02VA0153  PLC, FCS, DCS three control system difference

FCS is developed from DCS and PLC, FCS not only has the characteristics of DCS and PLC, but also a revolutionary step, and at present, the new DCS and the new PLC have a trend to move closer to each other, the new DCS has a strong sequence control function; The new PLC, in the processing of closed-loop control is not bad, and both can form a large network, DCS and PLC scope of application, has a great crossover.

1. Distinguishing key points

1. DCS

02VA0153 The key of DCS system is communication. You could say that the data highway is the backbone of the DCS. Since its mission is to provide a communication network between all the components of the system, the design of the data highway itself determines overall flexibility and security. The media of the data highway can be: a pair of stranded, coaxial, or fiber optic cables.

02VA0153 Through the design parameters of the data highway, you can basically understand the relative strengths and weaknesses of a particular DCS system.

(1) How much I/O information the system can process.

(2) How much control loop information related to control can be processed by the system.

(3) How many users and devices can be adapted (CRT, control station, etc.).

(4) How thoroughly the integrity of the transmitted data is checked.

(5) What is the maximum allowable length of the data highway?

(6) How many branches the data highway can support.

(7) Whether the data highway can support hardware produced by other manufacturers (programmable controllers, computers, data recording devices, etc.).

In order to ensure the integrity of communication, most DCS manufacturers can provide redundant data highways.

To ensure the security of the system. Using sophisticated communication protocols and error detection techniques, a communication protocol is a set of rules to ensure that the data transmitted is received and understood in the same way as the data sent.

02VA0153 At present, in the DCS system, two types of communication means are generally used, namely synchronous and asynchronous, synchronous communication relies on a clock to regulate the transmission and reception of data, and asynchronous network uses a reporting system without a clock.

2. FCS

02VA0153 There are three key points of FCS.

(1) 02VA0153 The core of FCS system is the bus protocol, that is, the bus standard.

A type of bus, as long as its bus protocol is determined, the relevant key technologies and related equipment are also determined, in terms of the basic principle of its bus protocol, all types of buses are the same, to solve the bidirectional serial digital communication transmission as the basic basis. However, due to various reasons, the bus protocols of various buses are very different.

In order to meet the interoperability requirements of fieldbus and make it a real open system, the original IEC international standard, the user layer of fieldbus communication protocol simulation, clearly stipulates that the user layer has the device description function. For interoperability, each fieldbus device is described by device description DD. DD can be thought of as a driver of the device, which includes all the necessary parameter descriptions and operation steps required for the master station. Because DD includes all the information needed to describe device communication and is independent of the master station, it enables true interoperability of field devices.

Whether the actual situation is the same as above, the answer is no. At present, there are 8 types of fieldbus international standards, and the original IEC international standard is only one of the 8 types, and the status of the other 7 types of buses is equal. The other 7 types of buses, regardless of their market share, each bus protocol has a set of software and hardware support. They can form systems, they can form products. The original IEC fieldbus international standard is an empty shelf with neither software support nor hardware support. To make these buses compatible and interoperable is almost impossible in its current state.

02VA0153 From the above, can we arrive at such a picture: Open fieldbus control system interoperability, in terms of a specific type of fieldbus, as long as the bus protocol of the type of fieldbus, the product is open and interoperable, in other words, no matter what manufacturer’s product, it is not necessarily the product of the fieldbus company, as long as the bus protocol of the bus is followed, the product is open between the products. And with interoperability, you can form a bus network.

(2) The foundation of the FCS system is the digital intelligent field device.

The digital intelligent field device is the hardware support of the FCS system and is the foundation. The reason is very simple. The FCS system executes the field bus signal system of two-way digital communication between the automatic control device and the field device. If the field device does not follow the unified bus protocol, that is, the relevant communication protocol, does not have the digital communication function, then the so-called two-way digital communication is just an empty phrase, and it cannot be called a fieldbus control system. Moreover, a major feature of the fieldbus is to increase the field level control function. If the field device is not a multifunctional intelligent product, then the characteristics of the field bus control system do not exist, the so-called simplified system, convenient design, conducive to maintenance and other advantages are also virtual.

(3) The essence of FCS system is on-site information processing.

For a control system, whether using DCS or fieldbus, the system needs to handle at least as much information, in fact, after using fieldbus, you can get more information from the field. The amount of information in the fieldbus system has not been reduced, and even increased, while the cables to transmit the information have been greatly reduced. This requires, on the one hand, greatly improving the ability of cable transmission information, on the other hand, allowing a large amount of information to be processed on-site, reducing the information roundtrip between the site and the control room. It can be said that the essence of fieldbus is the field of information processing.