PLC control and DCS control system are not concepts at the same logical level.
PLCs are named according to their functions, while DCSs are named according to their architecture.
In principle, a PLC can be used to form a DCS.
Of course, there are still performance differences between the two, and it depends on the specific product and needs.
Differences between DCS control systems and PLC control systems:
DCS is a "distributed control system," while PLC (programmable logic controller) is merely a control "device." The difference lies in the distinction between a "system" and a "device." A system can perform the functions and coordination of any device , while a PLC device only performs the functions required by its own unit.
The DCS network is the central nervous system of the entire system . DCS systems typically use the international standard protocol TCP/IP. It is a secure, reliable, dual-redundant, high-speed communication network with better system scalability and openness . In contrast, PLCs generally operate as individual small systems, and when communicating with other PLCs or host computers, they typically use a single-network structure, and their network protocols often do not conform to international standards.
In a DCS overall solution, operator stations all have the functions of engineer stations. After the program is downloaded, the stations are closely linked. Any station, any function, and any controlled device are interlocked and coordinated. In contrast, a system consisting of interconnected PLCs has a loose connection between stations (PLCs) and cannot achieve coordinated control.
The DCS design incorporates numerous scalability interfaces, making it easy to connect to or expand external systems. Once the entire system is built using a PLC, it is difficult to add or remove operator stations at will.
To ensure the safety and reliability of the equipment controlled by the DCS , the DCS employs dual redundant control units. When a critical control unit fails, a corresponding redundant unit seamlessly switches over to take over as the working unit in real time, ensuring the safety and reliability of the entire system. Systems connected by PLCs require dual PLCs for redundancy.
Updating various process control schemes is a fundamental function of DCS (Distributed Control System). When a scheme changes, engineers only need to compile the modified scheme at the engineer's station and then execute the download command. The download process is completed automatically by the system and does not affect the operation of the original control scheme. Various control software and algorithms within the system can improve the control precision of process objects. However, for systems composed of PLCs, the workload is extremely large. First, it is necessary to determine which PLC needs to be edited and updated. Then, the program must be compiled using the corresponding compiler. Finally, a dedicated machine (reader/writer) is used to transmit the program to that PLC one-to-one. During system debugging, this significantly increases debugging time and costs and is extremely detrimental to future maintenance.
The difference in control precision is significant. This explains why systems entirely composed of PLCs are rarely used in large and medium-sized control projects (over 500 points). In a DCS system, all I/O modules have CPUs, enabling quality assessment and scalar transformation of acquired and output signals, allowing for hot-swapping and random replacement in case of faults. In contrast, PLC modules are simply electrical converters without intelligent chips; a fault in a PLC causes the entire corresponding unit to malfunction . A Distributed Control System (DCS) integrates computer technology, control technology, communication technology, and CRT technology to monitor, control, operate, and manage production processes.
The similarities between DCS and PLC lie in the basic configuration of software settings, but DCS is more powerful, for example, in database generation, historical data generation, graphic generation, report generation, and control configuration.
PLCs typically control relatively simple objects, while DCSs are different; they can control all the equipment in an enterprise.
There is no one controlling the other. The PLC can send signals to the DCS, and vice versa. The two cooperate with each other, which mainly depends on the requirements of the equipment.
