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RS422 / RS485 loopback self test

Thierry Venet
Prodigy 40 points
Other Parts Discussed in Thread: MPC509

Hello everybody,

On my board we need to have RS422 interface (microcontroller with UART + RS422 transceiver) that can do selftest. So the microcontroleur have to control a switch behind the RS422 transceiver to do the loopback on demand (switch the TX pair to the RX pair)

I wonder if I can do this with analog switch ? Maybe the common mode voltage behind the transceiver will be a problem ?

Best regards,

Thierry

  • 0
    Guru 317480 points

    It would be possible to switch the TX and RX on the logic side.

    But if you need to test the RS-422 side, then you indeed need a switch that can handle all voltages that might actually appear on the bus. This typically requires a power supply covering that range; I am not aware of a suitable analog switch that can handle voltages beyond the power supply rails (the MPC509's resistance is too high), but try asking on the switches forum. It might be possible to use a (solid-state or mechanical) relay.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points

    Hello Clemens,

    Thanks for the answer. Sure It may be possible to do this on the RS422 side with a mechanical relay, but now we try to avoid this kind of component on embedded board. With solid state relay, maybe but I'm not sure.

    I think you're right, the need for a high voltage power supply is a problem, it may be a better choice to test it on the logic side.

    Best regards,

    Thierry