Do you know, biggest difference between DCS and PLC

How to choose between Programmable Logic Controller ( PLC ) and Distributed Control System ( DCS ) should be analysed on a case-by-case basis, because different applications have different requirements for the control system. When we communicate with customers, we can start from the following aspects!

       PLC or DCS



  1. From switching quantity control to sequence control and transportation processing, it is a multi-function such as continuous PID control from bottom to top, and PID is in the interrupt station.
  2. One PC can be used as the master station, and multiple PLCs of the same type can be used as the slave stations.
  3. One PLC can also be used as the master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. This is more convenient than using a PC as the master station: when there is user programming, it is not necessary to know the communication protocol, just write it in the format of the manual.
  4. The PLC grid can be used both as an independent DCS and as a subsystem of the DCS.
  5. PLC is mainly used for sequential control in industrial processes, and the new PLC also has closed-loop control functions.


  1. Distributed control system DCS set

4C (Communication, Computer, Control, CRT) technology in one monitoring technology.

  1. Large system with top-to-bottom tree topology, in which communication is the key.
  2. PID in the interruption station, the interruption station connects the computer with the field instrumentation and the control device is a tree topology and a parallel continuous link structure, and there are also a large number of cables from the interruption station to the field instrumentation in parallel.
  3. Analog signal, A/D—D/A, mix with microprocessor.
  4. One instrument is connected to I/O by a pair of lines, and is connected to the local area network LAN by the control station
  5. DCS is a three-level structure of control (engineer station), operation (operator station), and field instrument (field measurement and control station). For large-scale continuous process control, such as petrochemical, etc.

       How to choose between PLC and DCS system

How to choose between Programmable Logic Controller (PLC) and Distributed Control System (DCS) should be analysed on a case-by-case basis, because different applications have different requirements for the control system.

The control system platform has an impact on the way an automated system meets the needs of optimizing production, maintaining availability, and capturing data. A lack of foresight in choosing a control system can also affect future expansion, process optimization, user satisfaction, and company profits.

In addition to some basic guidelines (such as how to control the process), the design team must also consider various factors such as installation, scalability, maintenance, maintenance and so on.

At present, although PLC systems may be the most cost-effective for small equipment, DCS systems provide more economical scalability and are more likely to achieve a higher initial return on investment.

A PLC is an industrial computer used to control manufacturing processes such as robotics, high-speed packaging, bottling, and motion control. Over the past 20 years, PLCs have added more functions, creating more benefits for small factories and installations. 

PLC usually operates as a stand-alone system, but it can also be integrated with other systems to realize the connection between each other through communication. Since each PLC has its own database, integration requires some level of mapping between controllers. This makes PLCs especially suitable for small applications that do not require much expansion.

DCS systems disperse controllers in automation systems and provide common interfaces, advanced controls, system-level databases, and easy-to-share information. Traditionally, DCS has been mainly used in process technology and relatively large plants, where large system applications are easier to maintain throughout the life cycle of the plant.

PLC is developed from the principle of relay control. It stores instructions for performing logical operations, sequence control, timing, counting, and operations, and controls various types of machinery or production processes through digital input and output operations. 

The control program compiled by the user expresses the technological requirements of the production process, and is stored in the user program memory of the PLC in advance. When running, the content of the stored program is executed one by one to complete the operations required by the technological process.

     Comparison of PLC and DCS engineering analysis

There is a program counter in the CPU of the PLC that indicates the storage address of the program step. During the program running process, the counter is automatically incremented by 1 after one step is executed, and the program is sequentially executed from the initial step (step number zero) to the final step (usually the end instruction).), and then return to the starting step to cycle operations. The time required for PLC to complete one cycle operation is called a scan cycle. For different types of PLCs, the cyclic scan period is between 1 microsecond and several tens of microseconds. Loop operations such as the program counter, which DCS does not have. This is also what makes PLCs less redundant than DCSs.

DCS is developed on the basis of operational amplifiers. All the relationships between functions and process variables are made into function blocks (some DCS systems are called extruded blocks). The main difference between the performance of DCS and PLC is in the logic calculation of switching quantity and the operation of analog quantity. Even if the two penetrate each other later, there is still a difference.

After the 1980s, in addition to logic operations, the algorithm functions of PLC control loops have been greatly enhanced, but PLC uses ladder diagram programming, and analog operations are not intuitive in programming, and programming is more troublesome. However, in terms of solving logic, it shows the advantage of being fast, in the order of microseconds, it takes less than 1 millisecond to solve a 1k logic program. It treats all inputs as switch quantities, 16-bit (and 32-bit) as an analog quantity.

The DCS regards all inputs as analog quantities, and 1 bit is the switch quantity. Solving a logic is on the order of hundreds of microseconds to several milliseconds. For PLC to solve a pid operation in tens of milliseconds, which is comparable to the operation time of DCS.

In terms of grounding resistance, PLC may not be very demanding, but DCS must be below a few ohms (usually below 4 ohms). Analog isolation is also very important.

For a system with the same number of i/o points, the cost of PLC is lower than that of DCS (about 40% can be saved). PLC does not have a dedicated operation station, and the software and hardware it uses are universal, so the maintenance cost is much lower than that of DCS. If the controlled object is mainly equipment chain and there are relatively few loops, it is more appropriate to use PLC.

If it is mainly analog control and there are many function operations, it is best to use DCS. DCS is much better than PLC in terms of redundancy of controllers, i/o boards, communication networks, etc., some advanced operations, and special requirements of the industry. Because PLC adopts general monitoring software, it is easier to design the management information system of the enterprise.

PLC and DCS systems are generally applicable to discrete and process manufacturing, respectively. A discrete manufacturing facility that uses a PLC system, generally consisting of separate production units, primarily used to complete the assembly of parts, such as labeling, filling, or grinding. Process manufacturing facilities, often using automated systems, produce to recipe rather than piece in a continuous and batch process. Large continuous processing facilities, such as oil refineries and chemical plants, use DCS automation systems. Hybrid applications often use both PLC systems and DCS systems. Selecting a controller for an application requires consideration of the size of the process, scalability and future update plans, integration needs, functionality, high availability, and return on investment over the entire life cycle of the plant facility.

       Relevant factors that influence how a decision is made

Process scale: How many input/output (I/O) points are required? Small systems (<300 I/O points) may have less budget, so a PLC system is more suitable. It is not easy to apply the DCS system to smaller projects. On the contrary, it can play its function better in large factory applications. Due to having a global database, the DCS system is easier to manage and upgrade, and any changes are global.

Upgrade plan: Smaller industrial processes can accommodate PLC systems, but if the process needs to be expanded or upgraded, more PLC hardware and databases will need to be added, and separate maintenance will be required. This is a time-consuming, labor-intensive process and prone to errors. The DCS system is easier to upgrade, such as user trust management from a central hub, so it is easier to maintain and maintain.

Integration needs: For stand-alone installations, PLC systems are ideal. When a factory is configured with multiple PLC systems, the requirement for interconnection arises. This is generally difficult to achieve because the data usually needs to be mapped using the communication protocol. There is of course no problem with integration, but when there is a change requirement, the user’s trouble will come: once a PLC system is changed, it may cause the two PLCs to fail to communicate normally, because the data mapping is affected. For the DCS system, no mapping is needed at all, and configuration changes are just a simple process; the controller comes with the system.

High availability: DCS systems can provide redundant configurations for processes that require high availability.

Efficiency and ease of redundancies are critical to keeping costs within budget.

Functional Requirements: Certain industries and facilities require a historian, streamlined alarm management, and a central control room with a common user interface. Some require the integration of manufacturing execution systems (MES), advanced controls and asset management. These applications are built into the DCS system (see Figure 3), making it easy to add to automation engineering applications without adding separate servers or increasing integration costs. In this respect, DCS systems are more economical, and can increase productivity and reduce risk.

Lifecycle ROI: Facility needs vary by industry. For smaller-scale process engineering, there is no need for expansion and integration with other process areas, so the PLC system has a better return on investment. The DCS system may have higher installation costs, but from the perspective of the whole life cycle, the increased production and safety benefits brought by the DCS system will offset part of the cost. Balancing short-term needs with long-term vision is critical to operational certainty and improved plant operations and maintenance.

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