For the question itself, some answers mentioned it. Reliability, scan time, with discrete points, with analog output points, scalability, these can be regarded as the basis for the division of PLC system levels. For example, the PLC with the core of LOGIC SOLVER certified by the corresponding safety integrity level, and the relay also has the corresponding SIL level, and the PLC used for SIS is generally considered to be advanced.
Let me say a few words about DCS, PLC, and SCADA.
Let me talk about my own conclusion first: DCS, PLC, and SCADA are different things, different concepts, and basically have no mutual affiliation.
The evolution process of DCS is from the combination instrument of the pneumatic unit to the electric single-loop controller, and then to the DCS. It is aimed at process control. There was a concept of centralized control in the middle, but it was found that the risk was concentrated and unacceptable, so it developed DCS, a distributed control system. The hardware structure is field instruments, junction boxes, marshalling cabinets (sometimes not available), I/O Card cabinets, controller cabinets (sometimes combined with I/O card cabinets), power cabinets, relay cabinets, safety barrier cabinets (for intrinsically safe systems), etc. The current DCS system can also complete logic judgment, sequential control and other functions. DCS usually also has communication interfaces with other PLCs, SIS, and upper data networks.
PLC, the evolution process is from solid state logic, to relay group, to programmable logic controller Programmable Logic Controller. Its original intention is just like its name, dealing with logical judgment, sequence control, batch, etc. Later, PID control was expanded (it can do part of DCS). The evolution of PLC is not necessarily accurate, but the difference with DCS is still clear. There will be no large-scale chemical equipment using PLC as the core process control, and only SIL-certified PLC as SIS and individual control of some equipment packages.
SCADA, like its name, is mainly used for data acquisition and monitoring. For example, the monitoring system of the MCC system of the electrical specialty of a chemical plant. Another example is that the gate stations of pressurization and pressure regulation stations along the long-distance oil and gas pipeline will centralize the data into a control center through the SCADA system. The main difference is that it is intended to collect and display information, while PLC and DCS should output control signals. I am not familiar with SCADA.
PLC and DCS are two things with different origins and different specialties. Although they have developed into each other so far, their scope of application is still very different.
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