The problem reflects: a set of equipment, equipped with S7-400 system, a TP 270 touch screen placed on the console, transmitting information through DP / MPI mode, the strange problem is: often plc and TP screen can not establish a connection, think of All the places that think that there are interferences are inspected, and I want to find a way to shield them. Sometimes I can solve the problem, the communication is normal, but I don’t know what causes the same fault, and sometimes the communication establishment and interrupt conversion frequency is very fast, and I will connect for a while. It will be disconnected for a while. Product Description Diaphragm Seal Systems,Absolute Pressure Gauge,Analog Pressure Gauge,Manometer Pressure Gauge,Accutire Pressure Gauge Changshu Herun Import & Export Co.,Ltd , https://www.herunchina.com
There is also a DC drive unit 590+, several Emerson inverters, and the rest are conventional appliances. The power supply system has TE and PE, and is isolated from each other.
How to completely solve the problem of communication connection interference?
Solution: It may be interference from the speed control device or grounding. The grounding should be strictly separated from the control ground and the protective ground, and the control system should be grounded at one point.
The problem is reflected: The influence of DC speed regulation is there, but the communication is only a point-to-point application, and the original Siemens RS-485 plug and 6XV1830-0EH10 cable are used. The grounding system has been separated, and the grounding of the communication line is a separate data ground. The cabinet of the PLC system is connected to the cabinet (metal cabinet, assembled by the original manufacturer), and the power supply system is zero and ground.
The problem is reflected: Serious enough to find the S7 statistical newsletter,
Solution: How far is the S7-400 from the TP? Is the line close to the line of the inverter? Does the inverter communicate with the PLC?
The problem is reflected: the linear distance from PLC to TP270 is about 20 meters, the length of the cable in the communication cable is about 35 meters, the PLC cabinet is next to the DC drive cabinet, and the DC drive cabinet is next to the low voltage cabinet (the hydraulic station motor and the main Motor fan contactors, protectors, low-voltage switches, micro-breaks, etc., there are several inverters), all cables are in a cable trench, a larger part of the parallel placement; neither the inverter nor the DC drive PLC realizes network communication, all running commands and speed values ​​are output by PLC, and PLC is connected with related equipment through cable.
Solution: Is the inverter and TP communication line shielded? If the wiring and shielding of the inverter are not handled well, it may cause heavy interference.
Is it possible to temporarily put the TP next to the PLC, or temporarily pull a communication cable (do not put the cable trench). If there is no problem, it is a problem of interference.
The problem is reflected: the inverter power is not large, 5.5Kw, but the inverter does not communicate with the TP. The PLC and TP communication cable is a special cable for Siemens. The shield is connected to the data ground, but this cable is placed in parallel with a number of high-current cables, and the length is about 15 meters.
I have placed the TP next to the PLC. If there is no problem, I am going to replace a communication cable today, and I am away from the power cable. I don't know if it works. The terminal resistance of the RS485 bus plug of the PLC terminal is not connected, and the terminating resistor of the TP terminal is connected.
Solution: If only the point-to-point communication between PLC and TP, both parties should access the terminating resistor.
The problem is reflected: the method proposed last time is used, and both ends are connected to the terminating resistor, but the communication interruption is also frequently prompted, and the power cable is replaced after the overhaul time.
Summary: Through experiments, it can be confirmed that the root cause of interference is that the communication cable is placed in parallel with the cable with multiple high currents (especially the input and output cables of the inverter) in the same cable trench, and the distance is very close.
This system is relatively simple, just point-to-point communication. The simplest experimental method is to temporarily put two devices together, or temporarily pull a communication cable (do not put cable trenches). If the interference disappears, it must be the interference introduced by the cable wiring. You can use an oscilloscope to observe the waveform of the digital ground (5-pin) of the A and B lines of RS-485. If there is strong interference, you can see the waveform of the interference signal.
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