PLC and DCS: What's the difference?

5, PLC and DCS system scalability and compatibility of control products on the market, whether DCS or PLC, there are many manufacturers in the production and sales. For PLC systems, there is generally no or little need for expansion, because PLC systems are generally used for equipment. In general, there are few compatibility requirements for PLC, such as two or more systems that require resource sharing, which is also very difficult for PLC. And PLC generally use a dedicated network structure, such as Siemens MPI total linear network, and even adding an operator station is not easy or costly.

DCS in the process of development is also the manufacturer's own system, but most of the DCS system, such as YOKOGAWA Yokogawa, Honeyville, ABB, etc., although the system internal (process level) communication protocol is not the same, but the operation level network platform is the same choice of Ethernet network, using standard or deformed TCP/IP protocol. http://diangon.com This provides very convenient scalability. In this kind of network, the controller and the computer exist as a node, as long as the network reaches the place, you can arbitrarily increase or decrease the number of nodes and arrange the node position. In addition, based on windows OPC, DDE and other open protocols, the system can also be very convenient communication, in order to achieve resource sharing.

6, PLC and DCS database

DCS generally provides a unified database. In other words, once a data exists in the database in the DCS system, it can be referenced in any situation, such as in the configuration software, in the monitoring software, in the trend chart, in the report... The database of the PLC system is usually not unified, and the configuration software and monitoring software and even the archiving software have their own database. Why is it often said that the S7 400 of Siemens is called DCS when it is more than 414? This is because Siemens' PCS7 system uses a unified database, and PCS7 requires controllers to be at least S7 414-3 and above.

7, PLC and DCS time scheduling

The PLC program generally cannot run according to the preset cycle. The PLC program is executed once from beginning to end and then again from the beginning. (Now some new PLCS have improved, but there are still limits on the number of task cycles) and DCS can set task cycles. For example, quick tasks. The same is the sampling of the sensor, the change time of the pressure sensor is very short, we can use 200ms task period sampling, and the lag time of the temperature sensor is very large, we can use 2s task period sampling. In this way, the DCS can reasonably schedule the resources of the controller.

8, PLC and DCS network structure

Generally speaking, DCS usually uses a two-layer network structure, the first layer is a process-level network, and most DCS use their own bus protocols, such as Yokogawa Modbus, Siemens and ABB Profibus, ABB CAN bus, etc. These protocols are based on standard serial transmission protocols RS232 or RS485. The field IO module, especially the analog amount of sampled data (machine code, 213/ scan cycle) is very large, and there are many field interference factors, so the network standard with large data throughput and strong anti-interference ability should be adopted. The bus structure based on the asynchronous communication mode of RS485 serial port meets the requirements of field communication.

The sampled data of I/O is converted to shape data or real data by the CPU and transmitted on the operation-level network (Layer 2 network). Therefore, the operation-level network can adopt the network standard of moderate data throughput, fast transmission speed and convenient connection, and because the operation-level network is generally arranged in the control room, the requirement of countering interference is relatively low. So using standard Ethernet is the best choice. TCP/IP protocol is a standard Ethernet protocol, generally we use 100Mbit/s communication speed.

The task of the PLC system is relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a layer of network structure. Either the process level network and the operation level network are merged together, or the process level network is reduced to internal connections between modules. PLCS make little or no use of Ethernet.

9, PLC and DCS application object scale

PLC is generally used in small automatic control places, such as the control of equipment or a small amount of analog control and interlocking, and large applications are generally DCS. Of course, this concept is not very accurate, but very intuitive, we are used to more than 600 points of the system called DCS, less than this scale called PLC. Our heat pump and QCS, horizontal product supporting control system is generally called PLC.

10, DCS and PLC market situation and development direction

In the field of thermal automation, the main plant control system uses DCS almost without exception. PLCS are used in auxiliary workshops. The main reason is that the early DCS system is very expensive, people think that the operation of the auxiliary workshop can be interrupted, the reliability requirements are not very high, and the analog control requirements are less, from the point of view of reducing costs, often choose PLC to build the control system. The boiler, steam engine and generator control system, requiring long-term stable and reliable operation, the signal contains a considerable proportion of analog, from the performance of the system, people have to choose expensive DCS.