The differences between DCS and PLC are:
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. This controller is referred to the PLC.
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.
What’s difference between PLC and DCS by its definition:
PLC is a controller or processor that can be programmed (programmable) whose function is to run (execute) logic functions. Logic means the discrete / sequence function is usually handled by the relay. From the beginning of the vendors who carry the name of the PLC is engaged in the business discrete / sequence control.
Regardless of the distributed control system (as opposed to the DDC = Direct Digital Control) is categorized as a DCS. In DDC all system controls are done in the central processor so that if it failed, the entire control plant will also failed. DDC is mostly used as a Regulatory Control. And from the beginning vendors that carry the name of DCS is to use the product as a regulatory control.
Different in functionality means that the DCS and PLC can not be implemented on the same application.
1. DCS is not a large PLC. Because system architecture of DCS and PLC are different.
2. DCS is not PLCs that integrated into one large system. "Controller" in the PLC is more intended as a "Logic Controller", while “Controller” in the DCS is more intended as a "Process Controller".
3. Both DCS and PLC is a configurable and reconfigurable.