The difference between DCS control system and PLC control: DCS is a “distributed control system”, while PLC (programmable controller) is only a control “device”, which is the difference between “system” and “device”. The system can realize the functions and coordination of any device, and the PLC device only realizes the functions of this unit.

DCS network is the central nerve of the whole system, and the international standard protocol TCP/IP is usually adopted by DCS system. It is a safe and reliable dual-redundant high-speed communication network, and the system has better expansibility and openness. Because PLC basically works for a single small system, when communicating with other PLCs or upper computers, the network form used is basically a single network structure, and the network protocol is often inconsistent with international standards. PLC has no good protection measures for network security.

DCS considers the scheme as a whole. The operator station has the function of engineer station. After the program of operation scheme is installed, there is a close joint relationship between stations. Any station, any function and any controlled device are interlocked and controlled by each other; However, the connection between stations (PLC and PLC) is a loose connection mode for the system composed of PLC connected with each other, which can not achieve the function of coordinated control.

A large number of expandable interfaces are reserved in the whole design of DCS. It is very convenient to connect the external system or expand the system. After the completion of the whole system connected by PLC, it is difficult to add or reduce operator stations at will.

In order to ensure the safety and reliability of the equipment controlled by DCS, DCS adopts a dual-redundant control unit. When an important control unit fails, the relevant redundant unit will be switched to the working unit in real time without disturbance to ensure the safety and reliability of the whole system. The system connected by PLC needs to be configured with double PLCs to achieve redundancy.

Updating various process control schemes is one of the most basic functions of DCS. When a scheme changes, engineers only need to execute the download command after the engineer station master compiles the changed scheme. The download process is completed by the system without affecting the operation of the original control scheme. Various control software and algorithms of the system can improve the control accuracy of process objects.

For the system composed of PLC, the workload is extremely huge. First, it is necessary to determine which PLC to edit and update, then use the corresponding compiler to compile the program, and finally use the special machine (reader and writer) to transfer the program to this PLC one-to-one. During the system debugging, the debugging time and debugging cost will be greatly increased, and it is extremely unfavorable for future maintenance.

There is a great difference in control accuracy. This determines why in large and medium-sized control projects (more than 500 points), the system connected by PLC is basically not used. All I/O modules of the DCS system are equipped with CPUs, which can realize the quality judgment and scalar conversion of the collected and output signals, and can be unplugged in case of fault and replaced randomly. The PLC module is only a simple electrical conversion unit, without intelligent chips. After the failure, the corresponding units are all paralyzed.