Currently, control systems using monitoring software such as WinCE and MCGS as the host computer software and PLCs as the slave computers have become a widely used control mode. This mode combines the user-friendly and intuitive interface of computers with the stability and flexible programming advantages of PLCs, leading the trend in computer control systems. Therefore, higher demands are placed on the data interaction and networking capabilities of PLCs. For example, data interaction is required between PLCs, between PLCs and computers, and between PLCs and intelligent devices. Especially in remote control scenarios with dispersed control points, the network functionality of PLCs is particularly important.
Common PLC networking methods can be broadly categorized into three types: based on general serial ports, based on dedicated buses, and based on Ethernet.
1 General-purpose serial port module
Network connectivity is achieved using a serial communication module, as shown in Figure 1. This network uses a computer-to-computer connection, with appropriate settings configured in the host computer's configuration software. No programming is required to communicate with multiple PLCs. Taking Mitsubishi's FX1S series PLC as an example, the RS232C/485 converter adapter is FX-485PC-IF, and the RS-485 communication board is FX1N-485-BD. This method is convenient and performs well, but the serial communication module is relatively expensive.
2. Based on dedicated bus
Currently, PLC manufacturers such as OMRON and Siemens provide dedicated network systems for their PLC products, such as OMRON's ControllerLink network and DeviceNet network. Due to the proprietary nature of these network systems, devices from different manufacturers cannot communicate with each other. Basically, if you choose a PLC from a certain manufacturer, all other supporting equipment must also be from that manufacturer, which results in relatively high costs and certain limitations in application.
3. Based on standard industrial Ethernet
The network is based on standard industrial Ethernet and is generally divided into three layers: the first layer is the host computer monitoring station composed of industrial control computers; the second layer is the industrial Ethernet composed of hubs, twisted pairs, and transceivers; and the third layer is the control station, which selects TCP/IP as the communication protocol and adopts a client/server mode to enable connection-oriented communication between the control station and the monitoring station.
The biggest advantage of this networking method is that it can utilize the existing factory LAN, improving overall utilization, and it offers high speed, with Ethernet communication rates reaching 100Mbps. If fiber optic transmission is used, the anti-interference capability is greatly enhanced, and the transmission distance can reach tens of kilometers. However, Ethernet cannot directly communicate with serial devices such as PLCs, requiring additional equipment for communication, which increases costs. This method is not commonly used in small and medium-sized control systems.
