Modbus Fault

What does SLF mean?

This fault signals an interruption in communication on the Modbus bus, preventing the drive from exchanging data with a Modbus master or remote display terminal. Causes often include physical bus issues or a disconnected remote display terminal when HMI command is enabled. This results in a loss of external control, monitoring, or parameter access.

• Broken, disconnected, or improperly wired RS-485 Modbus communication cable (e.g., A/B lines reversed or open).
• Incorrect Modbus slave address (Parameter P5-01) or baud rate (Parameter P5-02) configured in the drive or Modbus master.
• Missing or incorrectly placed 120 Ohm termination resistors at the physical ends of the Modbus bus segment.
• Excessive electrical noise or electromagnetic interference (EMI) on the communication lines, corrupting data packets.
• Modbus master controller experiencing a communication timeout or not actively polling the drive.

Click steps to track your progress.

1. 1

   1. Inspect the RS-485 Modbus communication cable for physical damage, loose connections, or incorrect wiring (e.g., A to A, B to B) at both the drive and master ends.

2. 2

   2. Verify that the drive's Modbus slave address (Parameter P5-01) and baud rate (Parameter P5-02) match the configuration settings in the Modbus master controller.

3. 3

   3. Check for the correct installation of 120 Ohm termination resistors at the two furthest physical ends of the Modbus bus segment.

4. 4

   4. Measure the voltage differential between the Modbus A and B lines (should fluctuate between 0-5VDC during communication) using an oscilloscope or specialized Modbus tester.

5. 5

   5. Ensure that no other device on the Modbus network is using the same slave address as the drive, causing address conflicts.