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PLC is becoming very close to a DCS. Of course, this does not apply to a small PLC which can have limited functionality. But a sophisticated PLC is very close to a DCS, and specially close or even better than a small DCS.
Nowadays, when we specify a Control System we define it as a "Process Control System" in order to place DCS and PLC as valid solutions.
PLC solutions include a PC based HMI (Human Machine Interface) while DCS refers to a set of DCS controllers and the DCS HMI.
Some of the difference or similarities are:
- Centralized database. This is natural for a DCS, but now PLCs offer also a kind of centralized database. HMI can access PLC database directly without the need to define another database for the HMI
- Programming language. This used to be Functional Block Diagram for DCS and Ladder Diagram for PLC. However now DCSs and PLCs comply with IEC 61113-1 standard which means they can be programmed in four different languages: Functional Block Diagram, Structured Text, Sequential Flow Chart and Ladder Diagram.
- Scan Time. Two facts are normally true. PLC are faster and PLC scan is variable. DCS scan time is 50 or 100 ms as standard. In the other hand PLC scan varies between 20 ms to over 300 ms. Although it is very easy for a PLC to have either a timed routine with a scan time of less than 10 ms or an interrupt routine which can be executed when an input occurs. However, some DCSs now have alternatives where logic can be executed at the I/O module level without waiting the normal scan time.
- Hardware. There is no specific difference. Just as an example Siemens PLC S7-400 plus the WinCC HMI is using exactly the same hardware as the Siemens DCS: PCS7. There is just a software difference.
- Centralized development. This is the difference users appreciate the mostly regarding DCS. Developed or system objects include features executed by the DCS controller and others which are executed at the HMI level, like graphics or data log. But PLC-HMI solutions are nowadays more and more integrated. For example alarms can now be processed at the PLC and reported in a producer-consumer model to the HMI.
We can discuss a lot regarding this topic. And be careful because some of the subjects are just commercial. As a matter of fact you can find DCSs where controllers are programmed with a different software tool than the HMI, and can be less integrated than a PLC-HMI solution!!
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