1. DCS is a "distributed control system", while PLC (programmable controller) is just a control "device", which is the difference between the two. The system can realize the function and coordination of any equipment, and the PLC equipment only realizes the function of this unit.
2. In terms of network, the DCS network is the central nerve of the whole system. It is a safe and reliable double-redundant high-speed communication network with good scalability and openness. Since PLC is basically for personal work, when communicating with other PLCs or host computers, the network form used is basically a single network structure, and the network protocol is often inconsistent with international standards. In terms of network security, PLC does not have good protection measures. We use dual redundancy of power supply, CPU, and network.
3. Considering the overall scheme of DCS, the operator station has the function of the engineer station, and after the operation scheme program is downloaded, each station is in a close joint relationship. Any station, any function and any controlled device are interlocked and coordinated. However, in a system consisting only of PLC interconnection, the connection between stations (PLC and PLC) is loosely connected, and the function of coordinated control cannot be realized.
4. DCS has a large number of expandable interfaces in the whole design, and it is very convenient to connect the external system or expansion system. After the completion of the entire system of PLC superposition, it is difficult to add or remove operator stations at will.
5. DCS safety: In order to ensure the safety and reliability of DCS control equipment, DCS adopts dual redundant control units. When an important control unit fails, the relevant redundant unit is switched to the working unit in real time without disturbance, thus ensuring the safety and reliability of the entire system. The PLC superimposed system basically has no redundant concept, let alone redundant control strategy. Especially when one of the PLC units fails, the entire system must be stopped before it can be replaced, maintained, and reprogrammed. As a result, the DCS system is one level higher than its safety and reliability.
6. System software. Updating various process control schemes is the most basic function of DCS. When a solution changes, the engineer only needs to compile the changed solution on the engineer 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. The various control software and algorithms of the system can improve the control accuracy of the plant required by the process. For a system composed of PLCs, the workload is extremely enormous. First of all, it is necessary to determine which PLC to edit and update, then compile the program with the corresponding compiler, and finally use a special machine (reader) to send the program to this PLC one-to-one. During the commissioning of the system, the commissioning time and cost will be greatly increased, which is extremely detrimental to future maintenance. Control accuracy varies widely. This determines why PLC connected systems are not used in large and medium-sized control projects (more than 500 points).
7. Module: All I/O modules of DCS system are equipped with CPU, which can realize the quality judgment and scalar conversion of the collected and output signals, hot swap and plug in case of failure, and random replacement. The PLC module is just a simple electrical conversion unit, there is no smart chip, and the corresponding units are all paralyzed after the failure.
8. Now the functions of high-end PLC and DCS are almost the same. DCS has a powerful ability to regularly scan networks and distributed databases, and is good at computational and analog processing.
9. PLC is also divided into large, medium, small and micro PLC, of which the micro one is only sold for a few hundred to 2000 pieces, the number of points is also few, the large one can bring thousands of points, the computing power is similar to DCS, but the multi-machine networking function is weak. Now both technology platforms are similar, but the focus is different.
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