Why is PLC communication failure so difficult?

Why is PLC communication failure so difficult? It is because the communication itself is not easy, and many PLC veterans are more afraid of communication! The reasons for the difficulty are as follows: 1. Communication involves at least two ends These two ends may not be the same product, so a wide range of knowledge is required. For example, if we add a slave station to a DP network, if it is a Siemens PLC or a Siemens slave station, it can be found directly in the hardware configuration, which is the smoothest. If it is a Siemens PLC, a third-party slave station, you have to search for the GSD file all over the world, and then pour it into the programming software to find and hang it for configuration. If it is another PLC or a third-party slave station, in addition to looking for the GSD file, it is possible to open the GSD file and write the parameters in it into the master station program one by one. You need to understand the meaning of each parameter and choose the one you communicate with. Appropriate value, it is possible to pass. If it is the MODBUS RTU communication between the computer and the PLC, in addition to setting the port and network parameters on the computer side, you also need to set various parameters and address mapping in the PLC. Among the many actions, if there is a mistake, the result will be “no way”! To start over again. 2. The communication is a weak electric signal, which is very easy to be interfered The maximum communication signal is only about 10V. Compared with the big ones like 220V and 380V, it is very easy to be interfered. Therefore, there are many protection rules for communication cables, collectively referred to as EMC rules, such as: shielding grounding, equipotential, the farther away from strong electricity, the better, away from interference sources, etc. We are also smart about control, try to choose differential communication with strong anti-interference ability, choose double-shielded cables, and try to isolate optical fibers between different buildings. At that time, it was a done deal, and we could only endure it silently. 3. The frequency of communication is high and the signal changes quickly, so it is difficult to measureThe communication speed is often measured according to KHz and MHz, which is difficult for not only a multimeter but also an ordinary oscilloscope to capture. Even if you capture it, it is a series of high and low waveforms with distortion, how to interpret them with the naked eye?

It can only be captured, waveform evaluated, and protocol analyzed with the help of a professional protocol analyzer. These are special equipment abroad and cannot be as popular as a multimeter. You don’t need to care about this one at all, you can only rely on a few red lights on the module to roughly judge and explore.

4. Summary With the development and popularization of Ethernet technology and optical fiber technology, the reliability of the network is becoming more and more High, we should try our best to replace the original field bus with optical fiber and Ethernet, and avoid digging holes for ourselves. For example: Profibus-DP is upgraded to PROFINET, MODBUS RTU is upgraded to MODBUS TCP, and the original thin cable CONTROLNET is upgraded to Ethernet port CONTROLNET. Network communication is becoming more and more important in the current PLC system. For larger control systems, the master station often only has one power supply, CPU, and a bunch of communication boards, and all I/Os are in the peripheral electrical cabinet. Therefore, the content of communication is an unavoidable problem for our friends who are engaged in PLC. Friends who are getting started should first chew on the protocol of Ethernet communication. With a dazed look on his face, he couldn’t get along in front of Party A!