Fault Finding Guides · Free PDF
Modbus and HART Communication Fault Finding Guide
A one page guide to Modbus RTU, Modbus TCP and HART communication problems.
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A one page A4 guide to print for the workshop or keep on your phone in the field. The full content is also shown below.
Download PDF A4 · 53 KBBefore you start
- Permit, isolation and control room informed.
- Put the loop in manual or inhibit trips as authorised.
- Use certified test equipment in hazardous areas.
- Record as found readings before changing anything.
Quick checks
- Settings match on both sides?
- Termination and bias correct?
- Exception code returned?
- Use a frame decoder to check bytes.
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Symptoms, causes and fixes
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| No response | Wrong address, baud, parity; A/B swapped; device off | Check settings and polarity; one device at a time |
| CRC errors | Noise, missing termination, stubs, wrong baud | Terminate both ends; daisy chain; check baud |
| Wrong values | Register offset of one, word order, scaling | Try address −1; swap words; check scaling |
| Intermittent timeouts | Poll too fast, long cable, weak bias | Increase timeout; check biasing |
| Modbus TCP no connect | IP, subnet, port 502 blocked, unit ID | Ping; check firewall; unit ID |
| HART no communication | Loop resistance below 250 Ω, wrong points, address | Add resistance; connect across load; poll address |
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Exception codes
| Code | Meaning |
|---|---|
| 01 | Illegal function |
| 02 | Illegal data address |
| 03 | Illegal data value |
| 04 | Server device failure |
Related Tools, Procedures and Calculators
Frequently Asked Questions
Why does Modbus return exception 02?
The register address is not valid in the device; check the map and the one offset.
Why are my floats wrong?
Word order differs between devices; try swapping registers.
Why can my HART communicator not connect?
Usually too little loop resistance.
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