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RS485 loop back possible?

Stephen Thomas
Intellectual 710 points
Other Parts Discussed in Thread: SN65HVD1785

Hi,

I want to test an RS485 port Txd Rxd Only

I was thinking of building another board with an RS 485 transceiver ..But this transceiver should send back what ever data it receives.

so i can test the RS485 port of my DUT by connecting it to my board.

looking http://www.ti.com/lit/ds/symlink/sn65hvd1785.pdf

I see the transceiver has both driver enable and receive enable..

Is it possible to wire the DE and RE so that both are active at the same time..

Will it work?

Thanks for any help.

  • 0
    TI__Mastermind 21800 points

    Hi Stephen,

    The two most common ways to handle the DE and /RE pins are the following:

    1. Tie the two pins together and control them with a microprocessor. In this case either the receiver or the driver are enabled. But never both.
    2. Tie the /RE pin low, so the receiver is always active, and then control the DE pin to a microprocessor to control when the driver is on or off.

    Please let me know if you have any other questions.

    Thanks,

    John

  • 0 in reply to John Griffith
    Intellectual 710 points

    Hi John,

    I completely agree with what you are suggesting. But my application scenario is little different

    I've a functional RS485 port (two wire) on a board with some microprocessor. (I want to do a loop back test on this board).What ever you suggested is good for normal transmission and reception.In this board the RE and DE pins are tied together and controlled by the microprocessor.

    No hardware change on this board is acceptable. So how to perform a loopback test.?

    That why i was thinking of some external hardware to connect to this board to enable loopback test.

    Thanks for any suggestions.

  • 0 in reply to Stephen Thomas
    TI__Mastermind 21800 points

    Hi Stephen,

    If you are looking at the SN65HVD1785 device, then the driver pins and receiver pins are connected internally. This is because the device is half-duplex. Therefore, if you tie /RE low, the DE pin high, and the send data on the D pin, you will see the same stream of data come back on the R pin some propagation delay later. I think all you would need to do to test the board while not disturbing the system is to disconnect the node while testing the loopback. Unfortunately, it sounds like you have tied the pins together and cannot make any changes to the board.

    Is this a one-time test you are trying to run, or are you going to want to do this test like on production every time the board is assembled. If it’s a one-off, then you could lift one of the enable pins, let’s say the /RE pin and tie it low, then you could run the test.

    If this is a test that needs to be run over and over, the best thing I can think of is just to have a receiver test board made. You could then do point to point testing on all the boards made. The DUT (that you want loopback for) can be the transmitter and the receiver test board can be a monitor. You can compare the D signal on the DUT to the R signal on the monitor board for the test.

    Are any of these a possibility for you?

    Thanks,

    John