[GreenKeys] Teletype (telephone) Local Loops

Mon May 25 22:23:45 EDT 2020

Paul, et al,

Anytime you're looking for an out of print book let me strongly suggest you
check out abebooks.com.  It's actually a company that lists books from
thousands of small book sellers and libraries.

The book your looking for is only $5.00 with free shipping:

https://www.abebooks.com/servlet/SearchResults?sts=t&cm_sp=SearchF-_-home-_-Results&kn=american+telephone+and++telegraph&an=&tn=A+Training+Course+Text&isbn=

They have lots of  very inexpensive books now as lots of brick and mortar
libraries are clearing out old titles at virtually no cost.  I've also in
the past gotten the internally published and sold to employees complete
History of the Bell System (a six or seven volume set, each book about 1K
pages).  I also got Winston Churchill's six volume HIstory of WWII set
first edition British printing for a reasonable price.

Marc Howard

On Mon, May 25, 2020 at 4:22 PM Jim Haynes <jhhaynes at earthlink.net> wrote:

> Yeah, we have it simple in our ham shacks, but it could be simpler yet.
>
> Telegraph test boards were pretty awful in the early days because each
> jack went to a wire on a pole that went somewhere.  If the wire broke
> or fell down the board man had to find a spare wire to replace it, and
> that spare wire might not have anywhere near the same resistance as
> the one he was patching around.  So loop currents had to be adjusted.
>
> There is also the real vs. virtual aspect of telegraph circuits.  In
> old railroad days the loop simply passed through all the stations along
> the line, so any one of them could send to all.  But with business use
> of telegraphy the lines went from telegraph/telephone office to office,
> with side branches to the customer sites.  So it became the practice to
> use repeaters in the offices to feed the customer sites, the repeaters
> creating a virtual loop through all when the actual circuit might be wire
> part way and carrier part way and didn't go anywhere near the customer's
> premises.  And you might want to use polar on the main line for signal
> quality reasons, but it takes some trickery for a polar circuit to be
> operable from any station along the route.
>
> Western Union developed a way to simplify things they called PCH for
> Polar Centralized Circuit Handling.  See Western Union Technical Review
> 8:1, January 1954, 8:3, July 1954, and 12:1, January 1958.  Incidentally
> Western Union Technical Review in my mind is about equal in importance
> to the AT&T "green book".  In a PCH office every circuit has a send leg
> and a receive leg, and things are standardized so that any pair of legs
> can be connected to any other pair without having to adjust currents.
>
> I'm less familiar with the Bell System approach to the same problem, but
> it involved a concept of "hubbing" so that circuits could be patched
> together without consideration of loop current.  Both the Bell and W.U.
> schemes included repeaters that could be used to create virtual loop-
> through multistation circuits.
>
> Then when we got transistors things were developed operating at lower
> voltages, typically plus and minus 6 volts as in MIL-STD-188 and
> EIA RS-232.  The 60ma loops where they still existed were quite
> separate from the test boards that ran the low voltage stuff.
>
> For a while I operated my station on a voltage basis rather than a
> current loop basis.  This involved vacuum tubes as selector magnet
> drivers, so I had 0 volts for mark and -40 volts for space.  An
> inconvenience of this was that I couldn't just plug in a plain TTY
> machine into the system without having a selector magnet driver and
> keyboard signal generator set aside for the purpose.
>
> Still later, when we got good high voltage transistors, I started on
> a similar system using +/- 6 volts.  But I am just too lazy to have
> fully carried it out, so I still had a mixture of current loops and
> voltage signals.  And every manufactured TU that came along was
> built for current loop and had to have an adapter or be reworked to
> fit into the voltage scheme.  But it sure simplifies switching - no
> need to break into a loop to add a printer, nor to keep every printer
> in a loop so it doesn't run open, nor to allow two different senders
> to send to a hub provided they don't both do it at the same time.
>
> Jim W6JVE
>
>         ---
>
>         "Ya can argue all ya wanna, but it's dif'rent than it was."
>         "No it ain't! No it ain't!  But ya gotta know the territory."
>                 Meredith Willson, The Music Man
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