[GreenKeys] Teletype near Toronto

[Bill Horne]

On 04/29/2011 10:53 AM, Jim Haynes wrote:
>  On Thu, 28 Apr 2011, Bill Horne wrote:
> >
> > I guess my real question is whether the 32/33 could be directly
> > connected to a DC loop circuit without added equipment ...
> >
>  Sure. I should let the Bell company guys answer this one instead of
>  trying to answer it myself, but perhaps the reason Bell Labs wanted
>  current loop on the basic machines was just so that they could be
>  operated over existing current loop circuits. Some of those Bell
>  Labs guys were pretty old. Maybe they didn't forsee that modems
>  would come to dominate the transmission end of things.
>

Well, since I was one of the Bell guys, I'll guess at some of the 
advantages:

   1. The operating companies didn't have to install or maintain extra
      equipment, which was a _BIG_ deal for Ma Bell: after all,
      "Everything you need and nothing you don't" was her claim to fame.
   2. Pull and Plug compatibility for companies retiring older equipment.
   3. "Mark" and "Space", signaling, which dated back to telegraph
      circuits, was well-understood, already in place, and reliable.
   4. The technicians were already trained for loop signaling, all along
      the line.
   5. An RS-232 interface only works with a computer, and the design
      decisions were made decades before microprocessors were invented.
   6. Vacuum tubes required AC power, at a time when many buildings
      still ran on DC.

I'd bet there are other reasons I haven't thought of, but I _do_ 
remember that the first "103" modem I ever bought, which was the kind 
that had rubber cups for a standard telephone handset, had a 
current-loop interface built in to the same connector as the RS-232 
cable used.

Bill, W1AC
-- 

"Behind the podium there
  Debbie grabbed my derrière
  'I'd like to thank my producer,
  and Jesus Christ'"
    - Loudon Wainwright III