I have a RS-485 device that returns serial ASCII constantly. As the device might near very high voltage or electromagnetic areas, I need to ensure the RS-485 to RS-232 adapters I've been supplied with are MUXing the A and B line correctly, or cheating by using the GND to A signal only. This may sound odd, but my test results show my problem.
I checked the device by logic analyzer, and GND to A is the usual serial data, and GND to B is the inverted version : All good.
I plugged in my adapter in the following ways:
(a) GND to GND, A to T/R+ and B to T/R- : Data is good. (b) GND to GND, A to T/R+ and B is removed : Data is good. (c) GND to GND, B to T/R- and A is removed : No data. (d) GND to GND, B to T/R+ and A is removed : No data. (e) GND to GND, A to T/R- and B is removed : No data.
If the adapter was MUXing properly, wouldn't it be watching the differential between A and B, so returning no data if: Either A or B was unplugged, Or A and inverse B didn't match?
If this is the case I would imagine (b) shouldn't return any data, but it does.