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The definition of a spec of RS485 interface

John11513
Expert 5480 points
Other Parts Discussed in Thread: SN65HVD72

Hi Team,

I noted all RS485 interface device has a spec, 'delta VOD, change in magnitude of driver differential-output voltage', I am not very clear about the definition of this spec. Could anyone can help me clarify? How it is defined? what effect does it have?

Thank you,

John

  • 0
    TI__Expert 5400 points

    John,

    Good question!

    In the RS-485 standard, you have the following test circuit:

    With a logic high output, you have a positive differential output voltage, VOD.  With a logic low output, you have a negative differential output voltage, VOD*.

    The specification you mention, Δ|VOD|, is defined as | |VOD| - |VOD*| |.  The RS-485 standard specifies that this value must be less than 200mV.

    For the SN65HVD72, for example, Δ|VOD| is specified to be within +/- 50mV, so it easily meets the standard.

    For an overview of this and other RS-485 specifications, I recommend this application note.

  • 0 in reply to Bart Stiller
    TI__Expert 5480 points

    Hi Bart,

    So quickly reply! Thank you for answer. It is very helpful.

    Another question, what effect will it have if the delta VOD is high. Or in other words, what advantages customer will benefit when using SN65HVD72 which has much lower delta VOD?

    Addtionally as you mentioned the delta VOD is specified must be less than 200mV (this is also the specification of most of RS485 devices), however, it is said (in the application note you mentioned) the specification requires delta VOD should less than 0.4V (page 5), which one is correct?

    Thank you,

    John

  • 0 in reply to John11513
    TI__Mastermind 28265 points

    Hi John,

    Bart has recommended an excellent application note. Figure 5, you have mentioned above, is valid for RS-422 circuits only. the appropriate figure for RS-485 is Figure 12.

    Basically an RS-485 driver is a H-Bridge driver. Depending on the logic state at input D, on or the other transistor cross-pair is active. When both branches in the H-bridge have equal resistance, the design is very balanced and DVod is small. If the h-bridge has large differences in resistance, the bridge is imbalanced and Vod changes with the input logic state. This can lead to noise radiating from your RS-485 link into adjacent circuitry.

    Hope this helps.

    Regards, Thomas