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ISO7231C: Input protection

Ian Benton
Prodigy 170 points
Part Number: ISO7231C

Tool/software:

I need to connect the ISO7231 to a rather dreadful interface. It consists of RXD and TXD from microprocessor pins via 1k resistors and 0V and 5V supplies. The data rate is 19200 baud. The interface is on a piece of equipment containing a buck regulator outputting 48V @ 100A, so there is plenty of noise superimposed on all four wires.

(I would have much preferred RS485 by twisted pair)

How should I best protect the ISO7231 which is at the far end of a 10m 4-core cable which plugs into the interface? My first attempt was a Thévenin terminator of 4.7k to 5V and 1.8k to ground, but I am wondering if that will affect the noise margin, and a 100nF capacitor between 5V and 0V.

I then added a ferrite sleeve and a 100nF capacitor between the far-end-of-the-cable ground and my ground at the other side of the isolator, which seemed to tidy up the input waveform at the isolator.

  • 0
    TI__Genius 17705 points

    Hello,

    Thank you for your question.

    Please give me an additional 24 hours to respond.

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    TI__Mastermind 29375 points

    Hello Ian, 

    Thank you for reaching out and thank you for your patience. 

    As you have noticed, a differential protocol with a twisted pair cable is the recommended solution to noisy systems above, as you have already mentioned. ISO7231 is not recommended for off-board communications like this and, as a result, may not have the drive strength to communicate across a 10m cable in a noisy environment.

    The approach you mentioned above (termination + shielding) is probably the most effective solution. However, please note that it is not recommended to have any electrical connection that crosses the isolation barrier.

    Best,
    Andrew