[GreenKeys] Modem with 20mA current loop interface

[Ralph Mowery]

There are two main reasons for the higher voltages.  Also a reason for a 60 ma loop current.

In simple terms the higher voltage is for the selector magnets that act directly from the loop.  It makes them pull in faster.  It has to do with the ammount of inductance in the circuit.  The more inductance, the slower the current can rise and pull in the magnets.  This distorts the relationship of the pulses.  Also you want to have the coils in parallel and use the 60 ma current for the same reason.

For the old mechanical switch contacts (keyboard) the higher voltge tends to burn off the oil and you get a beter connection.

The newer teletypes have their own internal loop and it is just activated by the external signal.  As there is not much inductance in the external loop as a general rule the rise time of the current is not that important.  

For just hooking a couple of machines together there is not much of a problem, but when going over the radio there are other things that can add some distortion.  Every little bit helps.

  ----- Original Message ----- 
  From: drlegendre . 
  To: Jim Haynes 
  Cc: Green Keys 
  Sent: Monday, December 15, 2014 8:31 PM
  Subject: Re: [GreenKeys] Modem with 20mA current loop interface


  Ah OK, so it's down to +two+ criteria, then? 


  1) SS vs. simple EM selector relays
  2) Length of cable run


  So then..


  The devices with SS inputs don't have the issue of inductive rise-time in the relay coils, so they don't need any kind of "high" voltage - in this case, 120V - to drive them (closed / on) quickly. The only reason they might need higher voltages (say, 24-48V) would be due to the length of the loop. 


  Conversely, the devices with simple EM relays need the higher voltages just to overcome the inductive rise vs. time - to say nothing of what might be required to cope with the length of loop run, which is a separate matter. 


  Am I getting this right? Please let me know -  Like many others, I'm quite interested to learn the gritty work-a-day details of current-loop signaling. 



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