PLC

1)DCS: Earlier DCS performed the control functions of analog panel instruments. It uses multi-variable proportinal Integral Derivative (PID) control to perfom any control. PLC: PLC dealth with contacts I/P and O/P by performing boolean logic caliculations. 2) DCS: Offers extra controllers, networks & I/Os. Which are "Built-in", also offers extra diagnostic features. PLC: All functions are defined by the user by using a "Written Logic" 3) DCS: It is provided with HMI (Interface console). PLC: We have to purchase HMI/SCADA systems seperately 4) DCS: We can access data (Process variable) of any particular instrument by using the Tag number. PLC: We have to remember the address or location of the hardware of that instrument. 5) DCS: Offers fractional second. However some DCS's provide interrrupt/Event triggered logic for high speed applications. PLC: PLCs are fast. They run Input-Compute-Output Cycle in milliseconds. 6) DCS: Multitasking operation is possible. You can download and run other application aside from real time control. Still do fraction of second control. PLC: Multitasking is possible but costly. 7) DCS: Data processing errors are not there in the central database of DCS PLC: There will be Data procesing error while (register exchange mechanism) getting data from and to various PLCs. 8) DCS: Bottle necking and timing problems are less. PLC: If so mnay PLCs try to communicate with too many PLCs result in bottle necking and timing problems. These are the major differences and however some more minor differences are there. We have to ecide based on our application and type od I/p, O/P's. If requiredd more info search on the net or just leave a message here in this forum. Good Luck.

http://en.wikipedia.org/wiki/Programmable_logic_controller

http://en.wikipedia.org/wiki/Distributed_Control_System

Post #2 has good info so I will only add a note that may be of interest. We use both DCS and PLC in the same system. The DCS is normal control and the PLC provides a secondary system to ensure control of Safety Critical devices (isolation valves, overflow control, fired heater fuel systems , etc) within that control. Both systems have UPS power so that in the event of an upset the shutdown devices have a stand alone system (PLC) that will function to bring a safe controlled interlock.

Make that post #1 and #2. Both are good info.

Post#2 is also right but if we go on adding, the list will go on and on. We also use many things like Ciruit breakers, Isolators fot the safe running of PLC or DCS apart from the UPS. where UPS is required for continuous power supply for the PLC or DCS.