Allen-Bradley Micro820 Fault — Communication Timeout
What This Fault Means
A communication timeout indicates the Micro820 lost contact with a configured peer device (Modbus TCP slave, EtherNet/IP target, or serial Modbus RTU device) for longer than the configured timeout period.
Common Causes
- Network cable disconnected or damaged
- Modbus slave device powered off or not responding
- IP address conflict on the network
- Serial wiring fault (RS-485 A/B swap, missing termination resistor)
- Timeout parameter too short for the network latency
Recommended Fix
- Ping the target device IP from a laptop on the same network to confirm reachability.
- For Modbus RTU, verify RS-485 wiring: A↔A, B↔B, and 120Ω termination at both ends.
- Check the Micro820's IP configuration and subnet mask in CCW.
- Increase the communication timeout in the MSG instruction configuration.
- Check for duplicate IP addresses using an ARP scan.
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