Andy, is it possible that another device is occupying the RS485 bus or has forgotten to release it after using it? I've seen the exact condition you describe when another unit forgot to release the RS485 line. I've also seen it happen when the 232/485 convertor had died.

My suggestion:

1. Try the test with the same two devices right next to each other with RS232, if possible.

2. If #1 works, try the test with the same two devices right next to each other with RS485.

3. If #2 works, add any additional units to the RS485 bus until it fails. If it fails when you add a new unit to the bus, the culprit is probably the last unit *or* the next-to-last unit that you added to the bus.

You might want to verify that, at the electrical level, all 232/485 converters are releasing the line. I was driven crazy once when everything seemed to check out but the 485 network failed to work with multiple units together. Going through steps 1-3 above lead me to the conclusion that it was a hardware problem in one of the converters--which it was.

Craig Steiner