FCS is the fieldbus control technology, and the two are the same thing.
FCS is developed by plc; in other industries, FCS is developed by dcs, so there are inextricable links between FCS and PLC and DCS, and there are essential differences.
FCS is developed by DCS and PLC. FCS not only has the characteristics of DCS and PLC, but also takes a revolutionary step. At present, the new DCS and the new PLC have a tendency to move closer to each other. The new DCS has a strong sequence control function; the new PLC is not bad in dealing with closed-loop control, and both can form a large network. The scope of application of DCS and PLC has been greatly crossed.
The key to DCS systems is communication. It can also be said that the data highway is the spine of the distributed control system DCS. Because its mission is to provide a communication network between all components of the system, the design of the data highway itself determines the overall flexibility and security. The media for the data highway can be: a pair of twisted wires, coaxial cables, or fiber optic cables.
Through the design parameters of the data highway, you can basically understand the relative advantages and weaknesses of a particular DCS system.
(1) How much I/O information the system can handle.
(2) How much control loop related information the system can handle.
(3) How many users and devices (CRT, control station, etc.) can be accommodated.
(4) How to completely check the integrity of the transmitted data.
(5) What is the maximum allowable length of the data highway?
(6) How many branches can be supported by the data highway.
(7) Whether the data highway can support hardware (programmable controllers, computers, data recording devices, etc.) produced by other manufacturers.
To ensure the integrity of communications, most DCS manufacturers can provide redundant data highways.
In order to ensure the security of the system, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules to ensure that the transmitted data is received and understood to be the same as the transmitted data.
At present, two types of communication means are generally used in the DCS system, that is, synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate data transmission and reception, and the asynchronous network uses a reporting system without a clock.
·FCS
There are three key points in FCS.
(1) The core of the FCS system is the bus protocol, that is, the bus standard.
As described in the previous section, one type of bus, as long as its bus protocol is determined, the relevant key technologies and associated equipment are also determined. As far as the basic principle of the bus protocol is concerned, all kinds of buses are the same, and all are based on solving the two-way serial digital communication transmission. However, due to various reasons, the bus protocols of various types of buses vary greatly.
In order to make the fieldbus meet the interoperability requirements and make it a truly open system, in the user layer of the IEC international standard and the fieldbus communication protocol model, it is clearly stated that the user layer has a device description function. In order to achieve interoperability, each fieldbus device is described by a device description DD.
The DD can be thought of as a drive for the device, which includes all the necessary parameter descriptions and the operational steps required by the primary station. Since the DD includes all the information needed to describe the communication of the device and is independent of the primary station, the field device can be made truly interoperable.
Whether the actual situation is consistent with the above, the answer is no. The current fieldbus international standard has eight types, and the original IEO international standard is only one of eight types, and the status of the other seven types of buses is equal. The other seven buses, regardless of their market share, each bus protocol has a set of software and hardware support. They can form systems and form products, and the original IEC fieldbus international standard is an empty shelf with neither software support nor hardware support. Therefore, to achieve mutual compatibility and interoperability of these buses, it is almost impossible in the current state.
Through the above, can we come up with an image of the interoperability of an open fieldbus control system, which is open to a specific type of fieldbus as long as it follows the bus protocol of this type of fieldbus And interoperability. In other words, no matter what manufacturer's products, nor one is the product of the fieldbus company, as long as the bus protocol of the bus is followed, the products are open and interoperable, and the bus network can be formed.
(2) The basis of the FCS system is the digital intelligent field device
The digital intelligent field device is the hardware support of the FCS system. It is the basis and the reason is very simple. The FCS system performs the two-way digital communication field bus signal system 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, and does not have digital communication function, then the so-called two-way digital communication is just an empty talk, and can not be called a field bus control system. Another point, a major feature of the fieldbus is to increase the level of control at the site. If the field device is not a multi-functional intelligent product, then the characteristics of the field bus control system do not exist. The so-called simplified system, convenient design, and good maintenance are also virtual.
(3) The essence of FCS system is information processing on-site
For a control system, whether it is DCS or a fieldbus, the system needs to process at least as much information. In fact, after using the fieldbus, you can get more information from the scene. The amount of information in the fieldbus system has not decreased or even increased, and the cable for transmitting information has been greatly reduced. This requires on the one hand to greatly improve the ability of the cable to transmit information, on the other hand, to allow a large amount of information to be processed on site, reducing the information between the scene and the control room. It can be said that the essence of fieldbus is the on-siteization of information processing.
Reducing information round trips is an important principle in network design and system configuration. Reducing information round trips often brings the benefits of improved system response time. Therefore, in the network design, the nodes with large amount of information exchange should be placed in the same branch.
Cables that reduce information to and from the system are sometimes contradictory. At this time, we should still make choices based on the principle of saving investment. If the response time of the selected system allows, you should choose a cable-saving solution. If the response time of the selected system is relatively tight, and a little reduction in the transmission of information is sufficient, then the solution for reducing the information transmission should be selected.
Now some field instruments with fieldbus are equipped with many function blocks. Although the performance of different functional blocks of different products will be slightly different, there are many functional sub-functions on a network branch. Which function block on the field instrument is selected is the problem to be solved by the system configuration.
The principle of considering this problem is to minimize the round-trip of information on the bus. It is generally possible to select the function block on the meter that outputs the most information related to this function.
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