Using Relays on RS-422 signals

Thread Starter

Ramkrishna

Joined May 23, 2008
4
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
 

kancee

Joined May 23, 2008
2
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.:)
 

AlexR

Joined Jan 16, 2008
732
But a electromechanical relay cannot get on and off as fast as the diff line.
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.
 

Thread Starter

Ramkrishna

Joined May 23, 2008
4
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.:)
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.
 

Thread Starter

Ramkrishna

Joined May 23, 2008
4
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.
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.
 

AlexR

Joined Jan 16, 2008
732
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.
 
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