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SN65HVD72: The definition about Vb-Va in RS485 communication and AISG 3.0

Marvin Feng
Intellectual 1340 points
Part Number: SN65HVD72

Hi Team,

In SN65HVD72 datasheet, it defines Vod=Va-Vb. see the below.

However, for AISG3.0 standard, it defines Vod=Vb-Va. see the below.

https://aisg.org.uk/files/AISG-Base-standard-AISG-v3-0-2-1.pdf

If the definition of Vod is reversed, the connection will be reversed.

Can you clarify which definition is correct? Or both are correct?

For this kind of reversed definition, we need to reverse the connection, correct?

Thanks!

BR

Marvin

  • 0
    TI__Guru 62920 points

    Hi Marvin,

    The naming convention for RS-485 can be kind of confusing.  The RS-485 standard defines A and B lines so that the bus is considered in a “binary 0/ON” condition when VA is greater than VB and in a “binary 1/OFF” condition when VB is greater than VA.

    However, pretty much all RS-485 transceivers for the last 30+ years have followed a seemingly opposite convention in their pin naming: a logic high signal applied to the transmitter input results in a positive differential voltage from A to B (i.e., VA > VB) and a logic low signal applied to the transmitter input results in a negative differential voltage from A to B (i.e., VB > VA).  (The receiver logic works in the same way.)

    For this reason, most applications use the “A” pin of the transceiver (i.e., the non-inverting bus input/output) to represent the “B” line that is described by TIA/EIA-485 and vice-versa.

    So, I believe the differences you are seeing between these two documents are just a difference in naming and you should not need to reverse the connections.

    Regards,
    Max