A PLC network consists of multiple subnets. The communication process of each subnet is determined by the communication protocol. The communication method is the core of the communication protocol. Communication methods include access control and data transmission. What are the working principles of the three common PLC communication methods?
1. Cyclic I/O communication mode
Periodic I/O is typically used for remote I/O in communication-mode PLCs. The I/O link in the chain operates in master mode. The PLC remote I/O master is the master station, and all I/O devices of other remote stations are slave stations. The master station sets up a "remote I/O buffer" using a mailbox structure. It is divided into several sub-boxes, with one sub-box corresponding to each slave station. Each sub-box is divided into two grids, one for transmitting and the other for receiving. The master station communication processor uses a periodic scanning method, exchanging data with each slave station sequentially, sending the data from the corresponding sub-box's grid to the master station, and the slave station reads the data and puts it into the receiving grid of the corresponding sub-box. In this way, the master "remote I/O buffer" is refreshed periodically.
2. Global I/O communication mode
Global I/O communication mode is a serial shared memory area communication mode, mainly used for communication between PLCs and link area communication modes.
The communication principle of global I/O mode is shown above. In the PLC network, each I/O area of each PLC is divided into a link area, and each link area uses an email structure. Sending and receiving areas with the same number share the same address space. One is the sending area, and the other is the receiving area. Communication is achieved through broadcasting. Data from PLC1's first transmission area is broadcast on the PLC network within that first transmission area. Upon receiving the data, PLC2 and PLC3 store the data in their respective receiving areas (number 1).
3. Master-slave bus communication mode
The master bus communication mode, also known as the 1:N communication mode, refers to a PLC subnet with N stations, of which only one is a master station and the rest are slave stations.
1:N communication mode uses centralized access control technology to allocate bus access, typically through a roundtable discussion. The so-called cyclic checklist is a list of slave device number sequences configured in the master station. The master station queries the slave devices according to the order of the checklist to see if they are using the bus, thereby allocating bus usage rights.
For sites with high real-time requirements, the slave number can appear multiple times in the polling table to grant the site higher communication priority. In some 1:N communication scenarios, the polling table method is combined with the interrupt method. Urgent tasks can interrupt normal periodic polling to gain priority.
Token bus communication, also known as N:N token bus communication, refers to a PLC subnet with N stations in the same state, independent of master and slave stations. Alternatively, these N stations can be considered master stations.
The station acquiring the token has two data transmission modes: no-acknowledgment data transmission mode and acknowledgment data transmission mode. In no-acknowledgment mode, the station acquiring the token can immediately send data to the destination station, completing the communication process upon transmission. In acknowledgment mode, the station acquiring the token does not complete communication after sending data to the destination station. The entire communication process only ends after the destination station obtains the token and sends an acknowledgment frame to the sending station. The latter significantly increases response time and reduces real-time performance.
4. Floating Master Station Communication Method
Floating master communication, also known as floating master communication N:M communication mode, is suitable for bus-structured PLC networks, meaning there are M stations on the bus, including M stations. N(N
The N:M communication mode employs a combination of token bus and master-slave bus access control technology. First, N master stations form a logical ring. Through the token flow within the logical ring, bus access rights are distributed among the N master stations; this is the meaning of a floating master station. The master station that obtains bus access rights will decide which stations to communicate with during its token holding period based on its own behavior. Master stations are typically equipped with a polling table, which can be used to filter information from other stations.
What are the working principles of the three common communication methods for PLCs? Data transmission using FX3U, FX2N, FX2NC, FX1N, and FX0N programmable controllers can be established on an N:N basis. Using this network communication, they can link data within a small-scale system.
