No Communication?
- Check RS232 wiring against pinout
- Verify baud rate, parity, stop bits match PLC
- Confirm correct COM port and driver selected
- Verify station number / node address
- Test with known good cable
Intermittent or Unstable Communication?
- Inspect cable shielding and grounding
- Check for electrical noise sources
- Verify proper termination (if applicable)
- Confirm stable power to PLC
Incorrect Data or Values?
- Verify correct prefix (X, Y, I, M, T, C, L, D, B, V, Z, W)
- Confirm correct address range
- Check bit vs word access type
- Ensure correct scaling or data interpretation
Timer or Counter Issues?
- Use correct prefixes (TS, TP, TI, CS, CP, CI)
- Confirm read-only vs read/write behavior
- Verify PLC logic is updating the values
Read Works but Write Fails?
- Confirm register is writable
- Verify PLC is in correct mode (run/monitor)
- Check value format (integer vs other type)
- Ensure address is valid for writing
No Response from PLC?
- Confirm PLC is powered and running
- Verify communication module installed and configured
- Check correct physical port is used
- Try alternate port or cable
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