A netizen consulted me about methods to handle communication interference. Below are our exchanged emails (text slightly modified), hoping to help everyone.
Q&A Netizen:Our unit has a set of equipment equipped with an S7-400 system, and a TP 270 touch screen placed on the operation console. Information is transmitted via DP/MPI. The strange issue is that the PLC often fails to establish a connection with the TP screen. We have checked all possible interference sources and tried to shield them. Sometimes it resolves the issue, and communication is normal, but for unknown reasons, the same fault recurs. Moreover, the frequency of establishing and interrupting communication is very fast; it connects for a while and then disconnects.
There is also a DC drive device 590+ in the system, several Emerson frequency converters, and the rest are conventional electrical appliances. The power supply system has TE, PE, and they are isolated from each other.
I’d like to ask: How can we completely resolve the communication connection interference issue?
Liao:I suspect it might be interference from the speed control device or grounding issues. Grounding should strictly separate control ground and protective ground, with the control system grounded at one point. Netizen:The impact of DC speed control is indeed there, but our communication is just a point-to-point application, and we are using the original Siemens RS-485 plug and 6XV1830-0EH10 cable. The grounding system has already been separated, and the grounding of the communication line is done separately as data ground. The PLC system’s casing is connected to the cabinet (metal cabinet, installed by the original manufacturer), and our power supply system combines neutral and ground. Today it has become so severe that we cannot find the S7 communication statistics, which is quite a headache. Liao:What is the distance between the S7-400 and the TP? Are their lines close to the frequency converter’s lines? Is there communication between the frequency converter and the PLC? Netizen:The straight-line distance from the PLC to the TP270 is about 20 meters, and the communication line is approximately 35 meters in the cable channel. The PLC cabinet is right next to the DC drive cabinet, and beside the DC drive cabinet is the low-voltage cabinet (which contains contactors, protectors for the hydraulic station motor, main motor fan, low-voltage switches, miniature breakers, etc., as well as several frequency converters). All cables are in one cable trench, with a significant portion placed parallel; the frequency converter and DC drive do not communicate with the PLC, and all operational commands and speed values are output by the PLC, which connects to related equipment via cables. Liao:Do the communication lines of the frequency converter and TP have shielding? If the wiring and shielding of the frequency converter are not handled well, it may cause significant interference.Can you try temporarily placing the TP next to the PLC or pulling a temporary communication cable (not in the cable trench)? If there are no issues, it indicates that it is an interference problem. Netizen:The frequency converter is not very powerful, at 5.5 kW, but there is no communication between the frequency converter and the TP. The PLC and TP communication cable is a dedicated Siemens cable, with the shielding layer connected to the data ground, but this cable is placed parallel to several high-current cables for about 15 meters.
We have already placed the TP next to the PLC, and there are no issues. Today, we plan to replace the communication cable and keep it away from power cables; I wonder if it will be effective. The terminal resistance of the RS485 bus plug at the PLC end is not connected, while the terminal resistance at the TP end is connected.
Liao:If it is just point-to-point communication between the PLC and TP, both sides should connect terminal resistors.
Netizen:We applied the method you suggested, connecting terminal resistors at both ends, but there are still frequent communication interruption prompts. We will wait until maintenance time to replace the power cable and check the effect.
Summary:Through experiments, it can be confirmed that the fundamental cause of interference is the communication cable being placed parallel to multiple high-current cables (especially the input and output cables of the frequency converter) within the same cable trench and at a very close distance.
This system is relatively simple, just point-to-point communication. The simplest experimental method is to temporarily place the two devices together or pull a temporary communication cable (not in the cable trench). If the interference disappears, it is definitely due to the cable wiring introducing interference. An oscilloscope can be used to observe the waveform of the RS-485 A and B lines against the digital ground (pin 5); if there is significant interference, the interference signal waveform can be seen.
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