When Mitsubishi PLC inverter Modbus communication problems, you can follow the following steps to troubleshoot and solve:
1. Check communication parameter Settings
Inverter side setting:
Confirm the setting of frequency converter communication parameters, including communication rate (such as 19200), data length (such as 7 bits), stop bit (1 bit), and verification method (such as parity check).
Check whether Pr.117 (PU communication station number), Pr.118 (communication rate), Pr.119 (data length and stop bit), Pr.120 (verification mode), and Pr.549 (communication protocol Modbus-RTU) are correct.
PLC side Settings:
Ensure that the 485 serial port Settings of the PLC are consistent with those of the frequency converter, including the protocol format (Modbus_RTU), verification mode, and baud rate.
2. Check the communication lines
Connection mode:
Ensure that the 485 two-wire system is used for cable connection, and the cable quality is good, and there is no open or short circuit.
Check the setting of the terminal resistance, which should usually be dialed to 110Ω.
Line length and interference:
Ensure that the length of the communication line is within the allowable range to avoid signal attenuation due to excessive length.
Check whether interference sources, such as strong electromagnetic fields or high-frequency devices, exist around the circuit, and take shielding measures if necessary.
3. Check the programming and instructions
Programming logic:
Confirm PLC programming logic is correct, including Modbus master station instruction writing and register address allocation.
Check whether the function code is used correctly, such as read hold register (function code 03), write hold register (function code 06), etc.
Instruction execution status:
Monitor the communication execution status register for abnormal conditions such as communication errors or timeouts.
According to the value of the communication execution status register, the corresponding error handling measures are taken.
4. Check the hardware and rectify the fault
Hardware failure:
Check whether the communication module of PLC and inverter is working properly. If it is damaged or faulty, it should be replaced in time.
Troubleshooting:
Use communication test tools or software to conduct communication test on PLC and inverter to determine whether there is communication failure.
based on test results, troubleshoot and solve problems step by step.
5. Other precautions
Communication Protocol version:
Check whether the PLC and inverter use the same Modbus communication protocol version, such as Modbus RTU or Modbus ASCII.
Power and grounding:
Check whether the power supply of PLC and inverter is stable and whether the grounding is good to avoid interference with communication caused by power fluctuations or poor grounding.