There is no difference between PLC and DCS. It can be said that DCS includes PLC.

PLC and DCS have developed to this day, in fact, they are moving closer to each other. Strictly speaking, PLC and DCS can no longer be distinguished in the same way, and the concepts between them have often become blurred. Now, let's discuss the similarities between them.

1. In terms of function.

PLC already has analog control functions, and some PLC systems have even very powerful analog processing capabilities, such as Siemens' S7 400, AB's Control Logix, and Schneider's Quantum system. DCS also has very strong logic processing capabilities. For example, we have realized all possible process interlocks and equipment linkage start and stop on AC800F.

2. From the perspective of system structure.

The basic structure of PLC and DCS is the same. PLC has been fully transplanted to computer system control today, and traditional programmers have long been eliminated. PLCs for small applications generally use touch screens, while PLCs for large-scale applications use computer systems. Like DCS, the controller and IO station use the field bus (generally based on the RS485 or RS232 asynchronous serial communication protocol bus mode). If there is no expansion requirement between the controller and the computer, that is, when only one computer is used, this bus communication will also be used. However, if more than one computer is used, the system structure will be the same as DCS, and the host computer platform will use Ethernet structure. This is one of the reasons why the concept of PLC and DCS is blurred after large-scale.

3. Development direction of PLC and DCS

Small-scale PLCs will develop towards more specialized usage, such as more targeted functions and application environments, etc. The boundaries between large-scale PLCs and DCSs will gradually fade until they are completely integrated.

DCS will continue to develop in the direction of FCS. In addition to the more decentralized control system, the core of FCS is particularly important for instruments. FCS applications abroad have already developed to the instrument level. The control system only needs to deal with signal acquisition and provide human-machine interface and logic control. The entire analog control is decentralized to the field instrument. There is no need for traditional cables to connect the instrument and the control system. The field bus is used to connect the entire instrument system.

In short, the difference between PLC system and DCS (distributed control system) is getting smaller and smaller, and a process control system can also be constructed using PLC.