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ISOW1412: DE/RE pin behavior

Nathan Jensen
Prodigy 60 points
Part Number: ISOW1412

Tool/software:

I am working on a RS485 application with the ISOW1412. I have other communications that need to happen over the serial bus, but I am having trouble getting those to work properly when connected to the D and R outputs. I have RE/DE shorted to GND. Based on tables 10-2 and 10-3, I see that this will mean the ISOW1412 is driving R high, thus blocking any communications on that line, and that even with the EN/FLT pin pulled low this will still be the case. From this table I assume that in order to be able to have other devices use this line, RE will have to be pulled high.

However, for the line connected to D, I am also unable to drive signals on that line which I have not yet found behavior on the datasheet to explain. I tried grounding EN/FLT, which did not work (and based on the heat produced I'm guessing this is not intended operation). When I have D connected to the bus, I am unable to communicate with another serial device. I attached scope traces showing that when D is connected, the other device is unable to pull that line low, but when I disconnect D it works normally (see the amplitude differences between the two traces).

Any design insights would be much appreciated. Hopefully I am missing something obvious!

Thank you

-Nathan

  • +1
    Guru 273570 points

    D is a high-impedance input, so it should not affect other signals. But it has a clamping diode to VIO; are you powering down the isolator?

  • +1 in reply to Clemens Ladisch
    Prodigy 60 points

    I have VIO connected to +3.3 V, and VDD I have tried with both +5V and not connected. I did check the EN pin, which I left floating (internal pull-up), and it is sitting around 2.1 V, which seems a little low but from the datsheet it doesn't seem like that should have much of an effect.

    However, I tried again today and I am unable to recreate the waveform on D - it looks good now. This is a little strange as I had seen that behavior a couple of times now without it working. I will now try pulling up the RE pin to ensure that enables communication on the R net to work as well.

  • 0 in reply to Nathan Jensen
    TI__Guru 53330 points

    Hi Nathan,

    Thank you for reaching out.

    I am not sure if I understood your problem correctly, it looks like you were observing incorrect voltage levels on D pin. D is an input pin and the device doesn't produce anything here. If you were trying to connect multiple signals at the same time to D then that could cause intermediate voltages.

    EN/FLT should be pulled HIGH to enable device operation. When this pin is LOW, all input/output functions are disabled and device is not expected to operate normal.

    Based on your latest response, it looks like everything is fine. But if you still facing any issues, please provide a schematic showing ISOW1412 and all the connections to the device. Please also explain the issue observed and the test conditions so that we provide feedback. Thanks.


    Regards,
    Koteshwar Rao