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COMMUNICATION • FX5U / GX WORKS3

Why is my FX5U RS-485 device not communicating?

Check the selected serial channel, polarity, common/reference connection, station number and the complete serial format before changing the PLC program.

Practical explanation

A and B terminal naming is not consistent across every manufacturer, so confirm both manuals rather than relying on the letters alone.

Communication settings are configured for the actual FX5 port/module in GX Works3.

Check in this order

  1. 1Confirm CH1/CH2/CH3/CH4 hardware assignment.
  2. 2Compare baud, data bits, parity and stop bits.
  3. 3Verify slave address and protocol.
  4. 4Check termination and cable topology.

Last reviewed: 2 October 2026

Official source: MELSEC iQ-F FX5 User's Manual (Serial Communication)

Related verified answers

Which channel numbers are used for FX5U serial communication?How many Modbus slaves can an FX5U master connect?

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