Red Lion ERR-301: Red Lion G3 Series Error Code ERR-301

Symptoms

All Modbus RTU tags show 'Bad Quality' and the red communication bar is visible.

RS-485 A/B lines reversed at the terminal block, causing signal polarity inversion.
Modbus slave ID mismatch between HMI register configuration and PLC/RTU setting.
Termination and biasing resistors missing, causing signal reflections on long cable runs.

Step 1: Swap the A and B wires at one end and observe if communication recovers.
Tags return to 'Good Quality' within 5 seconds after polarity correction.
Step 2: Poll the slave with a USB-to-RS-485 adapter and Modbus scanner tool at the HMI location.
Slave responds correctly with the expected slave ID and register map.
Step 3: Install 120 Ohm termination resistors at both ends of the RS-485 bus and verify bias.
Signal edges are clean on oscilloscope and CRC error rate drops to 0%.

Correct A/B polarity, match Modbus slave IDs, and install proper termination and biasing.