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A distributed control system provides a more specific approach towards process automation. A DCS system is used mainly in a control system where a large process is to be controlled.
Whereas a PLC is a gereneric device, it can be used for automating systems with4 to a few hundered I/O.
Basically a DCS is used to overcome problems faced by a PLC .
But a PLC is more widely used in any factory
Some of the differences are :
1- PLC is using for factory automation such as automobile factory and DCS for process automation such as petrochemical industry .
2-In PLC th most I/O are digital . But in DCS are analogue .
3- In PLC the programing software's and monitoring software are separated from each others but in DCS all software's are embedded .
4- In PLC the programing done by lower level languages such as LAD-FBD but in DCS by higher level languages such as CFC.
5- Response speed in PLC is higher than DCS.
6- PLC is suitable for discrete control but DCS for loop control .
DCS(distributed control system)-By the name itself we understand that its a system which consist of controllers which are distributed. To more specific it consist of dependent and independent contollers like PLCs and inbulit SCADA(for monitoring and controlling remotely) which are centralised. DCS is highly reliable system as we can catagorise the plant processes and if any failure ocure it will not affect the whole process. So it is largely used in large scale industries like oil/gas ,nuclear plants etc.
PLC(programmable logic controllers)- These are controllers which consist of analog/digital i/o modules and can be used for specific process. They can be programmed independendly with an independent CPU . They don't have any inbuilt monitoring and user controlling interface. For monitoring and controlling the devices which are connected to PLCs, we have to connect the PLCs to interface units like HMI(human machine interface), SCADA etc via communication protocols available.
Conclusion
DCS is reliable and consist of PLCs itself, inbuilt SCADA which are centralised.
PLCs which consist CPU, analog/digital i/o ,communication modules ,are used for specific processes .
the (PLC) programmable logic controller is a specialized processor/ computer with all the hardware and software to do a specific automation task, say running a plant/factory
the(DCS) Distributed Control System is a system of dividing plant or process control into several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity
plc is a programmable logic controller which can handle the binary input and output of the logic statement which are stored in its memory.its a digitally operating system which are logic stored in its memory by programming system.DCS:-Distributed control system its control all types of process variable at a time through this we can control the whole plant,we can see the status of plant,and handling yhe unit.
The main difference between Programmable Logic Controller (PLC) and Distributed Control System (DCS) are:
The number of I/O in the system. DCS is dealing with very large no. of I/O with less cost. also DCS comes with it's SCADA always. if DCS stopped, all system is stopped. if the system has less No. of I/O, then PLC is better from cost view. Also SCADA is optional. if system is controlled by no of PLC. if PLC is down. others continue working Normally. Historical, main difference was that DCS deal with Analogue signals but PLC deal with Discrete digital system. Now DCS has Digital Modules and PLC has Analogue modules.
Besides, PLC has a processor and input and output cards (I/O card could be digital and/ or analog) Processor has the software - basis the input it receives from field devices it gives out the output commands to control devices of the field. DCS is a group of individual smart controllers (again having embedded software and connections with field devices) doing their specified operation (duty) but interconnected.
Plc was use only in single looping working on simple logics for controling single machine. But the DCS system is more advance it is using for multi propose it is maintain more then one machine it is inface with GUI system to control and system to and area.
PLC is used in small processes compared with DCS system which is used in large processes with thousnds of i/o like power plants or heavey industris
in some cases PLC is used to control certain process and send some important data to DCS system for whole plant control
also, DCS system is consistes of multi processors with redendant ones. all of these processors are communcate with each other using one of famous communcation protocols like PROFIBUS or MODBUS
some modern DCS systems' processros are a PLC connected with each other using PROFIBUS protocol like SIEMENS SPPA-T3000 DCS
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