: Long daisy-chain runs require exactly two
| Step | Action | What to look for | |------|--------|------------------| | 1 | Check LED indicators on slave | Power LED? Comm LED blinking? | | 2 | Swap cables with a known good device | Does another slave work on same line? | | 3 | Use a loopback test | Short TX+ to RX+ on master – do you receive your own echo? | | 4 | Monitor with a serial sniffer (e.g., Wireshark + COM proxy, or hardware tap) | See if master transmits, and if slave responds at all. | | 5 | Increase timeout temporarily to 5-10 seconds | Does it work now? → Slave is slow or network is delayed. | | 6 | Test slave independently with a simple tool (e.g., Modbus Poll, QModMaster, or mbpoll CLI) | Eliminates master software as cause. | modbus poll timeout error
When a Modbus poll timeout error occurs, you may notice the following symptoms: : Long daisy-chain runs require exactly two |