RS-422 chip questions

Discussion in 'General Electronics Chat' started by ghall426, Oct 28, 2012.

  1. ghall426

    Thread Starter Member

    Jul 22, 2009
    11
    0
    I am building a circuit using the AM26C32 quad receiver, however I am only using 3 of the 4 lines. My questions are:

    1. Do I need to do anything with the A & B differential inputs that I'm not using to keep them from floating to stop any potential oscillations?

    2. Do the driver and receiver need to share a common ground, or are the differential pairs all that is needed between the 2 chips?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    AM26C32 inputs are already pulled up or down. You can leave them not connected.

    RS-422 and RS-485 systems would normally be grounded via chassis or earth ground.
    No, you do not need an extra ground connection if both ends have good ground connections.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    It's a much more robust design practice to include a ground connection between the transmitter and receiver.

    *most* of the time you can get away with using the system ground, but many nasty intermittent RS-485 problems can be traced to the lack of a good ground.

    Below is a great reference for system design.

    http://www.canarysystems.com/nsupport/ArtScienceRS485.pdf
     
  4. ghall426

    Thread Starter Member

    Jul 22, 2009
    11
    0
    Thanks for the replies. One more question:

    I've seen on other RS-422 receiver chips that a 100-ohm resistor is required between the differential inputs. The AM26C32 datasheet doesn't mention this, so is it still required?

    If so, why does it have to be an external resistor? Why can't they take care of this internally?
     
  5. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    The 100 ohm resistor is a line terminator to match the impedance of the cable, preventing signal reflections.

    It should be the same resistance as the cable impedance.

    If you have a multi-drop network, the mid-cable nodes typically do not have a terminating resistor.
     
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