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.

Any ideas?

  • \$\begingroup\$ Why do you need RS232? \$\endgroup\$ – Jasen May 2 '20 at 6:18
  • \$\begingroup\$ RS-485 is, as you note, a differential connection. There is no multiplexing involved, as normally defined i.e. sending multiple independent signals over the same connection. RS-485 sends a single differential signal over a pair of wires. \$\endgroup\$ – Graham Nye May 2 '20 at 18:07

No, inside a typical RS485 reciever there's a resistive divider as a result disconnected lines will behave as-if they have a voltage somewhere between ground and vcc.

With one of the data wires disconnected it will work or not depending on what voltages the signal is using.


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