• What is DCS and PLC
  • Difference Between DCS and PLC
  • Similarities Between DCS and PLC
  • System Architecture of DCS and PLC

What is DCS and PLC

DCS System

Distributed Control System (DCS) used to control complex, large distributed applications or large continuous process plants where high reliability and security is important.

The DCS is Controlling many autonomous controllers that handled many continuous operations, mainly using analog control.

A DCS would be better used in an environment where there are large I/O counts.

DCS also claims that while safety is a top priority, DCS is the most reliable system because the manufacturer supplies both control and supervisory equipment.

The DCS is a process oriented system that uses closed-loop control.

DCS communication between field level and the processor level can be any type that may be compatible with the components. ( Ethernet, Profibus DP, Ethernet CAT, Fiber optic)
Components at field level would be devices such as RTD, transmitters, switches, valves, remote etc.

PLC System

Programmable Logic Controller (PLC) was a replacement for panels of relays.

The I/O modules can be miles away from the CPU and connected with data cables so that the PLC is not limited to a single cabinet.

PLC will work best in small production environment so there is minimum I/O count.

The PLCs are programmed using an application running on a standard  desktop or laptop PC.

The communicate with the PLC using Ethernet or a proprietary communication bus,

Depending on the manufacturer, most manufacturers seem to have settled on ethernet and USB in recent years.

Difference Between DCS and PLC

DCS’s were designed to control processes, not discrete operations. PLC was in discrete control of manufacturing processes. Whole discrete logic used to be implemented with relay circuitry.

PLC is that it is easier for plant personnel to implement and configure internally than a DCS, and PLC is cheep as compare to DCS when I/O < 1000  for PLC and I/O> 1000 for DCS.

PLC will control the machine and DCS will control the whole plant.

The PLC requires minimum human interface and DCS requires maximum human interface due to maximum control loops.

PLC scan rate is 10-50ms and DCS scan rate is 80-500ms because DCS has many controllers but we can adjust scan time in DCS but PLC has fix scan time.

Similarities Between DCS and PLC

DCS is designed or made available to the user in such a way that configuration is to be done as only one functional block unlike PLC where complete programming is to be implemented using any one of the various languages available in the system . Now, functional blocks are also available in PLC systems.

PLC and DCS both send, receive and process data from field to control system.

One DCS is connected to many PLCs systems.

When DCS started emerging in the market, idea was to supply DCS with whole bunch of hardware and software packages, PLC were not suppose to be in packages but competition with DCS vendors forced the PLC manufacturers to offer necessary all other software and packages.