Up to now, there is no strict boundary between DCS and PLC. In the eyes of most people, the large system is DCS, and the small system is called PLC. Of course, it's not impossible to say that, but it's not right. Now let's re-establish this concept.

1. From the perspective of development:
DCS evolved from the traditional dashboard monitoring system. Therefore, DCS is inherently more focused on the control of the instrument, such as YOKOGAWA CS3000, DCS system does not even have the limitation of the number of PIDs (PID, proportional-differential-integral algorithm , which is the standard algorithm for closed-loop control of regulating valves and inverters, usually PID The number determines the number of control valves that can be used).
PLC developed from the traditional relay circuit. The original PLC did not even have the ability to process analog quantities. Therefore, the PLC emphasized the logic operation ability from the beginning.
2. From the aspect of system scalability and compatibility:
There are many control products on the market, no matter whether DCS or PLC, many manufacturers are producing and selling. For PLC systems, there is generally no or very little need for expansion, because PLC systems are generally used for equipment. Generally speaking, PLCs rarely have compatibility requirements. For example, two or more systems require resource sharing, which is also very difficult for PLCs. Moreover, PLCs generally use a dedicated network structure, such as Siemens' MPI bus network, and even adding an operator station is not easy or costly.
In the process of development of DCS, each manufacturer has established its own system, but most of the DCS systems, such as Yokogawa F9342BA, Honeywell, ABB, etc., although the internal communication protocol (process level) of the system is not the same , but the operation The high-level network platform unanimously chooses Ethernet, using standard or modified TCP/IP protocol. This provides a very convenient scalability. In this kind of network, both the controller and the computer exist as a node. As long as the network reaches the place, the number of nodes can be added or deleted and the location of nodes can be arranged at will. In addition, based on open protocols such as OPC and DDE of the Windows system, various systems can also communicate easily, and resource sharing has been realized.