PLC network is composed of several sub-network compound, the sub-network communication process is determined by the communication protocol, and communication is the core of the communication protocol content. The communication mode includes access control mode and data transmission mode. The so-called access control (also known as access control) refers to how to obtain the right to use the shared communication media, and data transmission means that a station has access to the use of communication media, how to transfer data. In the case of
1. Cycle I / O communication mode
Cyclic I / O communication is commonly used in remote I / O links of PLCs. The remote I / O link works in master / slave mode, the PLC remote I / O master is the master, and the other remote I / O units are slaves. In the main station to set up a “remote I / O buffer”, the use of the mailbox structure, divided into several sub-boxes and each slave – a correspondence, each sub-box and then divided into two cells, a grid sent, one Grid management. The main station in the communication processor using the cycle scan mode, in order to exchange data with the slave, the corresponding sub-box to send the grid of data sent to the slave, read data from the slave into its corresponding points Box of the receiving grid. This cycle, so that the main station in the “remote I / O buffer” to be periodically refreshed. In the case of
In the main station, the CPU unit of the PLC is responsible for the scanning of the user program. It is processed in a cyclic scan mode. Each cycle has a time for I / O processing. When it is used for local I / O units and remote I / O Buffer for read and write operations. PLC CPU unit for the user program periodic cycle scan, and PLC communication processor for each remote I / O unit periodic scan is asynchronous. Although the PLC CPU unit is not directly on the remote I / O unit to manipulate, but because the remote I / O buffer to obtain periodic refresh, PLC CPU unit on the remote I / O buffer read and write manipulation, it is equivalent to direct access The remote I / O unit. This communication is simple and convenient, but to occupy the PLC I / O area, it is only applicable to a small amount of data communication. In the case of
2. Global I / O communication mode
The global I / O communication mode is a serial shared memory mode, which is mainly used for communication between PLCs with link areas. In the case of
In the PLC network of each PLC I / O area to draw a piece as a link area, each link area using the mailbox structure. The same number of send and receive area size of the same, occupy the same address segment, one for the sending area, the other are the receiving area. Using broadcast communication. PLC1 1 # send area of ​​the data in the PLC network broadcast, PLC2, PLC3 listen to it after receiving it down into their own 1 # receiving area. PLC2 2 # send area data broadcast in the PLC network, PLC1, PLC3 it received down into their own 2 # receiving area. PLC3 3 # send area data broadcast in the PLC network, PLC1, PLC2 it received down into their respective 3 # receiving area. Obviously through the above-mentioned broadcast communication process, PLC1, PLC2, PLC3 in the link area of ​​the data is the same, this process is called the equivalent process. The data in the link area of ​​each PLC in the PLC network is consistent by equivalent communication. It contains both the data sent to their own, but also contains the data sent by other PLC. As the size of each PLC link area, occupied by the same address, each PLC as long as access to their own link area, that is, access to other PLC link area, it is equivalent to exchange data with other PLC. So that the link area has become a real shared memory area, shared area to become the exchange of data exchange PLC. In the case of
The link area can be updated asynchronously (equivalent), or it can be refreshed by synchronous mode. Asynchronous mode refresh and PLC in the user program has nothing to do, by the PLC communication processor in order to broadcast communications, cycle, so that all of its link area to maintain the equivalent; synchronization mode is updated by the user program to send the link area to send instructions A refresh, this way only when the link area of ​​the send area data changes when the refresh. In the case of
In the global I / O communication mode, the PLC directly reads and writes the link area with the read and write instructions. It is simple, convenient and fast. However, it should be noted that writing to an address in a PLC is the same address in other PLCs. Can be read and manipulated. As with the periodic I / O mode, the global I / O mode also occupies the PLC’s I / O area, and therefore only applies to a small amount of data communication. In the case of
3. Master – slave bus communication
Master-slave bus communication is also known as 1: N communication  means that the bus structure of the PLC subnet has N stations, of which only one master station, the other is from the station. In the case of
1: N communication using centralized access control technology to allocate bus access, usually using the polling table method. The so-called polling table is a sort order from the machine number, the table is configured in the master station, the master station in accordance with the order of the order of the polling table to ask the station to see if it wants to use the bus, so as to achieve the allocation of bus use the goal of. In the case of
For real-time requirements of the relatively high station, you can kill the table in the wheel so that it appears several times from the machine number, giving the station a higher communication priority. In some 1: N communication, the polling table method is used in conjunction with the interrupt method, and the emergency task can interrupt the normal cycle polling and obtain the priority. In the case of
1: N communication mode when the bus access from the bus after the two types of data transmission. One is only agreed to master and slave communication, do not agree from the communication, from the station and the slave to exchange data, must be transferred by the master station; the other is to allow both master and slave communications also agreed from the communication, from the bus to obtain the bus After the first arrangement of the master and slave communication, and then arrange their own communication with other from the station. In the case of
4. Token bus communication
Token bus communication mode, also known as N: N communication means that the bus structure of the PLC subnet has N stations, their status is equal to the master and slave points, it can be said that N stations are the main station. In the case of
N: N communication using token bus access control technology. In the physical bus to form a logical ring, so that a token in the logical ring in accordance with a certain direction in order to get the token to obtain the right to use the bus station. The token bus access control limits the token holding time of each station to ensure that each station has access to the bus usage and provides priority services during the loop of the token cycle, so the token bus access control Has a good real-time. In the case of
There are two types of data transmission, that is, no response data transmission mode and response data transmission mode. When the non-response data transmission mode is adopted, the station that obtains the token can send the data to the destination station immediately, and the communication process is completed. The communication process is completed. When the response data transmission mode is adopted, the station After the data is not complete the communication, you must wait for the destination station to obtain the token and send the response frame to the sending station, the entire communication process is over. The latter than the former response time increased significantly, real-time decline. In the case of
5. Floating master station communication mode
Floating main station communication mode, also known as N: M communication mode, suitable for bus structure of the PLC network, refers to the bus has M stations, where N (N <M = the main station, the rest is from the station.
N: M communication mode using the token bus and master and slave bus combined access control technology. First, the N main station to form a logical ring, through the token in the logical ring in turn activities, in the N master between the distribution of bus use, which is the meaning of the floating master. The master station that obtains the bus usage right then follows the master-slave mode to determine which stations to communicate with during its own token hold time. Generally, there is a polling table in the master station, which can be polled according to the other master station number and slave number arranged on the polling table. The master station that received the token for the user’s random communication task can be prioritized before or after the polling. In the case of
The master station that obtains the bus usage right can communicate with the destination station in a variety of data transmission modes, with the fastest of no answerless connection. In the case of
6. CSMA / CD communication mode
CSMA / CD communication is a random communication mode, suitable for the bus structure of the PLC network, the bus on the status of the same station, there is no master and slave points, the use of CSMA / CD access control, that is, Listen to “. In the case of
CSMA / CD access control mode can not guarantee that in a certain period of time, PLC network on each station can get the right to use the bus, so this is a real-time access control can not guarantee the way. But it uses a random way, the method is simple, and the joints, as long as the bus is busy on the Internet, communication resource utilization is high, so in the PLC network CSMA / CD communication method for the upper production management subnet. In the case of
CSMA / CD communication mode of data transmission can be used to connect, no connection, a response, no response and broadcast communication in each of the communication speed and reliability requirements can be selected. In the case of
These are PLC network commonly used in communication, in addition to a small number of PLC network using other means of communication, such as token ring communication and so on. In addition, in the recently introduced PLC network, often a variety of communication methods integrated configuration in a sub-network, which is the future trend of technology development.

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