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Original question:

SN65C1167: Asking for the SN65C1167 driving capability support

SN65C1167: Extending the SPI bus for long-distance communication Article

Ian Templeton
Prodigy 40 points
Part Number: SN65C1167
Other Parts Discussed in Thread: SN65C1168

In this article Extending the SPI bus for long-distance communication, a digital isolator is used to provide a stable ground reference. But other than the short paragraph there isn't more detail on this. Can you please provide more information regarding the purpose and when to use the digital isolator. It appears a similar question was posted to the forum (SN65C1168: RS-485 compatibility - Interface forum - Interface - TI E2E support forums). But there was no mention of a digital isolator for the application where the bus length was 18 inches. I am expecting a SPI bus cable length of about 10 meters in a noisy environment, any further detail would be greatly appreciated!

  • 0
    Prodigy 40 points

    To clarify, I am asking this question with respect to SN65C1168, this is the IC I am planning to incorporate in my design.

  • 0
    Guru 316920 points

    You need an isolator if the (possible) ground offset is larger than the receivers can handle, i.e., if the signal goes outside the supported common-mode range of ±7 V, relative to GND at the other end.

  • +1
    TI__Guru 52262 points

    Hi Ian,

    The point of the isolator is help mitigate large Ground Potential Differences (GPD) found in industrial systems where the GPD can exceed the +/-7V allowance dictated by RS-422 (it is -7V to 12V in RS-485 - the article states +/-7V for both which is incorrect - but the sentiment remains). They help break up ground loops especially around equipment that could be noisy. You should you use an isolator when the ground potential differences could exceed the rating on the part as this is the most secure way to do so. It aims to prevent damage due to conditions that are typically found in industrial systems.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson.