Using Relays on RS-422 signals

Discussion in 'General Electronics Chat' started by Ramkrishna, May 23, 2008.

  1. Ramkrishna

    Thread Starter New Member

    May 23, 2008
    4
    0
    Hello,
    Please suggest whether electromechnical or Solid state relays can be used on the differential lines (RS-422/485).
    The idea is to loop back the Tx signals to Rx line for performing BIST.

    Please provide your comments.

    Regards,
    Ramkrishna
     
  2. kancee

    New Member

    May 23, 2008
    2
    0
    But a electromechanical relay cannot get on and off as fast as the diff line.
    so u will not get the actual data(that would be bad idea)
    but if u insist try a buffer so as not to load the line.
    If all that u want is loop back the data...u could use a low drop diode..Schottky diode.:)
     
  3. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    Why do you assume the relay would be pulsing?

    I take it that Ramkrishna just wants to use a relay to loop Tx(a,b) lines to the Rx(a,b) in which case I can't see anything wrong with using a relay to do the job. In fact many V.11/RS422 modems do use a relay to perform a loop-back.
     
  4. Ramkrishna

    Thread Starter New Member

    May 23, 2008
    4
    0
    kancee,
    Thanks for your replay.
    The relay need not On and Off every time. The diff o/ps are normally connected to the external system through DPDT relay (NC lines) and whenever it is required to perform loop back, diff tx lines switched to NO lines which are connected to diff Rx lines.
     
  5. Ramkrishna

    Thread Starter New Member

    May 23, 2008
    4
    0
    Yes! exactly. But my concern is whether I can use Solid state relay insted of Reed relay for differential signals. If use solid state relays, will this have any effect on the differential signals electrical characterstics?.

    As I understand, one disadvantage of Solid state relay is, it is more prone to EMI/EMC issues as it involves electronic switching.

    Thanks.
     
  6. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    I didn't think that a solid state relay would have enough contact to do the job.
    I guess you could use a analogue switch but why complicate things. Why not use a simple relay? It's easy to use and should be very reliable in this application.
     
  7. Ramkrishna

    Thread Starter New Member

    May 23, 2008
    4
    0
    Thanks for your suggestion AlexR.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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