[GreenKeys] Teletype (telephone) Local Loops

[ad7i]

Sorry for the long-winded backgrounding before posing my question…



I was fortunate to start my career with "the phone company" when I was 16
in a small central office at an independent telephone company in Washington
State.  A few years later, when I was in college, I was able to move on to
a larger company for a summer job where I worked the toll test board (this
was for General Telephone around 1977, which was later absorbed into
Verizon and so on).  Our primary test equipment for our voice circuits
(which was anything that didn't connect to the local CO Step-by-Step
switching equipment on the floor below us) was a level meter (AC RMS
voltmeter calibrated in dBm), tone generator and a high impedance amplified
speaker.  Most voice circuits sounded to the ear as either good or had
terrible distortion and/or noise.  Our repair MO was to determine if the
problem was in our own office and then swap gear until the problem was
resolved, *or* point to the office to the East or West and suggest it was
their problem or with the cable between us.



One time I pick up a trouble ticket for a data circuit from the in-basket.  I
then found the circuit card in the file cabinet and made my way to the jack
field to monitor the circuit with the speaker to see what the tones sounded
like.  But before I couldget there the senior guy told me not to bother
with a speaker as that circuit was a "WU DC teletype" circuit and other
than resistance measurements of the loop (which would require coordination
with a technician at the customer site and/or the WU Central Office) there
wasn't much that we could do.  If I remember right he told me to call WU
and let them take the lead on solving the problem.  If WU needed our help
because of a bad local loop or cross connects in our office, wait for them
to call us back and ask for our assistance (I don't recall why we got the
initial trouble ticket instead of WU, but that wasn't uncommon because even
when different interconnect companies were involved in a circuit or service
General’s management wanted the customer to see all services as coming from
"the phone company" and then internally we’d figure out who to contact to
get the problem resolved.  For this circuit I think we (General) provided
one pair of wires from our wire-frame to WU’s customer site and another
pair of wires from our wire-frame to the WU central office (which was three
blocks down the street from us).  The pairs were cross-connected on our
wireframe after passing through our test board's jack field.



Which leads me to my question (sorry to take so long to get to the
question) which has to do with DC teletype loops (up to 3 or 4 miles in
length) over dry* pairs leased from the telephone company.   In the case of
long DC loops for TTY service I assume that the current limiting was done
via a large wattage resistor located in the same building as the voltage
source (and not at the customers location).  Is that correct or usually
correct?



I know that many TTYs include a polar relay for recovering polar signals
from the line.  But what did the DC signaling look like from the sending
station going toward the polar relay at the customer's location?  Was there
an attempt at the sending side to maintain “line balance” so that adjacent
telco pairs didn’t get clicking crosstalk? For example was MARK sent as +130V
on the tip wire and -130V on the ring wire, and SPACE sent as -130V on the
tip wire and +130V on the ring wire?  Or was it something like MARK sent as
-48V on the tip wire and GND on the ring wire, and SPACE sent as GND on the
tip wire and -48V on the ring wire -- maybe with the feed passing through a
common mode choke or balance transformer?



Were there cases when the telco local loop was connected directly to the
selector magnets and the sending side was single-ended, much like the TTY
loops in our own ham stations, but over a much greater distance?  Was there
telco crosstalk problems when doing this?



I'd be grateful for whatever knowledge members have to share on the topic
of DC signaling over telco lease dry pairs.



73, Paul, ad7i



*as I recall, to those in the phone company the term “dry pair” meant that
the phone company didn’t provide any equipment or sealing current for the
circuit (although the circuit usually passed thorough a jack field in our
toll test board for diagnostic testing when needed).  It was just a pair of
wires and our only obligation was to ensure that the line was balanced,
didn’t have foreign voltage sources (foreign battery was our term) or audio
noise and that the resistance of the loop wasn’t greater than X (also load
coils in or out of the pair, as the customer requested).  It –was– expected
that the customer would do something to cause DC to flow in the circuit, if
for nothing more that to maintain a “sealing current”.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20200525/5ffbd140/attachment.html>