DCS (distributed control system) and PLC (programmable logic controller) are very common in our industry, so how much do you know about them?

Up to now, there is no strict boundary between DCs and PLC. In the eyes of most people, DCS is used for large-scale control and PLC is used for small-scale control. That's not accurate. Now let's re-establish this concept.

So what is the difference between DCs and PLC?

Development

DCS is developed from the traditional instrument panel monitoring system. Therefore, DCS pays more attention to the control of instruments from the congenital point of view. PLC developed from the traditional relay circuit. The original PLC even had no analog processing capacity. Therefore, PLC emphasized the logic operation ability from the beginning.

System scalability and compatibility

There are many control products in the market, no matter DCS or PLC, many manufacturers are producing and selling. For the PLC system, there is generally no or little need for expansion, because the PLC system is generally used for equipment. Generally speaking, PLC has few compatibility requirements. For example, two or more systems require resource sharing, which is also very difficult for PLC.

In the process of DCS development, each manufacturer has formed its own system, but most DCS systems, such as Siemens, abb, Honeywell, Ge, Schneider, etc., although the internal (process level) communication protocols of the system are different, the operation level network platform invariably selects Ethernet and adopts standard or modified tcp/ip protocol. This provides convenient scalability.

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 case, such as in the configuration software, in the monitoring software, in the trend chart, and in the report. The database of PLC system is usually not unified. Configuration software, monitoring software and even archiving software have their own database.

Time scheduling

Generally, the PLC program cannot run according to the preset cycle. The PLC program is executed from beginning to end and then from the beginning. (now some new PLCs have been improved, but the number of task cycles is still limited). DCs can set task cycles. For example, fast tasks. It is also the sampling of the sensor. The change time of the pressure sensor is very short. We can use a 200ms task cycle for sampling, while the lag time of the temperature sensor is very large. We can use a 2S task cycle for sampling. In this way, DCs can reasonably dispatch the resources of the controller.

network structure

Generally speaking, DCS usually uses a two-layer network structure. The first layer is a process level network. Most DCS use their own bus protocols, such as PROFIBUS of Siemens and abb, can bus of ABB, MODBUS of Schneider, etc. these protocols are based on the standard serial port transmission protocol RS232 or RS485. The field i/o module, especially the sampling data of analog quantity (machine code, 213/ scanning 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 RS485 serial port asynchronous communication mode meets the requirements of field communication.

The sampling data of i/o is converted into shaping data or real data by CPU and transmitted on the operation level network (layer 2 network). Therefore, the operation level network can adopt the network standard with moderate data throughput, fast transmission speed and convenient connection. At the same time, since the operation level network is generally arranged in the control room, the requirements for anti-interference are relatively low. Therefore, standard Ethernet is the best choice.

The work task of the PLC system is relatively simple, so the amount of data to be transmitted is generally not too large, so the common PLC system is a one-layer network structure. The process level network and the operation level network are either merged, or the process level network is simplified into an internal connection between modules. PLC does not or rarely use Ethernet.

If you are interested in PLC and DCS products, you can contact us directly, and we will provide you with answers to any questions in the shortest time.