MODBUS FAULT

What does SLF mean?

This fault indicates an interruption in communication on the Modbus bus or a disconnected remote display terminal when [HMI command] (LCC) is enabled. If communication is not restored, remote control and monitoring capabilities will be lost, potentially leading to operational delays or safety issues if critical information cannot be exchanged with the control system.

• Interruption or disconnection of the Modbus communication cable (RS-485 A/B lines) between the drive and the master device (e.g., PLC, HMI).
• Incorrect Modbus communication parameters configured in the drive (e.g., P7-01 (Modbus address), P7-02 (Baud rate), P7-03 (Parity) not matching master settings).
• Incorrect termination resistors (120 Ohm) missing or improperly configured at both ends of the Modbus segment.
• Electrical noise or interference on the Modbus communication lines due to unshielded cables or improper routing near power cables.
• Faulty Modbus port on the drive or the master communication module.

Click steps to track your progress.

1. 1

   1. Inspect the Modbus communication cable (RS-485, typically green/red wires or A/B) for physical damage, loose connections at terminal blocks (e.g., RJ45 connector, screw terminals).

2. 2

   2. Verify drive parameters P7-01 (Modbus address), P7-02 (Baud rate), P7-03 (Parity) match the settings of the Modbus master device (PLC or HMI).

3. 3

   3. Check the presence and correct placement of 120 Ohm termination resistors at the drive's Modbus terminal block (e.g., activated via DIP switch or wired externally) at both ends of the segment.

4. 4

   4. Use a Modbus sniffer tool or oscilloscope to monitor activity on the RS-485 bus and diagnose communication frames and signal integrity.

5. 5

   5. Ensure Modbus cable is shielded twisted pair (e.g., Belden 3105A or similar) and routed away from high-voltage power cables to minimize electrical noise.

6. 6

   6. If using an external HMI, verify that the HMI command source parameter (e.g., LCC, P1-02) is correctly set to Modbus and not a local HMI or terminal input.