DCS and PLC have been competing for 40 years, let’s take a look at the differences and applications first!

The topic of DCS and PLC has been debated in the industry for at least 40 years. To understand what everyone is debating, it is necessary to understand the differences between DCS and PLC, objectively understand these fundamental differences, and rationally choose to apply DCS and PLC based on products and needs.

1. DCS focuses on the overall control system, while PLC focuses on local logic control

Why is this? Who stipulated this? Why is it so divided? This is because DCS has evolved from analog instruments and is primarily based on analog signals. DCS focuses on process control, which can achieve complex control such as PID, feedforward, cascade, multi-level, and selective control, emphasizing the accuracy of continuous process control; PLC has developed from electrical relays, mainly focusing on digital quantities. PLC is oriented towards the general industrial control field and has strong universality. Generally, PLC only has PID function, and its control accuracy is not as high as DCS.

2. Differences in communication between DCS and PLC

DCS adopts the internationally recognized TCP/IP protocol, also known as the Internet interface, which has good universality and scalability; However, PLC communication is diverse (such as PPI, Internet, Device net, modbus, Profibus, USB, etc.), and it is often difficult to arbitrarily add or reduce operator stations after PLC is built.

In terms of network security, a large number of technical personnel in the Internet field are engaged in network security work, and there are currently no good protection measures for PLC communication security.

DCS adopts a holistic approach, and the operator station also has the function of an engineer station. After the installation of the operating program, there is a close joint relationship between stations. Each station, each function, and each controlled device are interlocked and controlled in a coordinated manner; The system composed of PLCs connected to each other is loosely connected and cannot achieve coordinated control function between PLCs.

3. Differences in reliability between DCS and PLC

DCS is dual redundant and can achieve undisturbed switching. The I/O modules of DCS are equipped with CPUs, which can judge the quality of collected and output signals and perform scalar transformation. In case of module failure, they can be plugged and replaced randomly with power; Most PLCs have no redundancy, while dual path PLCs have higher costs. The PLC modules are simple electrical conversion units without intelligent chips, and all corresponding units will be paralyzed after module failure. At present, PLC can achieve redundancy, hot standby, and live plugging, and can complete the majority of DCS functions.

DCS and PLC each have their own advantages, and they are also mutually penetrating and shaking hands, so discussing which one is better between DCS and PLC now is no longer meaningful. For example, Siemens’ SIMATIC PCS7 system is a new system that combines PLC and DCS.

4. The difference in number of points between DCS and PLC

There are many DCS points, usually above 500 points; The number of PLC points is relatively small, usually around 100, and control systems connected entirely by PLC are not used for points above 500. The cost of PLC systems will also be high. In the DCS update control scheme, engineers execute the download command on the engineer station to compile the control scheme. The program download process is automatically completed by the system and does not affect the operation of the original control scheme. To change the control scheme of a PLC, it is necessary to first clarify which PLC it is, compile the program with a compiler, and then transmit it to the corresponding PLC. Debugging takes a long time and is costly, which is not conducive to future maintenance.

5. Differences in software

The DCS system has powerful software packages and can also develop software tailored to process parameter configuration and control algorithm requirements. It has formula functions, and in terms of software, DCS is more convenient to apply than PLC.

Should DCS or PLC be used in industrial sites?

The choice of these two options mainly depends on the investment situation and where they are used. For example, PLC is more suitable for logic control and protection of boiler burners than DCS, while DCS is more suitable for systems such as steam turbine regulation with numerous analog probes. Don’t blindly worship DCS and ignore PLC!