Can't get modbus working on new LinuxCNC box

Finally made a breakout cable to find... the Modbus is returning perfectly valid packets. The code composes packets out and gets a response but the code's just timing out.

I don't see how to add that. My x200_vfd.comp user code doesn't pass a timeout to libmodbus, that's internal protocol stuff. Libmodbus says 0.5s, and if that's actually the time used then there's no way a response would be later than that.

I don't know how to modify and recompile libmodbus. I'd like to in order to figure out why the responses aren't counting. I can't think of a likely prob that would be specific to this setup.

Well I have mixed logic there.

We know we can send requests to libmodbus, it sends them out. Then libmodbus calls an error-printing task, but only passing a text string. All the error handling code is encapsulated in libmodbus.

So, the x200_vfd compile seems to "work". AFAIK if the Modbus packet was composed incorrectly, the VFD would not respond. As a slave, it only responds to complete commands for its specific address. I do see a response coming back. So the libmodbus code was called correctly from x200_vfd. Then it fails to get the response inside libmodbus, but nothing about the x200_vfd code should interfere with it. It makes no calls.

I'm thinking that perhaps libmodbus doesn't like this specific PC hardware. I did try the USB-RS232 cable but I now think that one probably doesn't produce valid RS232 levels anyways. But, Putty shows that Debian and the physical hardware can talk.

One thought that came to mind is libmodbus might be measuring time incorrectly. If it was somehow something invalid, it might register as timed out instantly. If so this wouldn't affect Putty, it'd be a general hardware-specific code glitch.

Sorry to labor this point. I need to get this installed and this is the roadblock I can't get around