What causes fault code VFD-COMM1 (VFD Communication Loss — Modbus RTU)?

RS-485 cable disconnected or damaged — check terminal block connections. Baud rate mismatch between VFD and PLC/HMI — both must match exactly. Modbus address conflict — two devices with same slave address on bus. Missing or incorrect termination resistor on RS-485 bus — 120 ohm at each end. PLC communication module faulted or port disabled. RS-485 cable run too long without repeater — max 1200m/4000ft for RS-485

How do I fix fault code VFD-COMM1?

Verify RS-485 wiring: A(+) to A(+), B(-) to B(-), shield grounded at one end only. Confirm baud rate, parity, stop bits, and slave address match on both ends. Check for termination resistors — 120 ohm between A and B at first and last device on bus. Use PLC Modbus diagnostic counters to check for CRC errors, timeouts, and retries. If multiple devices on bus, disconnect all except VFD and test — add back one at a time. Check comm loss action parameter on VFD — set to fault, coast, or hold last speed based on application safety

VFD Communication Loss — Modbus RTU

VFD-COMM1

VFD lost serial communication with the PLC or HMI via Modbus RTU (RS-485). Drive entered fault or last-speed-hold state.

high vfd plc

vfd communication modbus rs-485 serial

Probable Causes (ranked)

RS-485 cable disconnected or damaged — check terminal block connections
Baud rate mismatch between VFD and PLC/HMI — both must match exactly
Modbus address conflict — two devices with same slave address on bus
Missing or incorrect termination resistor on RS-485 bus — 120 ohm at each end
PLC communication module faulted or port disabled
RS-485 cable run too long without repeater — max 1200m/4000ft for RS-485

Fix Steps

1 Verify RS-485 wiring: A(+) to A(+), B(-) to B(-), shield grounded at one end only
2 Confirm baud rate, parity, stop bits, and slave address match on both ends
3 Check for termination resistors — 120 ohm between A and B at first and last device on bus
4 Use PLC Modbus diagnostic counters to check for CRC errors, timeouts, and retries
5 If multiple devices on bus, disconnect all except VFD and test — add back one at a time
6 Check comm loss action parameter on VFD — set to fault, coast, or hold last speed based on application safety

Field Trick

RS-485 polarity is backwards on half the devices out there. If you can't get comms, swap A and B wires at one end. There's no standard — what Rockwell calls A(+), Siemens calls B(+). Just try both.

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