T wrote:
In article ,
"Jim Thompson" wrote
in message ...
Two lines coming in...
How much risk is it to assume that both "Tips" are essentially earth
ground potential?
Probably not a good idea to do that. Back in the days when the
subscriber phones had local batteries it was important. However
with the advent of no battery phones in the early 1900's it made
little difference which side was grounded. My father was an installer
and was taught in the 40's that ring/tip red/green were not critical.
Even if the tip were always ground that ground may not be the same
as the electric company's ground in your home/office as the phone
co often pounds in their own ground rod which can be many feet,
and volts, away from the electric ground rod.
Art
Well it wasn't critical until the 1960's when DTMF aka Touch Tone made
the scene. The early 35 type DTMF dials had to have the correct polarity
to function properly.
It was telco's early trick to prevent people using Touch Tone if they
didn't pay for it.
I was a "victim" of this once. I had just moved into a new apartment, back
when the Telco guy had to come in to hook up the phone. I finally got all
moved in, and picked up the phone to call whoever to tell them I was moved
in, and the touch-tone didn't work. I got dial tone, but when I pressed the
touch-tone buttons, all it did was blank the dialtone, and go back to
dialtone when I let the button up.
I "flashed the hookswitch (switchhook?)" - you know, in the old movies,
when they'd rattle the button or rattle that hook where the earpiece hung
on the _really_ old phones - "clickety, clickety, Hello, Operator?" - ten
times, because I knew that that was how the old dial phones worked - it
simply interrupted the current loop - and finally the operator answered.
I told her what the problem was, and she said, "Oh, sounds like polarity."
They sent out a Telco guy, who swapped the red and green leads, and I had
touch-tone. If they'd told me that that was all it needed, they could
have saved a service call, but I was but a mere customer.
I suppose I could have figured it out - I was a tech at the time, after
all, but hey, I think it was illegal to mess with your own phone back in
those days.
Cheers!
Rich