I used libmodbus for an older LinuxCNC 2.7 on preempt-rt to talk to an X200 VFD through an Adam isolated 232-485 converter. That worked.
Changing to a fanless NUC with LinuxCNC 2.8 on Debian. It also has a built-in RS232 serial DB9. Just one slave device, ID#1. But LinuxCNC can't see the VFD. If I launch LinuxCNC from a terminal, the text output says Modbus Connection Timed Out.
I scoped these. libmodbus is able to send out, correct baud, and get a response from the X200 that passes back through the 485-232 bridge and I can see on the Rx-in on the DB9. But libmodbus doesn't see the response and times out.