Narrowly speaking, DCS is mainly used for process automation, PLC is mainly used for factory automation (production lines), and SCADA is mainly 

targeted at wide area needs, such as oil fields and thousands of miles of pipelines. From the perspective of computers and networks, they are unified. 

The main difference lies in the application requirements. DCS often requires advanced control algorithms. For example, in the refining industry, PLC has 

high processing speed requirements because it is often used in interlocking and even fault safety systems. SCADA also has some special requirements

such as vibration monitoring, flow calculation, peak shaving and valley adjustment.

SCADA is the scheduling management layer, DCS is the plant management layer, and PLC is the on-site equipment layer

PLC system, also known as programmable controller, is suitable for measurement and control in industrial sites. It has strong on-site measurement 

and control functions, stable performance, high reliability, mature technology, wide use, and reasonable price.

The DCS system, also known as the scattered labor control system, is an internationally advanced large-scale control system of the 1990s. It is 

suitable for industrial sites with multiple measurement and control points, high measurement and control accuracy, and fast measurement and 

control speed. Its characteristics are decentralized control and centralized monitoring, with network communication capabilities, strong measurement 

and control functions, reliable operation, easy expansion, convenient configuration, and easy operation and maintenance. However, the system is expensive.

SCADA system, also known as distributed data acquisition and monitoring system, belongs to small and medium-sized measurement and control systems. 

It combines the strong on-site measurement and control capabilities of PLC systems and the networking and communication capabilities of DCS systems

with a high cost performance ratio.

SCADA and DCS are one concept, while PLC is one product, and the three are not comparable

PLC is a product that can form SCADA and DCS.

DCS developed from process control, PLC developed from relay logic control systems, PLC is equipment, DCS and SCADA are systems.

Key differences between DCS and PLC

1. DCS is a "distributed control system", while PLC (programmable logic controller) is just a control "device", the two are the difference between 

"system" and "device". The system can achieve the functions and coordination of any device, while the PLC device only implements the functions 

possessed by this unit.

2. In terms of network, DCS network is the central nervous system of the entire system. It is a safe and reliable high-speed communication network 

with dual redundancy, and the system has better scalability and openness. PLCs, on the other hand, work as individuals and communicate with other 

PLCs or upper computers using a single network structure. The network protocols often do not comply with international standards. In terms of network 

security, PLCs do not have good protection measures. We adopt dual redundancy of power supply, CPU, and network.

3. DCS overall consideration scheme, the operator station has the function of an engineer station, and the relationship between stations is closely 

linked after installing the operation scheme program. Any station, any function, or any controlled device is interlocked and coordinated for control

A system composed solely of interconnected PLCs has a loose connection between stations (PLCs), which cannot achieve coordinated control functions.

4. DCS has a large number of expandable interfaces in the entire design, making it very convenient for external or extended systems. After the entire 

system connected by PLC is completed, it is difficult to arbitrarily add or reduce operator stations.

5. DCS security: 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 switch to working units in real time without disturbance, ensuring the safety and reliability of the entire system. 

The system overlapped by PLC basically has no concept of redundancy, let alone redundant control strategy. Especially when one of its PLC units fails, the 

entire system has to be shut down for replacement and maintenance, and reprogramming is required. So the DCS system is one level higher in terms of 

safety and reliability.

6. The system software updates various process control schemes, which is one of the most basic functions of DCS. When a scheme changes, engineers 

only need to compile the modified scheme on the engineer station and execute the download command. The download process is automatically completed 

by the system and does not affect the operation of the original control scheme. Various control software and algorithms in the system can improve the 

control accuracy of process required control objects. For systems composed of PLCs, the workload is extremely large. Firstly, it is necessary to determine 

which PLC needs to be edited and updated, then use the corresponding compiler to compile the program, and finally use a dedicated machine 

(reader/writer) to transfer the program to this PLC one-on-one. During system debugging, it greatly increases debugging time and costs, and is extremely 

unfavorable for future maintenance. There is a significant difference in control accuracy. This determines why systems connected entirely by PLCs are not 

commonly used in large and medium-sized control projects (over 500 points).

7. Module: All I/O modules in the DCS system are equipped with CPUs, which can be used to judge the quality of collected and output signals, perform 

scalar transformation, perform live plugging and unplugging in case of faults, and randomly replace them. The PLC module is just a simple electrical 

conversion unit without an intelligent chip, and after a fault, all corresponding units will be paralyzed.

8. Nowadays, the functions of high-end PLCs and DCS are almost the same. DCS has strong capabilities in scanning networks and distributed databases 

on a regular basis, and is also skilled in processing calculations and analog quantities.

9. PLCs are also divided into large, medium, small, and micro PLCs. Among them, micro PLCs are only sold for a few hundred to 2000 yuan, with fewer 

points, while large PLCs can carry thousands of points. Their computing power is similar to DCS, but their ability to connect multiple machines is weaker. 

Now both technology platforms are similar, but the focus is different.

Since DCS and SCADA are concepts? Actually, it should be understood as an architecture

PLC and DCS have gradually evolved from different product forms and application scenarios in the early days.

It can be said that PLC evolved and developed from relays in the manufacturing industry.

DCS evolved from process control instruments.

Each product has its own usage scenarios and characteristics, and with the advancement of technology, mutual penetration in the industry, and fierce 

competition.

Both PLC and DCS products are developing their own advantages while penetrating into each other.

It should be noted that the logic control function of PLC is very powerful, while loop control is not worth mentioning (you can take a look at slightly older 

PLC models), while DCS systems are exactly the opposite. This is due to the different internal processing mechanisms of the two systems.

What about now?

Established PLC manufacturers are all involved in DCS application fields such as Siemens and Rockwell, and their systems are nothing more than PLC 

plus network plus software plus enhanced controllers.

DCS manufacturers such as Honeywell have also integrated PLCs into their systems to enhance their logic control capabilities.

It should be noted that PLC is a type of controller;

DCS is a system architecture consisting of a controller, IO acquisition, network, and software.

SCADA system is another concept that presents a system with different application requirements.

As the name suggests, it is a distributed data collection and monitoring system. Its origin and application are different from the previous two. It is mainly 

used for data collection, such as power monitoring systems and oil pipeline monitoring systems. Its characteristic is that the control points are dispersed

and a system may cover thousands of functions within a radius (such as the oil and gas pipeline monitoring system in North America);

The communication structure is complex and incomparable to general control systems (from fiber optic to wireless and even satellite communication), and 

its basic unit is RTU.

There are two questions here:

1. To define a framework for a system;

2. Explain using the original system definition.

You should know that products are constantly evolving, and there is competition between different products. It is necessary to constantly learn from 

others' strengths while leveraging one's own advantages, which is integration.

The focus of SCADA is on monitoring and control, which can achieve some logical functions and is mainly used for upper level operations;

PLC simply implements logical functions and control, without providing a human-machine interface. Operations require the use of button indicator 

lights, HMI, and SCADA systems;

DCS combines the functions of both, but is generally used in larger systems and some systems with high control requirements, and is also more expensive 

in price;

The three systems permeate each other, and in some production enterprises, there are three systems included. SCADA serves as the upper level 

monitoring for production management, DCS realizes complex control, and PLC realizes single machine and simple control.

But with the development of technology, some manufacturers' PLCs can also achieve complex and precise control, gradually occupying the DCS market

Simply put, PLC+SCADA=DCS. As far as I understand it, they can achieve cross functional control, which is also based on the control of this system, to 

develop the advantages of the other for their own use.