Abstract: This paper mainly discusses theoretically some common token networks, Ethernet and FDDI in DCS communication networks, analyzes their advantages and disadvantages according to the requirements of industrial control, and finally publishes the author's personal opinions on the development of communication networks. the opinion of.
1 DCS Communication Network Overview DCS is an abbreviation of English Distributed Control System, Chinese translated into a decentralized control system. As we all know, DCS is the hub system for modern units to start, stop, and operate, and the key to distinguishing them from traditional control systems is to have a complete communication system that delivers a large amount of data in a timely and accurate manner to all DCS related subsystems. The various subsystems can be organically combined to complete the automation of the complex system and eventually make the entire DCS an organic whole.
The importance of the DCS communication system has led to the continuous efforts of various DCS manufacturers in the world to improve and develop the DCS communication network. Although various manufacturers have different DCS communication systems and network types due to different development perspectives, the development direction is the same, that is, to improve the real-time communication performance, increase the effective transmission bandwidth, and enhance system reliability. Comparing the major DCS findings, based on the consideration of the openness, ease of use, and reliability of the entire system, taking into account different requirements for different levels of communication, DCSs generally adopt a layered system and divide the entire communication system into multiple parts. The level, the communication speed and the network type of each layer are different. However, from the perspective of industrial control, a sound communication system should meet the following two requirements: A. The communication waiting time does not increase significantly due to the increase in communication load; B. The maximum communication delay should be controllable.
In general, all levels of the DCS communication system network are used within a relatively small area and all belong to the local area network. Therefore, various types of LANs are available for DCS. Typical are Token Ring and Ethernet. There are two major categories of networks. In recent years, with the emergence of various high-speed LAN technologies, FDDI networks and switched Ethernet have also been introduced into DCS. This article will analyze the advantages and disadvantages of these popular network structures and industrial control in combination with the characteristics of industrial control, and hope to help the majority of process control technicians.
2 Token Ring 2.1 Token Ring Communication Features The core of Token Ring communication technology is the use of controlled communication technology. This technology is similar to the early delivery polling technology, but it is not controlled centrally but by using a scale. Controls the sending rights of each node on the network for the token's special format frame, which passes [l] between nodes in a specific order. Since the time for each node to hold the token is limited, the time for waiting for the arrival of the token at any one of the online nodes is controllable. That is, there is a maximum waiting time, so any one of the online nodes is within a specific time interval. All have the opportunity to get the token for data transmission. This transmission characteristic is the certainty of the network transmission. In a deterministic transmission network, the maximum communication delay under any communication load can be controlled, and the communication waiting time does not rise significantly as the load increases. At the same time, due to the use of controlled communication technology, no data collisions occur on the network, which ensures high-quality and efficient data transmission. In this way, even under heavy load, the communication performance of the entire network is very good. Studies have shown that token networks can still guarantee good performance under 90% communication load [2]. In addition, the priority of various data frames can be set under Token Ring technology, which can ensure the timely delivery of key data under heavy load.
2.2 Token Bus Network According to the network topology structure, Token Ring Network can be divided into Token Ring Network and Token Bus Network. Token Ring technology is the earliest token network technology implemented. The entire loop is connected through a segment of point-to-point links, which facilitates the implementation of token transmission technology. However, the drawback of the Token Ring is that the trunk line coupler between all the nodes and the loop is an active device, and its reliability is lower than that of a passive bus network. In addition, the access of the nodes on the ring network is relatively inconvenient. The token bus network integrates the advantages of the bus network and the token ring network. It is physically a bus network, but it is also a token network logically. Therefore, it not only has convenient access and high reliability of the bus network. Advantages, but also has the advantages of no transmission of the token network transmission and deterministic maximum transmission delay, therefore, the token bus technology has been more and more widely used. The biggest drawback of the token bus network is that the media access layer algorithm is relatively complicated to implement.
2.3 Analysis of Token Network Advantages and Disadvantages Token networks are recognized as very suitable for communication networks used in industrial control systems because of their significant advantages such as transmission determinism, priority control and high performance under heavy load. Tomorrow, Token Ring technology is still the most widely used in industrial control. Compared with Ethernet, token network technology has a significant advantage: In the process of data transmission, token network technology uses the forwarding technology. After the data stream passes through each node, the waveform is reshaped and amplified, thus making the node The distance between transmissions has greatly increased. However, the total transmission delay thus increased is very small, for example, each node in the token ring only causes one bit of transmission delay.
The main drawback of Token Ring is that the entire network must introduce additional delays due to waiting for tokens under any load. This delay is relatively obvious under low load. In addition, there is also a significant disadvantage of Token Ring, which is that due to the high royalty costs, the TokenNet related hardware prices are relatively high, which greatly limits its application in ordinary networks.
3 Ethernet 3.1 Ethernet Communication Features Ethernet is a local area network jointly developed by Xerox Corporation, DEC Corporation and Intel Corporation. Its transmission media is very extensive. Various types of copper cables, twisted pairs and optical fibers can be selected depending on the situation. Wait. Since its inception, the development of Ethernet technology has been very rapid. The network transmission speed has gradually evolved from the early 10Mbps to the current 1000Mbps. The network mechanism has evolved from the early sharing type to the prevailing exchange type. The working mode of the network card has evolved from simplex. To full duplex.
The main communication features of Ethernet are random access, carrier sensing, collision detection, and collision contention. The current general Ethernet standard is IEEE802.3, which uses the CSMMCD (Carrier Monitor Multipoint Access with Collision Detection) transmission protocol. According to the transmission protocol, any node that needs to transmit data must first monitor the network. If the network is busy, it will insist on listening to the network. Once the network is idle, it will send data. During data transmission, it will continue to monitor. If a conflict is detected, it will immediately stop sending and send a message. Strengthen the interference signal of the conflict and notify all nodes that the network has already been in conflict at this time. At this point, the parties to the conflict actively retreat and wait for a period of time and then listen to the network again.
The analysis found that there are two main sources of Ethernet collisions: one is that at some point there may be more than one node listening to the network. Therefore, once the network is idle, multiple nodes may send data at the same time to cause a collision. The second is the node. Because there is a distance between transmission delays, when an existing node on the network transmits data but has not yet transmitted to a node that needs to send data, the node will listen to the network idle and send data, causing a collision. Therefore, in theory, the use of standard industrial Ethernet will inevitably cause conflicts. Studies have shown that conflicts increase sharply with the increase in communication load, but they are less at low load. More seriously, standard Ethernet may have an “islanding†phenomenon from a purely theoretical perspective. That is, some nodes may not be able to transmit data for a period of time because the parties to the conflict need to be random after the conflict occurs. Waiting for a while to re-listen to the network, but then it may continue to cause conflicts, resulting in a looping state of "listen-send-conflict-wait-and-review", which is quite unfavorable for the timely delivery of some important data. . IEEES02.3 stipulates that abandoning transmission processing is performed for data frames that have failed to be transmitted for many times, and reports are sent to upper layers at the same time, and upper layers determine whether to retransmit data frames.
From the above analysis, it is found that Ethernet has the characteristics of transmission uncertainty, and any variable cannot fully ensure that the transmission is successful in a suitable time. In addition, in order to resolve invalid frames, Ethernet specifies a minimum data frame length of 64 bytes, so when the transmitted data is very short; for example, only one character, it will cause a significant waste of bandwidth.
3.2 Switched Ethernet Traditional Ethernet has inevitably suffered from conflicts due to the use of bus competition. In order to effectively increase the transmission efficiency of Ethernet and reduce conflicts, bus competition needs to be reduced as much as possible. Switched Ethernet is based on the A new generation of Ethernet developed by this idea [2]. Switched Ethernet uses star wiring, and all nodes are connected to the ports of a switching hub. The switching hub contains a complex switching array. A transmission channel can be set up between any two ports. The data is transmitted at the nominal transmission speed. Compared with the traditional bus shared Ethernet, the switched Ethernet does not have bus competition and significantly improves the transmission efficiency of the entire system. It should be noted that switched Ethernet also cannot control the maximum transmission delay because multiple nodes may need to transmit data with the same node at the same time, but only one node can successfully establish a transmission channel with the destination node at this time. The remaining nodes can only wait.
3.3 Analysis of Ethernet Advantages and Disadvantages Ethernet has the advantages of easy installation and low price, and it has no time delay because it does not have to wait for the arrival of the token under low load. It is the most rapidly developing LAN technology and has a market share. Absolutely dominant position. The biggest drawback of Ethernet is the uncertainty of sending delay and low performance under heavy load.
4FDDI network 4.1 FDDI network communication characteristics FDDI is the abbreviation of English Fiber Distributed DataInterface, Chinese name is fiber distributed data interface [2], it uses single-mode fiber or multimode fiber as the transmission medium, the transmission rate is fixed at 100Mbps, communication The same mature token transmission technology is adopted, so the network is completely deterministic and can guarantee the timely delivery of important data without loss of data or reduced precision. Therefore, FDDI is a method that can be used to transfer process control information. High-speed, high-bandwidth network. In design, FDDI not only adopts full redundancy and fault-tolerance technology, but also fully considers the interconnection with various LANs and WANs. Therefore, mature standard hardware and software are used to connect with regular LANs, thus eliminating the use of common DCSs. The complex redundancy mechanism and special gateway requirements increase system reliability [2].
4.2 High Fault-tolerance Characteristics of the FDDI Network Compared with the ordinary token network, the outstanding advantage of the FDDI is its inherent full redundancy and high fault tolerance. According to IS09314.2, a standard FDDI network consists of two fiber rings with opposite data transmission directions. Under normal conditions, only one ring is in operation and the other ring is in standby. When the running loop fails, FDDI can be automatically reconfigured at the same time, whether it is a link failure or a site failure. At the same time, the backup loop is activated to allow the entire network to continue working. See Figure 1.
It can be seen that, compared with other redundant communication networks used in conventional DCS, FDDI not only allows one loop to fail, but also allows the entire communication system to communicate normally even when both loops fail, even in the ring. In the case of multiple faults on the road, the entire network can still be automatically configured to continue working in multiple separate small ring networks.
4.3 Advantages and disadvantages of the FDDI network The decisive advantages of determinism, high speed, redundancy, and high fault-tolerance make FDDI networks ideal for demanding industrial process areas and can meet the real-time requirements for automation of large-scale complex systems. In recent years, several typical DCSs have introduced FDDI as their backbone control network in their latest versions, such as ABB's PROCONTROLP, Hitachi's HIACS5000+ and Westinghouse's Ovation. The biggest drawback of the FDDI network is its high price, which greatly limits its scope of use. Until now, it is still mainly used in the backbone network, but it is rarely used in the local area network.
5 About the use of Ethernet in DCS In recent years, the use of Ethernet in the DCS backbone communication network has been steadily increasing. The underlying reason is that because of its low price, the transmission speed has steadily increased while the price is decreasing. By adopting widely used standard Ethernet products, DCS manufacturers can not only easily upgrade the performance of the entire communication system as needed, but also more importantly reduce the R&D investment of the manufacturers, reduce the overall system cost, and increase the competitive advantage. All of these are very important for companies that have little financial and technical strength, especially for China's DCS developers.
The fatal drawback of Ethernet is the uncertainty of transmission and low performance under heavy load. Therefore, in order to effectively reduce the transmission delay, the communication load of Ethernet must be reduced as much as possible. Studies have shown that 20% of communication load is an appropriate upper limit for Ethernet in industrial control systems. At this time, the average waiting time for the entire system is satisfactory. By rationally optimizing the data transmission system of the entire DCS, the data transmission between the nodes is reduced as much as possible, and at the same time, by adopting various effective transmission measures such as information compression technology and event reporting technology, the data traffic of the entire network can be further reduced. This can ensure that the entire network is at a lower communication load and improve the real-time performance of the entire system. It should be noted that Ethernet design must be selected to take into account the significant increase in the communication load under accident conditions. The communication system must have sufficient margins, and if necessary, various high-speed Ethernet networks can be used.
Analysis of the major DCS found that due to the different requirements of real-time nature of DCS different communication levels, many large DCSs divide the backbone communication network into two levels, the operator network and the control network, as shown in Figure 2. In this way, different network types and communication protocols can be used as needed to reduce overall costs. The analysis found that DCS adopting this layered structure generally uses the industry standard Ethernet and TCP/IP protocols in the operator network, but differs in the control network. Some vendors are cautious and still insist on using deterministic networks. For example, ABB's PROCONTROLP, Hitachi’s HIACS5000+, and Westinghouse’s Ovation all use FDDI as their control network, while others are slightly larger and use the same in their DCS control networks. Standard Ethernet, but a special communication protocol was developed for the special requirements of industrial control, in order to maximize the rapid transfer of important information, the typical representative of which is Siemens' TEI. EPERMXP and ABB's Advant.
Relatively speaking, in the introduction of Ethernet technology, the United States MCS company's steps are the largest, in its latest MAXl000 + system, the backbone communication network is not divided into two levels of operator network and control network, all adopt industry standard Ethernet and TCP /IP protocol. In order to effectively reduce system communication load and reduce transmission delay, 10 Mbps/100 Mbp switched Ethernet is specially adopted. Among them, 100 Mbps ports are used for cascade connection among switches, and 10 Mbps ports are used to connect each control cabinet and operator station. .
6 Conclusion Through the analysis of this paper, we can find that token network, Ethernet and FDDI have their own characteristics, and they have successful application performance in the field of industrial control. Each manufacturer can flexibly choose according to their needs. According to my point of view, from a development point of view, Ethernet and FDDI will be further used in the DCS backbone communication network: The advantages of Ethernet are its low cost, high speed, and good performance at low loads, which is a good one for small and medium-sized DCS. The ideal choice; FDDI as a deterministic high-speed, high-fault-tolerance network can meet the demanding requirements of large-scale and complex system automation and control. Of course, the last thing to note is that a mature DCS, the choice of communication network type is just an important aspect of the efficiency of the communication system, but it is not the only one. There are many other key technologies that can be used to improve the communication efficiency and communication of the entire DCS. Sex, such as network segmentation technology, information compression technology and event reporting technology, etc., the author will further discuss these issues in the future.
References 1 Xie Xiren, Chen Ming, Zhang Xingyuan, Computer Network, Electronic Industry Press, 1994
2UylessBlack, EmergingCommunicationTechnologies(2ndEdition), PrenticeHall, 1997
1 DCS Communication Network Overview DCS is an abbreviation of English Distributed Control System, Chinese translated into a decentralized control system. As we all know, DCS is the hub system for modern units to start, stop, and operate, and the key to distinguishing them from traditional control systems is to have a complete communication system that delivers a large amount of data in a timely and accurate manner to all DCS related subsystems. The various subsystems can be organically combined to complete the automation of the complex system and eventually make the entire DCS an organic whole.
The importance of the DCS communication system has led to the continuous efforts of various DCS manufacturers in the world to improve and develop the DCS communication network. Although various manufacturers have different DCS communication systems and network types due to different development perspectives, the development direction is the same, that is, to improve the real-time communication performance, increase the effective transmission bandwidth, and enhance system reliability. Comparing the major DCS findings, based on the consideration of the openness, ease of use, and reliability of the entire system, taking into account different requirements for different levels of communication, DCSs generally adopt a layered system and divide the entire communication system into multiple parts. The level, the communication speed and the network type of each layer are different. However, from the perspective of industrial control, a sound communication system should meet the following two requirements: A. The communication waiting time does not increase significantly due to the increase in communication load; B. The maximum communication delay should be controllable.
In general, all levels of the DCS communication system network are used within a relatively small area and all belong to the local area network. Therefore, various types of LANs are available for DCS. Typical are Token Ring and Ethernet. There are two major categories of networks. In recent years, with the emergence of various high-speed LAN technologies, FDDI networks and switched Ethernet have also been introduced into DCS. This article will analyze the advantages and disadvantages of these popular network structures and industrial control in combination with the characteristics of industrial control, and hope to help the majority of process control technicians.
2 Token Ring 2.1 Token Ring Communication Features The core of Token Ring communication technology is the use of controlled communication technology. This technology is similar to the early delivery polling technology, but it is not controlled centrally but by using a scale. Controls the sending rights of each node on the network for the token's special format frame, which passes [l] between nodes in a specific order. Since the time for each node to hold the token is limited, the time for waiting for the arrival of the token at any one of the online nodes is controllable. That is, there is a maximum waiting time, so any one of the online nodes is within a specific time interval. All have the opportunity to get the token for data transmission. This transmission characteristic is the certainty of the network transmission. In a deterministic transmission network, the maximum communication delay under any communication load can be controlled, and the communication waiting time does not rise significantly as the load increases. At the same time, due to the use of controlled communication technology, no data collisions occur on the network, which ensures high-quality and efficient data transmission. In this way, even under heavy load, the communication performance of the entire network is very good. Studies have shown that token networks can still guarantee good performance under 90% communication load [2]. In addition, the priority of various data frames can be set under Token Ring technology, which can ensure the timely delivery of key data under heavy load.
2.2 Token Bus Network According to the network topology structure, Token Ring Network can be divided into Token Ring Network and Token Bus Network. Token Ring technology is the earliest token network technology implemented. The entire loop is connected through a segment of point-to-point links, which facilitates the implementation of token transmission technology. However, the drawback of the Token Ring is that the trunk line coupler between all the nodes and the loop is an active device, and its reliability is lower than that of a passive bus network. In addition, the access of the nodes on the ring network is relatively inconvenient. The token bus network integrates the advantages of the bus network and the token ring network. It is physically a bus network, but it is also a token network logically. Therefore, it not only has convenient access and high reliability of the bus network. Advantages, but also has the advantages of no transmission of the token network transmission and deterministic maximum transmission delay, therefore, the token bus technology has been more and more widely used. The biggest drawback of the token bus network is that the media access layer algorithm is relatively complicated to implement.
2.3 Analysis of Token Network Advantages and Disadvantages Token networks are recognized as very suitable for communication networks used in industrial control systems because of their significant advantages such as transmission determinism, priority control and high performance under heavy load. Tomorrow, Token Ring technology is still the most widely used in industrial control. Compared with Ethernet, token network technology has a significant advantage: In the process of data transmission, token network technology uses the forwarding technology. After the data stream passes through each node, the waveform is reshaped and amplified, thus making the node The distance between transmissions has greatly increased. However, the total transmission delay thus increased is very small, for example, each node in the token ring only causes one bit of transmission delay.
The main drawback of Token Ring is that the entire network must introduce additional delays due to waiting for tokens under any load. This delay is relatively obvious under low load. In addition, there is also a significant disadvantage of Token Ring, which is that due to the high royalty costs, the TokenNet related hardware prices are relatively high, which greatly limits its application in ordinary networks.
3 Ethernet 3.1 Ethernet Communication Features Ethernet is a local area network jointly developed by Xerox Corporation, DEC Corporation and Intel Corporation. Its transmission media is very extensive. Various types of copper cables, twisted pairs and optical fibers can be selected depending on the situation. Wait. Since its inception, the development of Ethernet technology has been very rapid. The network transmission speed has gradually evolved from the early 10Mbps to the current 1000Mbps. The network mechanism has evolved from the early sharing type to the prevailing exchange type. The working mode of the network card has evolved from simplex. To full duplex.
The main communication features of Ethernet are random access, carrier sensing, collision detection, and collision contention. The current general Ethernet standard is IEEE802.3, which uses the CSMMCD (Carrier Monitor Multipoint Access with Collision Detection) transmission protocol. According to the transmission protocol, any node that needs to transmit data must first monitor the network. If the network is busy, it will insist on listening to the network. Once the network is idle, it will send data. During data transmission, it will continue to monitor. If a conflict is detected, it will immediately stop sending and send a message. Strengthen the interference signal of the conflict and notify all nodes that the network has already been in conflict at this time. At this point, the parties to the conflict actively retreat and wait for a period of time and then listen to the network again.
The analysis found that there are two main sources of Ethernet collisions: one is that at some point there may be more than one node listening to the network. Therefore, once the network is idle, multiple nodes may send data at the same time to cause a collision. The second is the node. Because there is a distance between transmission delays, when an existing node on the network transmits data but has not yet transmitted to a node that needs to send data, the node will listen to the network idle and send data, causing a collision. Therefore, in theory, the use of standard industrial Ethernet will inevitably cause conflicts. Studies have shown that conflicts increase sharply with the increase in communication load, but they are less at low load. More seriously, standard Ethernet may have an “islanding†phenomenon from a purely theoretical perspective. That is, some nodes may not be able to transmit data for a period of time because the parties to the conflict need to be random after the conflict occurs. Waiting for a while to re-listen to the network, but then it may continue to cause conflicts, resulting in a looping state of "listen-send-conflict-wait-and-review", which is quite unfavorable for the timely delivery of some important data. . IEEES02.3 stipulates that abandoning transmission processing is performed for data frames that have failed to be transmitted for many times, and reports are sent to upper layers at the same time, and upper layers determine whether to retransmit data frames.
From the above analysis, it is found that Ethernet has the characteristics of transmission uncertainty, and any variable cannot fully ensure that the transmission is successful in a suitable time. In addition, in order to resolve invalid frames, Ethernet specifies a minimum data frame length of 64 bytes, so when the transmitted data is very short; for example, only one character, it will cause a significant waste of bandwidth.
3.2 Switched Ethernet Traditional Ethernet has inevitably suffered from conflicts due to the use of bus competition. In order to effectively increase the transmission efficiency of Ethernet and reduce conflicts, bus competition needs to be reduced as much as possible. Switched Ethernet is based on the A new generation of Ethernet developed by this idea [2]. Switched Ethernet uses star wiring, and all nodes are connected to the ports of a switching hub. The switching hub contains a complex switching array. A transmission channel can be set up between any two ports. The data is transmitted at the nominal transmission speed. Compared with the traditional bus shared Ethernet, the switched Ethernet does not have bus competition and significantly improves the transmission efficiency of the entire system. It should be noted that switched Ethernet also cannot control the maximum transmission delay because multiple nodes may need to transmit data with the same node at the same time, but only one node can successfully establish a transmission channel with the destination node at this time. The remaining nodes can only wait.
3.3 Analysis of Ethernet Advantages and Disadvantages Ethernet has the advantages of easy installation and low price, and it has no time delay because it does not have to wait for the arrival of the token under low load. It is the most rapidly developing LAN technology and has a market share. Absolutely dominant position. The biggest drawback of Ethernet is the uncertainty of sending delay and low performance under heavy load.
4FDDI network 4.1 FDDI network communication characteristics FDDI is the abbreviation of English Fiber Distributed DataInterface, Chinese name is fiber distributed data interface [2], it uses single-mode fiber or multimode fiber as the transmission medium, the transmission rate is fixed at 100Mbps, communication The same mature token transmission technology is adopted, so the network is completely deterministic and can guarantee the timely delivery of important data without loss of data or reduced precision. Therefore, FDDI is a method that can be used to transfer process control information. High-speed, high-bandwidth network. In design, FDDI not only adopts full redundancy and fault-tolerance technology, but also fully considers the interconnection with various LANs and WANs. Therefore, mature standard hardware and software are used to connect with regular LANs, thus eliminating the use of common DCSs. The complex redundancy mechanism and special gateway requirements increase system reliability [2].
4.2 High Fault-tolerance Characteristics of the FDDI Network Compared with the ordinary token network, the outstanding advantage of the FDDI is its inherent full redundancy and high fault tolerance. According to IS09314.2, a standard FDDI network consists of two fiber rings with opposite data transmission directions. Under normal conditions, only one ring is in operation and the other ring is in standby. When the running loop fails, FDDI can be automatically reconfigured at the same time, whether it is a link failure or a site failure. At the same time, the backup loop is activated to allow the entire network to continue working. See Figure 1.
It can be seen that, compared with other redundant communication networks used in conventional DCS, FDDI not only allows one loop to fail, but also allows the entire communication system to communicate normally even when both loops fail, even in the ring. In the case of multiple faults on the road, the entire network can still be automatically configured to continue working in multiple separate small ring networks.
4.3 Advantages and disadvantages of the FDDI network The decisive advantages of determinism, high speed, redundancy, and high fault-tolerance make FDDI networks ideal for demanding industrial process areas and can meet the real-time requirements for automation of large-scale complex systems. In recent years, several typical DCSs have introduced FDDI as their backbone control network in their latest versions, such as ABB's PROCONTROLP, Hitachi's HIACS5000+ and Westinghouse's Ovation. The biggest drawback of the FDDI network is its high price, which greatly limits its scope of use. Until now, it is still mainly used in the backbone network, but it is rarely used in the local area network.
5 About the use of Ethernet in DCS In recent years, the use of Ethernet in the DCS backbone communication network has been steadily increasing. The underlying reason is that because of its low price, the transmission speed has steadily increased while the price is decreasing. By adopting widely used standard Ethernet products, DCS manufacturers can not only easily upgrade the performance of the entire communication system as needed, but also more importantly reduce the R&D investment of the manufacturers, reduce the overall system cost, and increase the competitive advantage. All of these are very important for companies that have little financial and technical strength, especially for China's DCS developers.
The fatal drawback of Ethernet is the uncertainty of transmission and low performance under heavy load. Therefore, in order to effectively reduce the transmission delay, the communication load of Ethernet must be reduced as much as possible. Studies have shown that 20% of communication load is an appropriate upper limit for Ethernet in industrial control systems. At this time, the average waiting time for the entire system is satisfactory. By rationally optimizing the data transmission system of the entire DCS, the data transmission between the nodes is reduced as much as possible, and at the same time, by adopting various effective transmission measures such as information compression technology and event reporting technology, the data traffic of the entire network can be further reduced. This can ensure that the entire network is at a lower communication load and improve the real-time performance of the entire system. It should be noted that Ethernet design must be selected to take into account the significant increase in the communication load under accident conditions. The communication system must have sufficient margins, and if necessary, various high-speed Ethernet networks can be used.
Analysis of the major DCS found that due to the different requirements of real-time nature of DCS different communication levels, many large DCSs divide the backbone communication network into two levels, the operator network and the control network, as shown in Figure 2. In this way, different network types and communication protocols can be used as needed to reduce overall costs. The analysis found that DCS adopting this layered structure generally uses the industry standard Ethernet and TCP/IP protocols in the operator network, but differs in the control network. Some vendors are cautious and still insist on using deterministic networks. For example, ABB's PROCONTROLP, Hitachi’s HIACS5000+, and Westinghouse’s Ovation all use FDDI as their control network, while others are slightly larger and use the same in their DCS control networks. Standard Ethernet, but a special communication protocol was developed for the special requirements of industrial control, in order to maximize the rapid transfer of important information, the typical representative of which is Siemens' TEI. EPERMXP and ABB's Advant.
Relatively speaking, in the introduction of Ethernet technology, the United States MCS company's steps are the largest, in its latest MAXl000 + system, the backbone communication network is not divided into two levels of operator network and control network, all adopt industry standard Ethernet and TCP /IP protocol. In order to effectively reduce system communication load and reduce transmission delay, 10 Mbps/100 Mbp switched Ethernet is specially adopted. Among them, 100 Mbps ports are used for cascade connection among switches, and 10 Mbps ports are used to connect each control cabinet and operator station. .
6 Conclusion Through the analysis of this paper, we can find that token network, Ethernet and FDDI have their own characteristics, and they have successful application performance in the field of industrial control. Each manufacturer can flexibly choose according to their needs. According to my point of view, from a development point of view, Ethernet and FDDI will be further used in the DCS backbone communication network: The advantages of Ethernet are its low cost, high speed, and good performance at low loads, which is a good one for small and medium-sized DCS. The ideal choice; FDDI as a deterministic high-speed, high-fault-tolerance network can meet the demanding requirements of large-scale and complex system automation and control. Of course, the last thing to note is that a mature DCS, the choice of communication network type is just an important aspect of the efficiency of the communication system, but it is not the only one. There are many other key technologies that can be used to improve the communication efficiency and communication of the entire DCS. Sex, such as network segmentation technology, information compression technology and event reporting technology, etc., the author will further discuss these issues in the future.
References 1 Xie Xiren, Chen Ming, Zhang Xingyuan, Computer Network, Electronic Industry Press, 1994
2UylessBlack, EmergingCommunicationTechnologies(2ndEdition), PrenticeHall, 1997
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