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Original question:

ISOW1432: RS485 to TTL conversion issue

ISOW1412: Communication Issue

Larisa Dummer
Prodigy 20 points
Part Number: ISOW1412

Tool/software:

Hello, 

I am using the ISOW1412 in half duplex and RS485 mode. I added the link to a post below because I have followed the circuit design guidelines from the datasheet and that post. 

The problem I am facing is that I send a message through the A/Y and B/Z pins and that message does not get through to the D/R/DE//RE. I have probed the signals with an oscilloscope and on the A/Y and B/Z it clearly shows the message being delivered to the ISOW1412. On the D/R/DE//RE side, no message appears when I probe any of those pins. I have ensured that the DE//RE is always listening, and not being held high. 

Also, the transceiver properly generates 3.3V on the isolated side.

Do you have any suggestions for how to troubleshoot this issue? Thank you!

e2e.ti.com/.../isow1432-rs485-to-ttl-conversion-issue

  • 0
    TI__Guru 54195 points

    Hi Larisa,

    Thanks for reaching out.

    I am assuming that you have tied DE and RE\ together to control as a single pin, at least this is done in the related E2E post that you are referring to. When DE and RE\ are tied together and controlled as a single pin, the driver and receiver can only be enabled one at a time and not both at the same time. This means that, the device is either transmitting or receiving and not both.

    Let's call this combined pin as DIR. When DIR = HIGH, driver is enabled and data is transmitted from D to Bus. When DIR = LOW, receiver reads data from Bus to R. Since the driver is disabled at this time, it can only read the data on the Bus that is coming from other nodes and not the one from D of its own.

    This implementation is a common practice in the industry where a product is only transmitting or reading the bus. Disabling the driver during idle and receiving times saves a lot of power.

    If you would like to read what is being transmitted, then I recommend that you make DE = HIGH and RE\ = LOW and not tie them together. In half-duplex, you have disable the driver if you want to read data from other nodes on the Bus.

    Let me know if this is clear and if you have any further questions, thanks.


    Regards,
    Koteshwar Rao