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A distributed control system (DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring.
A programmable logic controller (PLC) or Programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges,immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.
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.