human machine interface (HMI) for DCS systems provide the functions of status indication, alarm reporting, operator intervention in control action, and data storage and programming. Several levels or layers of operator interfaces are required to provide a reliable and maintainable system: equipment level, controller level, and supervisory level. At the controller and supervisory level, HMI may also provide capability to modify the controller program.

Plant Operation by thousands of Graphics

Real time display of Plant Abnormalities.

Easy Operation by a mouse, a keyboard etc. Fig: 4.3 Control room for a typical DCS system 18

Graphic display:This device usually provides analog display stations, analog trend recorder, and sometime video display for readout. The data can be displayed in any way the user requires. The data can be viewed from anywhere, not just on site. It is customer-configurable, object orientated and bit mapped, unlimited number of page and it has the resolution up to 1280 x 1024 with millions of colors.

Historical data storage and retrieval (HSR):This device is provided with significant computing power that is used to structure much of the incoming data and format it into a form for easy retrieval. Process variables are available for hourly, shift, daily and monthly average calculation and recording. All system event history such as process alarms, system status changes, and error messages are stored into the HSR. The user can see trends of past process conditions by calling historical data storage and retrieval.

Data highway:A data highway is a serial digital data transmission line connecting all other components in the system may consist of coaxial cable. Most commercial DCS allow for redundant data highway to reduce the risk of data loss. DCS can access a large amount of current information from the data highway.

4.2 TYPES OF DISTRIBUTED CONTROL SYSTEM There are basically four type of distributed control system:a)

Plant distributed control system

b)

Direct digital control system

c)

Remote terminal unit based distributed control system

d)

Programmable logic control based distributed control system

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4.2.1 PLANT DISTRIBUTED CONTROL SYSTEM (DCS) While the term DCS applies in general to any system in which controllers are distributed rather than centralized, in the power generation and petrochemical process industries it has come to refer to a specific type of control system able to execute complex analog process control algorithms at high speed, as well as provide routine monitoring, reporting and data logging functions. In most applications, the input and output modules of the system are distributed throughout the facility, but the control processors themselves are centrally located in proximity to the control room. These systems typically use proprietary hardware, software and communication protocols, requiring that both replacement parts and technical support be obtained from the original vendor. 4.2.2 DIRECT DIGITAL CONTROL SYSTEM (DDC) DDC systems are used in the commercial building heating, ventilation and air conditioning (HVAC) industry to monitor and maintain environmental conditions. They consist of local controllers connected to a network with a personal computer (PC) based central station which provides monitoring, reporting, data storage and programming capabilities. The controllers are optimized for economical HVAC system control, which generally does not require fast execution speeds. Their hardware and control software are proprietary, with either proprietary or open protocols used for network communication.

4.2.3 REMOTE TERMINAL UNIT (RTU) BASED DCS RTU-based systems are common in the electric, gas and water distribution industries where monitoring and control must take place across large geographical distances. The RTUs were developed primarily to provide monitoring and control capability at unattended sites such as substations, metering stations, pump stations, and water towers. They communicate with a central station over telephone lines, fiber-optics, radio or microwave transmission. Monitored sites tend to be relatively small, with the RTU typically used mainly for monitoring and only limited control. Hardware and software are proprietary, with either proprietary or open protocols used for data transmission to the central station.

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4.2.4 PROGRAMMABLE LOGIC CONTROL BASED DCS PLCs, can be networked together to share data as well as provide centralized monitoring and control capability. Control systems consisting of networked PLCs are supplanting both the plant DCS and the RTU-based systems in many industries. They were developed for factory automation and have traditionally excelled at high speed discrete control, but have now been provided with analog control capability as well. Hardware for these systems is proprietary, but both control software and network communication protocols are open, allowing system configuration, programming and technical support for a particular manufacturer’s equipment to be obtained from many sources. 4.3 NETWORK CONFIGURATIONS Commonly used network configuration includes star, ring and bus configurations. A logical network is defined as a group of interconnected devices that are communicating together with the same protocol. Different logical networks may be interconnected by using protocol converters or translators.

4.3.1 STAR TOPOLOGY In a star topology, each device on the network is connected to a central hub by a single communications circuit, as shown in figure 4.4. The hub performs the function of passing messages between devices. Types of devices that may serve as the hub of a star network include repeaters, switches and routers. The most common example of this topology is the Ethernet LAN used to interconnect all of the personal computers within an office environment. In this case, a dedicated cable is routed from the Ethernet port on each PC back to a switch or router somewhere in the office building. In a star network, loss of a single communication circuit affects only the single device at the end of that circuit, although loss of a hub device obviously affects the entire network. The star network has the highest installation cost per device.

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Fig: 4.4. Star Topology

4.3.2 RING TOPOLOGY In a ring topology, two communication ports are provided on each device and the network circuit makes a loop through all of the devices, with an open point, as shown in figure 4.5. Two-way communication allows messages to pass in either direction along the network. Messages must be passed through the communication a port of each device on the network, making it vulnerable to a break if a single device fails or is removed. If a means is provided to bridge the open point on failure of a particular device or circuit segment, this configuration can have high reliability at relatively low cost.

Fig: 4.5. Ring topology 22

4.3.3 BUS TOPOLOGY OR TAPPED TOPOLOGY In a bus/tapped (or multi-drop) network the communications circuit is tapped to be connected to each device so that the communication ports of the various devices are effectively electrically in parallel. The configuration in figure 4.6 typically represents the lowest installed cost per device. This configuration is commonly used for field device communications. In this all nodes have direct connections with each other. Failure of any nodes doesn’t affect the network, new nodes can be easily added data sender or receiver can be distinguish by addresses.

Fig: 4.6. Bus Topology

4.4 DIFFERENCE BETWEEN DCS AND PLC Since both DCS and PLC are control system used in industrial process control system however there are some differences between these two systems:

a) The difference between DCS and PLC is that, PLC is primarily a small scale control system which only executes digital function. When we say DCS, it is a network of

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PLC's, it is a large scale control system which is distributed all over the plant, and is located in a control room. b) DCS (Distributed Control System) is a control system that works using several controllers and coordinates the work of all these controllers. Each controller is handling a separate plant while the PLC (Programmable Logic Controller) is a controller which can be re-program back. If the PLC is only a stand-alone and not combined with other PLCs, it is called as DDC. It means PLC is a sub system of a large system called DCS. c) Controller in the DCS is more intended as a "Process Controller", while Controller in the PLC is more intended as a "Logic Controller". d) Distributed control system, have a redundant PLC for each PLC so that if primary system fails then its standby PLC takes over the charge. But PLC is basically oriented for one job only, if PLC fails then whole system will shut down. e) DCS is commonly used with process control systems as it is a combination of many control cards, PLCs and can manage a large scale system while PLC is commonly used with ON\OFF (digital) control and may be expanded with analog I\O modules for analog control and used for a unique task.

4.5 ADVANTAGES a) Depending upon the criticality of system redundant hardware is provided which will take over the function of the system which becomes faulty during the operation. b) In distributed control system graphical interfaces is also provided which makes the operator easy to see the control process with the help of pictorial representation and also the user can easily change the value by using input devices such as mouse or keyboard. c) Historical data storage and retrieval record the events that occur in the plant and stores the data in the form of logs, reports, trends so that data can be easily available in future evaluation. d) If it is require to install a new unit then it can be simply tapped with the predefined network rather than installing a new control system. Thus the installation cost gets reduced.

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4.6 DISADVANTAGES a) As distributed control system is a microprocessors based system so that programming them is not so easy. b) The centralized control system is alone capable of controlling the process in case of small scale industry, so it becomes expensive to install DCS in such type of industry. c) As DCS is distributed throughout the industry so it can be difficult to maintain all of them at a time.

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CONCLUSION

Distributed control system is one of the powerful control system used ever. Also it has the capability to stand alone and if it can be used in integration with other control systems like PLC’s and SCADA, then it gives a full control over the processes occur in an industry. Distributed control systems must be designed and commissioned with a mind to the overall operation and optimization of the system under control. It is important that the different parts of a distributed system can be interconnected via a capable communications network. As the amount of processing power available and the communications capability within controllers is increased there is more scope for the control system to optimize the operation of such plant by coordinating the operation of the system as a whole. The renewed emphasis on energy savings will make it important that we use these systems to their full potential.

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REFERENCES [1] Stout, T. M. and Williams, T. J. (1995), “Pioneering Work in the Field of Computer Process Control”, IEEE Annals of the History of Computing 17 (1). [2] Z. Tang, Z. F. Zeng, X.W.Zhou and J.Liu, “Distributed Control System”, computer measurement & control, 2007. [3] Process Control in Chemical industries, “Distributed Control System”, Chemical Engineering Dept, King Saud University pp.132-145, 2002. [4] J.M.Nogiec, E.Desavouret, D.Orris, J.Pachnik, S.Sharonov, J.C.Tompkins, K.Trombly, “A Distributed Monitoring and Control System”, Fermi National Accelerator Laboratory. [5] M.V. Pitke, “Design and Implementation of a Distributed Control System for General Purpose Applications”, Tata Institute of Fundamental Research pp-558-563. [6] IDC engineering and technologies, “Industrial Automation Book,” 1st edition vol.6 pp.1172, 9 August 2007.

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