What is the real-time communication technology of field bus

Abstract: What is the real-time communication technology information of fieldbus is provided by the excellent flowmeter and flowmeter production and quotation manufacturers. The time determinism of industrial control communication network means that the data through the transmission network must be transmitted from the source node to the sink node within a predetermined time. In a field bus control system, the measurement data collected from the field transmitter needs to be transmitted via the bus. More flowmeter manufacturers select models and price quotations. You are welcome to inquire. The following is the details of the real-time communication technology article of fieldbus. The time determinism of industrial control communication network means that the data through the transmission network must be transmitted from the source node to the sink node within a predetermined time. In a field bus control system, the measurement data collected from the field transmitter needs to be transmitted to the PID control module via the bus, and the calculation result is transmitted to the control valve. Since the set sampling time is determined, it is not subject to network transmission. The influence of other data, only in this way, can the PID control loop complete the control of this parameter with a fixed period. For such a system with real-time control requirements, the delay time from a node to another node in the network is usually required to be less than 2-4ms. 1. The main factors that affect the real-time performance of the field bus are as follows. ① Fieldbus data transmission rate: The system with high transmission rate has high real-time performance. Data transmission volume: The less data transmission volume, the better the real-time performance. Number of slave stations: The number of slave stations is small, the amount of data transmission is small, and the real-time performance is good. Master station data processing speed: The master station data processing speed is fast and the real-time performance is good. The way of controlling input and output: the stand-alone control way has better real-time performance than the field bus control way. Whether the communication goes through gateways, bridges or routers: The real-time transmission of data through these devices is poor. ②The size and complexity of the master application program. They will affect the response time of the system. Second, a variety of measures to solve the real-time nature can be adopted: ①Reduce the throughput of network communication to ensure that communication can be realized immediately once there is a communication requirement. For example, in the I/AS control system of Foxboro Company, the Ethernet communication adopted allows the number of connected nodes to be 1024, and the Ethernet adopts the CSMA/CD protocol. In essence, this protocol does not have the characteristics of communication determinism for the method of resolving communication conflicts, that is, after a node obtains the right to send, other nodes that need to send can only wait and constantly monitor whether the network is idle. Only when the network is idle can it compete for the right to send the communication. Due to the randomness of the listening latency, the determinism of the communication time cannot be guaranteed. To this end, the system stipulates that a maximum of 32 nodes can be connected, which greatly reduces the throughput of network communication and makes the time delay less than 2ms, which ensures the real-time requirements of network communication. Other measures to reduce traffic include methods such as increasing effective traffic. For example, a TCP/IP packet contains a 20-byte IP header and a 20-byte TCP header. After the TCP connection is established, the header of the packet is invalid communication and does not have to be repeatedly sent in each packet. , therefore, a small header can be reconstructed, reducing the transmission bytes of the communication. Typically, TCP/IP header compression is used to reduce packet header bytes to 10 bytes. ②Reduce the occurrence of communication conflicts and ensure the real-time nature of communication. For example, an intelligent hub with switching capabilities in a star network topology. Each node device hanging on the network resides in its own network segment, and all network segments are connected to the switching hub, which provides buffer memory and has the function of detecting which network segment needs to transmit data . Although there is a time delay (not more than 1ms) in this communication method, the communication conflict is minimized and meets the requirements of real-time communication. In addition, research on conflict resolution methods can also reduce packet delay. For example, practical Ethernet packet conflict resolution methods include CSMA/DCR, improved CSMA/MDCR, and CSMA/LDCR. CSMA/DCR adopts the deterministic binary tree addressing method, and adopts the preorder traversal method (PreorderTraversal) to resolve conflicts according to the node address. CSMA/MDCR adopts the sending time to divide the information notification and the information sending. After a node's information notification is successful, it can send several messages continuously to resolve the conflict, while CSMA/LDCR adopts the dynamic binary tree addressing method. When the information relaxation period is less than Information is sent only after a certain limit. Since these methods dynamically assign priorities, they can effectively reduce conflicts. ③ Adopt full-duplex communication mode and switched Ethernet.

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