Process Shutdown System
What's the main difference between shutdown PLC and DCS?
Main difference between PLC and DCS is their function. Distributed Control System (DCS) uses separate hardware to control the unit. It distributes the load to all the controller. PLC is processing all the logic inside its primary controller. If one DCS controller fails, it will only caused shutdown to only one unit of the process. If the PLC controller fails, it will shutdown all the process due to the reason that it only has one controller to process the sequence/logic (we put aside the redundancy here).
In term of hardware, both of them use different standard. DCS is following the general industrial standard which is not strict as compare to PLC which is using military standard. Specifically the respond time for DCS can be longer compare to safety PLC which requires 300ms.
In term of application, DCS generally being used in process control while PLC is being used mostly in batch process. This is due to the reason that DCS program runs in serial mode while PLC program runs in ladder.
The early DCS looked dramatically different from PLC. Initially operator use analog instrumentation for control while for safety they used relay system.
