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Chuangkong Education Siemens All-in-One Course Introduction
PLC networks are composed of several levels of subnetworks, and the communication process of each subnetwork is determined by the communication protocol, with the communication method being the core content of the communication protocol. Communication methods include access control methods and data transmission methods. The so-called access control method (also known as access control) refers to how to obtain the right to use the shared communication medium, while the data transmission method refers to how data is transmitted after a station has obtained the right to use the communication medium.
1. Periodic I/O Communication Method
The periodic I/O communication method is commonly used in remote I/O links of PLCs. The remote I/O link operates in a master-slave manner, where the PLC remote I/O master unit is the master station, and all other remote I/O units are slaves. A “remote I/O buffer” is established in the master station, which uses a mailbox structure divided into several sub-boxes corresponding to each slave station—each sub-box is further divided into two parts, one for sending and one for receiving. The communication processor in the master station uses a periodic scanning method to sequentially exchange data with each slave station, sending the data from the corresponding sub-box’s sending part to the slave station and reading data from the slave station into the corresponding sub-box’s receiving part. This cycle refreshes the “remote I/O buffer” in the master station periodically.
In the master station, the CPU unit of the PLC is responsible for scanning the user program, processing it in a cyclic scanning manner, where each cycle includes a period dedicated to I/O processing, during which it reads and writes to the local I/O units and the remote I/O buffer. The periodic cyclic scanning of the PLC’s CPU unit for the user program occurs asynchronously with the periodic scanning of the communication processor for each remote I/O unit. Although the PLC’s CPU unit does not directly operate on the remote I/O units, the periodic refresh of the remote I/O buffer allows the PLC’s CPU unit’s read and write operations on the remote I/O buffer to be equivalent to directly accessing the remote I/O units. This communication method is simple and convenient, but it occupies the PLC’s I/O area, thus is only suitable for communication of small amounts of data.
2. Global I/O Communication Method
The global I/O communication method is a serial shared memory communication method, primarily used for communication between PLCs with linked areas.
The communication principle of the global I/O method is shown in the above figure. In each PLC’s I/O area in the PLC network, a block is designated as the linked area, and each linked area uses a mailbox structure. The sending and receiving areas with the same number are of equal size and occupy the same address range, one being the sending area and the others being receiving areas. Communication is done using a broadcast method. PLC1 broadcasts the data from its 1# sending area on the PLC network, and PLC2 and PLC3 receive it and store it in their respective 1# receiving areas. PLC2 broadcasts the data from its 2# sending area on the PLC network, and PLC1 and PLC3 receive it and store it in their respective 2# receiving areas. PLC3 broadcasts the data from its 3# sending area on the PLC network, and PLC1 and PLC2 receive it and store it in their respective 3# receiving areas. It is evident that through the above broadcasting communication process, the data in the linked areas of PLC1, PLC2, and PLC3 are the same, a process known as the equalization process.
The equalization communication ensures that the data in each PLC’s linked area in the PLC network remains consistent. It contains both the data sent by itself and the data sent by other PLCs. Since each PLC’s linked area is the same size and occupies the same address range, accessing its own linked area is equivalent to accessing other PLCs’ linked areas, thus facilitating data exchange with other PLCs. This way, the linked area becomes a true shared memory area, serving as an intermediary for data exchange among PLCs.
The linked area can be refreshed asynchronously (equalization) or synchronously. Asynchronous refreshing is independent of the user program in the PLC, conducted by the communication processors of each PLC in sequence, continuously keeping all linked areas equalized; synchronous refreshing is initiated by a send instruction in the user program, and this method only refreshes when the sending area data in the linked area changes.
In the global I/O communication method, the PLC directly uses read and write instructions to operate on the linked area, which is simple, convenient, and fast, but it should be noted that a write operation to a certain address in one PLC can only be read in other PLCs. Like the periodic I/O method, the global I/O method also occupies the PLC’s I/O area, thus only suitable for small amounts of data communication.
3. Master-Slave Bus Communication Method
The master-slave bus communication method, also known as 1:N communication, refers to a bus-structured PLC subnetwork with N stations, where there is only 1 master station and the rest are slave stations.
The 1:N communication method uses centralized access control technology to allocate bus usage rights, usually employing a polling table method. The polling table is an ordered list of slave numbers configured in the master station, where the master station queries the slave stations according to the order in the polling table to see if they wish to use the bus, thereby achieving the allocation of bus usage rights.
For stations with higher real-time requirements, their slave numbers can appear multiple times in the polling table to grant them higher communication priority. In some 1:N communications, the polling table method is combined with interrupt methods, allowing urgent tasks to interrupt normal cyclic polling and gain priority.
In the 1:N communication method, once a slave station obtains the right to use the bus, there are two types of data transmission methods. One only allows master-slave communication, and does not permit slave-to-slave communication; if a slave station needs to exchange data with another slave station, it must go through the master station. The other allows both master-slave communication and slave-to-slave communication; after the slave station obtains the bus usage right, it first arranges master-slave communication, and then arranges communication with other slave stations.
4. Token Bus Communication Method
The token bus communication method, also known as N:N communication, refers to a bus-structured PLC subnetwork with N stations, where all stations are equal and there is no distinction between master and slave; in other words, all N stations are master stations.
The N:N communication method employs token bus access control technology. A logical ring is formed on the physical bus, allowing a token to circulate in a certain direction; the station that obtains the token gains the right to use the bus. The token bus access control method limits the token holding time for each station, ensuring that each station has the opportunity to gain bus usage rights when the token completes a full cycle, and provides priority services, thus offering good real-time performance.
The station that obtains the token has two data transmission methods: the non-acknowledgment data transmission method and the acknowledgment data transmission method. In the non-acknowledgment data transmission method, the station that obtains the token can immediately send data to the destination station, and once the sending is complete, the communication process is finished; in the acknowledgment data transmission method, the station that obtains the token sends data to the destination station but does not consider the communication complete until the destination station obtains the token and sends an acknowledgment frame back to the sending station. The latter has a significantly longer response time, reducing real-time performance.
5. Floating Master Communication Method
The floating master communication method, also known as N:M communication, is applicable to bus-structured PLC networks, indicating that there are M stations on the bus, of which N (N<m) and="" are="" master="" rest="" slave="" span="" stations,="" stations.
The N:M communication method combines token bus and master-slave bus access control technologies. First, the N master stations form a logical ring, and the token circulates in this logical ring to allocate bus usage rights among the N master stations, which is the meaning of floating master.
The master station that obtains the bus usage right then uses a master-slave method to determine which stations to communicate with during its token holding time. Generally, a polling table is configured in the master station, which can poll the other master and slave station numbers arranged in the polling table.
The master station that obtains the bus usage right can use various data transmission methods to communicate with the destination station, with the non-acknowledgment connectionless method being the fastest.
6. CSMA/CD Communication Method
The CSMA/CD communication method is a random access communication method suitable for bus-structured PLC networks, where all stations have equal status without distinction between master and slave, employing the CSMA/CD access control method, which means “listen before talk, talk while listening”.
The CSMA/CD access control method cannot guarantee that each station in the PLC network can obtain bus usage rights within a certain time period, making it a non-real-time access control method. However, it adopts a random approach, is simple to implement, and can seize the bus whenever it is free, resulting in high utilization of communication resources. Therefore, in PLC networks, the CSMA/CD communication method is suitable for upper-level production management subnetworks.
The data transmission methods of the CSMA/CD communication method can choose from connected, unconnected, acknowledgment, non-acknowledgment, and broadcast communication, depending on the required communication speed and reliability.
The above are the commonly used communication methods in PLC networks; additionally, a small number of PLC networks adopt other communication methods, such as token ring communication. Furthermore, in some PLC networks, multiple communication methods are often integrated and configured in a certain level of subnetwork, which is a trend in future technological development.
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Siemens All-in-One + TIA Portal + EPLAN Electrical Design Video Recordings for Sale at Low Price!
Chuangkong Education Siemens All-in-One Course Introduction
