Troubleshooting Guides · Free Tool
Modbus Communication Fault Finder
Choose the Modbus symptom and get likely causes in settings, RS485 wiring, register mapping and network configuration.
No responseCRC errorsWrong valuesTimeouts
SymptomNo response
Most likely causeAddress, baud or parity mismatch
First checkCompare settings
Causes in order of likelihood
| Cause | How to check | Fix |
|---|---|---|
| Address, baud or parity mismatch | Compare settings | Match settings |
| A and B wires swapped | Swap and retest | Correct polarity |
| Device not powered or wrong port | Check power and port | Correct |
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Before You Start
- Note the exact error: timeout, CRC, exception code.
- Confirm slave address, baud rate, parity and stop bits on both sides.
- Test with one device and a laptop master if possible.
Modbus Communication Fault Finder: All Symptoms
No response
| Likely cause | How to check | Fix |
|---|---|---|
| Address, baud or parity mismatch | Compare settings | Match settings |
| A and B wires swapped | Swap and retest | Correct polarity |
| Device not powered or wrong port | Check power and port | Correct |
CRC or framing errors
| Likely cause | How to check | Fix |
|---|---|---|
| Missing or extra termination | Check 120 Ω at both ends only | Correct termination |
| Star wiring or long stubs | Check topology | Daisy chain |
| Noise from power cables | Check routing and shield | Segregate; earth shield at one point |
Wrong values
| Likely cause | How to check | Fix |
|---|---|---|
| Register offset of one | Try address −1 or +1 | Correct the map |
| Word order of 32 bit values | Swap registers | Set word order |
| Scaling or data type | Check device map | Apply correct type and scale |
Exception code returned
| Likely cause | How to check | Fix |
|---|---|---|
| 01 illegal function | Check supported function codes | Use correct code |
| 02 illegal data address | Check register range | Correct address |
| 03 illegal data value | Check value or quantity limits | Correct request |
Modbus TCP cannot connect
| Likely cause | How to check | Fix |
|---|---|---|
| IP or subnet wrong | Ping the device | Correct IP settings |
| Port 502 blocked | Check firewall | Open port |
| Wrong unit ID through gateway | Check gateway mapping | Set unit ID |
Key insight: if you get an exception code the wiring works: the problem is the request. If you get nothing, look at settings and wiring.
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Frequently Asked Questions
Why is 40001 address 0?
Documentation uses 1 based numbers; the protocol is 0 based.
How do I fix CRC errors?
Check termination, topology, grounding and baud settings.
What does exception 02 mean?
The requested register does not exist in the device.
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