What is the difference between SCADA, DCS and PLC?

In a narrow sense, DCS is mainly used for process automation, PLC is mainly used for factory automation (production line), and SCADA is mainly used for wide area needs, such as oil fields and thousands of miles of pipelines. From the perspective of computer and network, they are unified. The reason why they are different is mainly because of the application requirements. DCS often requires advanced control algorithms. For example, in the oil refining industry, PLC requires high processing speed. Because it is often used in interlocking, even in fault safety systems, SCADA also has some special requirements, such as vibration monitoring, flow calculation, peak and valley regulation.


PLC system, namely programmable controller, is applicable to the measurement and control of industrial field. It has strong field measurement and control function, stable performance, high reliability, mature technology, extensive use and reasonable price.

DCS system, namely the centralized and decentralized control system, belongs to the international advanced large-scale control system in the 1990s. It is suitable for industrial sites with many measurement and control points, high measurement and control accuracy, and fast measurement and control speed. It is characterized by decentralized control and centralized monitoring, networking communication capability, strong measurement and control function, reliable operation, easy expansion, convenient configuration, and simple operation and maintenance, but the price of the system is expensive.

SCADA system, namely distributed data acquisition and monitoring system, is a small and medium-sized measurement and control system. It combines the two advantages of PLC system’s strong field measurement and control function and DCS system’s networking communication ability, with high performance price ratio.

SCADA and DCS are a concept, PLC is a product, and the three are not comparable.

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Differences between DCS and PLC

1. DCS is a “decentralized control system”, while PLC (Programmable Logic Controller) is just 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 can only realize the functions of this unit.

2. In terms of network, DCS network is the central nerve of the whole system. It is a safe and reliable high-speed communication network with double redundancy. The system has better expansibility and openness. Because PLC basically works as an individual, when communicating with other PLCs or upper computers, the network form adopted by PLC is basically a single network structure, and the network protocol is often inconsistent with international standards. In terms of network security, PLC has no good protection measures. We use dual redundancy of power supply, CPU and network.

3. The overall plan of DCS is considered. The operator station has the function of engineer station. After the operation plan program is installed, there is a close joint relationship between stations. Any station, any function, and any controlled device are interlocked and coordinated; However, the connection between stations (PLC and PLC) of a system composed of PLC interconnections is a loose connection mode, which can not achieve the function of coordinated control.

4. DCS has a large number of expandable interfaces in the whole design, and the external system or expansion system is very convenient. After the completion of the whole system connected with PLC, it is difficult to increase or reduce the operator station at will.

5. DCS safety: In order to ensure the safety and reliability of equipment controlled by DCS, DCS adopts dual redundant control units. When important control units fail, relevant redundant units will be switched to working units in a real-time and undisturbed manner to ensure the safety and reliability of the whole system. The system connected by PLC basically has no concept of redundancy, let alone redundant control strategy. In particular, when one of its PLC units fails, the whole system has to be shut down before it can be replaced, maintained and reprogrammed. Therefore, DCS system is one level higher than its safety reliability.

6. System software, updating various process control schemes is the most basic function of DCS. When a scheme changes, engineers only need to compile the changed scheme in the engineer station and execute the download command. The download process is automatically 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 the process required control object. For the system composed of PLC, the workload is extremely huge. First, you need to determine which PLC to edit and update, then use the corresponding compiler to compile the program, and finally use a dedicated machine (reader writer) to send the program to the PLC one-to-one. During the system debugging, debugging time and debugging cost are greatly increased, and it is extremely unfavorable for future maintenance. There is a big 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.

7. Module: All I/O modules of DCS system are equipped with CPU, which can realize the quality judgment and scalar transformation of acquisition and output signals, hot plugging and random replacement in case of failure. The PLC module is just a simple electrical conversion unit without intelligent chip, and the corresponding units will be paralyzed after the failure.

8. At present, the functions of high-end PLC and DCS are almost the same. DCS also has strong functions for regular scanning of network and distributed databases, and is good at processing computation and analog quantities.

9. PLC can also be divided into large, medium, small and micro PLC. The micro PLC only sells for hundreds to 2000 pieces, with few points. The large one can carry thousands of points. Its computing power is similar to that of DCS, but it is weak for multi computer networking. Now the two technology platforms are similar, but the focus is different.

To put it simply, PLC+SCADA=DCS. As far as I understand, they can cross each other in terms of control functions, which is also based on the control of this system to develop the advantages of the other party for their own use.


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