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RS485 - RS232 in one application - one circuit over two wire Connection - Software configurable

Walter Kotz81
Prodigy 10 points
Other Parts Discussed in Thread: TRS3221

How to combine RS232 and RS485 communication through to wires in one circuit design? It should be software configurable.

I've to delvelop a device which should communicate over a two wire connection either with a RS232 or with a RS485 protocol. I've following questions:

I've seen the "RS-485 / RS-232 Multiprotocol Transceiver Reference Design". Is it possible to use this solution for a two wire Connection?

If not, I want to ask following questions:

1) How can I design a circuit which is able to switch 120 Ohm Termination resistor in a transmission line?

2) Which RS232 transmitters are able to go in high impedance state if they are shuted down?

3) Which RS485 transmitters are able to go in high impedance state if they are shuted down?

3b) Which RS485 transimitters are able to switch a termination resister internal, too?

4) Are there other better aproaches?

Best regards

Walter Kotz
Austria

  • 0
    TI__Guru**** 155896 points

    Austria,

    I don't recommend software switching the 120 ohm on board. The load is only used at the ends of the cable and the software doesn't know if the connection is at an end or not. The person who runs the cable should add 120 ohm externally at the ends. If the software control device at the end of the cable is un-powered then the RS485 network goes down which is a terrible design. That's my opinion others may see it differently.

    Any RS232 device with a FORCEOFF or SD pin can disable the drivers.

    All RS485 devices should have a driver disable

    Not sure if we have RS485 with switchable termination.

    Here is a two line design. Note the TRS3221 caps should be changed for 5V. I also think the CPU should be 5V.
    RS232 & RS485 single channel.pdf