[GreenKeys] Teletype near Toronto

[Jim Haynes]

On Thu, 28 Apr 2011, Bill Horne wrote:
> I'm curious: did the private line model use electronic line current
> sensors, or where there polar relays in them? I trained on Model 32/33
> in 1976, but the class just barely covered the electronics: I remember
> there was a transistorized driver for the selector magnet, which took
> several hundred milliamps, but that's all they taught us.
>
> I know they could be strapped for 20 ma current loop, and probably for
> 60 ma (they say the memory is the second thing to go), but I never
> worked on a machine that was actually in use for private line: ours had
> 20 ma <> RS-232 converters on them, and they were attached to DEC computers.
>
To my knowledge all the 32/33/35 machines used 500ma selector magnets
and electronic drivers.  Why they used 20 or 60 ma current loop rather
than a voltage like RS-232 is unknown to me, but must be a misfeature
forced on them by Bell Labs.  Conversion between current loop and RS-232
required a separate box.

Now the one machine known to me that used polar relays was the Western
Union Telex model 32.  W.U. had several user interfaces based on real
wire connections, no modems, between the machine and the switching office.
This used mercury-wetted-contact relays when used on polar lines.
See Western Union Technical Review 17:4, October 1963, p. 134.

Another factor in all this is that RS-232 didn't come out until 1963 or 
so, and the 32/33/35 line were in full production already at that time. 
Those awful 101 Data Sets (modems) used in Dial TWX were low voltage 
current loop.  Nearly all the later modems, the 103s, the 202s, and so on 
used RS-232 or a forerunner of it.