A computer system in which a plurality of computers having independent functions and dispersed in different geographical locations are connected by a communication device and a communication line is called a computer network. PLc can be processed directly or through communication between a computer or a plurality of PLcs.
The devices form a network to realize information exchange; each PLC or remote I/O module is placed at the production site for decentralized control according to functions, and then connected by a network to form a distributed network with centralized management. Distributed network with adaptability, scalability and maintenance
It is widely used due to its advantages. Interconnection and communication are the core of the network. The topology, transmission control, transmission medium and channel utilization of the network are the four major elements of the network.
1. Network topology for data communication
In a network, a point that is interconnected by a transmission line is called a node, and a node can also be defined as an endpoint in the network that leads to any one branch, or to a common point of two or more branches. The manner and form in which each node is interconnected is called a network topology. Commonly used topologies are tree, star, bus, and ring.
(1) Tree structure
Tree structures are widely used in hierarchical distribution communication systems. Its characteristic is that the communication control software is relatively simple, and provides a concentration point for control and error processing. Sites in a higher position in the fabric control the data traffic of those sites located below it. Data transmission at the same level site is achieved through the transfer of the upper-level site. When a certain level of the site fails, the communication of its lower-level site will be paralyzed, and the communication of its upper-level site can still proceed, but it cannot communicate with the site. If a subordinate site connected to a higher-level site has more finger sets and data traffic is large, a "bottleneck blocking" problem occurs.
(2) Bus type structure
The bus type structure uses the bus to connect all the nodes. It is characterized in that all stations share a common communication bus, the bus is not closed, and it is easy to mount new sites or remove old sites. On the backbone link (bus), only two stations are allowed at any time.
Communication is carried out, but any two nodes can communicate directly through the bus, which is fast and has low delay overhead. When a site fails, it has less impact on the entire system. The communication protocol is simple, but sometimes there is contention control of the bus, which reduces transmission efficiency.
problem. Once the communication bus fails, the entire communication system will collapse. To solve this problem, a redundant bus is usually used. Communication media often use twisted pair, coaxial cable or optical fiber. This structure is especially suitable for industrial control. It is an industrial control finger sleeve.
A common topology in a local area network.
(3) Star structure
The star structure is connected to each node by a central node to form a dust-free cloth and a dust-free wiping cloth. Any two nodes in the network to be communicated are controlled and converted by the central control station. Its characteristics are: simple structure, easy control, clear data flow
Convenient for centralized development and resource sharing. However, because the central control station is responsible for the data exchange of the entire system, the dust-free cloth and the dust-free wipes have two major problems: “bottleneck blockage” and “danger concentration”. If using a redundant central control site approach
Will increase the complexity and cost of the system. It can be used in small systems and infrequent communication. The upper computer (also called the host computer, monitoring computer, central processing unit) communicates with each field processor (also called slave or lower computer) in a point-to-point manner.
It is a star structure. The star structure can use twisted pair as the transmission medium.
(4) Ring structure
The ring structure is formed by sequentially connecting the nodes in the ring network. A ring In most cases, information is passed from the source to the destination on the ring in one direction. Each station is connected to the network through a repeater, and the data is sent in packets.
Send, because multiple nodes share a loop, you need to control the dust-free cloth and dust-free wipe to determine when each station can send information on the ring. Each node has access to control sending and receiving. Logic, ring networks are commonly used up to 10Mbps
Twisted pair is used as the transmission medium. Its characteristics are: simple structure, easy to hook or remove nodes, and low installation cost. The task of each site is to receive the data sent by the neighboring previous site and then send the data to the next adjacent site. Between sites
Different transmission media and different transmission speeds are used. In the case of frequent data communication, its transmission efficiency is relatively high. However, if a site fails, it will block the information path, and the reliability will be poor. When there are many nodes, the transmission efficiency will be affected.
2. Media access control
Media access control, also known as transmission control, refers to the management and control of network channel possession. There are two ways to exchange information on a local network.
1 line switching, that is, there is a fixed physical channel between the sending node and the receiving node, and the channel is kept until the end of the call, such as the telephone system.
2 "Message exchange" or "packet exchange", this exchange method is to transfer the addressed data set from one conversion node to another conversion node until the destination station. There is no fixed physical channel between the sending node and the receiving node. If a node fails, then
The data group is sent to the destination node through other channels. This is somewhat like the way in which a parcel or telegram is delivered. Each addressed data set is similar to a postal packet, so it is called "packet exchange" or "message exchange."
At present, there are two types of transmission control for local area network applications, namely token transmission mode and contention mode.
