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Fault diagnosis

Answer a few questions to narrow down the cause. The steps follow the order technicians actually use: rule things out from the outside in, from cheap to expensive.

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What to do

Most likely a configuration or wiring mistake

  1. Check the port mode in the BIOS: RS-232, RS-422 or RS-485 — a port runs only one mode at a time
  2. Try SWAPPING the RS-485 A/B wires — the most common mistake, and it damages nothing
  3. Match baud rate, parity and stop bits against the device
  4. Check whether the Modbus address clashes with another device on the same line
  5. Long runs need 120Ω termination resistors at BOTH ends of the trunk

Need someone to take a direct look?

Describe the symptoms and what you have tried — an engineer will call back to help.

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Symptom: Most likely a configuration or wiring mistake

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Before opening the machine. Disconnect power, wait for capacitors to discharge, and photograph the current wiring. If the machine is under warranty, call us first — opening it yourself may void the warranty.