Telephones

Sep 3 21:56 UTC 1994

For the exchange of information about telephones and telephone circuitry.

response 2 of 99:

Sep 4 08:44 UTC 1994

I've never heard of a 5-wire block, however, phone lines are
pairs of wires, do you mean 5-pr on a block? 
  
IN any event, here is a -48VDC on a hot pair, when the voltmeter is
connected, red/black (most often), the red lead is tip and the black
lead is ring (and the v-meter reads -48). Notto worry too  much
about polarity these days though, almost all stuf is polarity independent.
  
If you are pretty close to downtown the DC voltage on a open circuit
could be as high as -56 VDC, but when there is a load onteh line (when
you lift the receiver to hear a dialtone) the voltage drops to the
5-8 VDC range, with 20-100 mA loop current.
  
Does this help?

response 4 of 99:

Sep 4 15:21 UTC 1994

If you come across a functioning 2-wire line, and get dial tone, but don't
know whose line it is, dial 0 and ask the operator for a "drop wire ident-
ification."  That's the jargon to obtain the phone number.

response 6 of 99:

Sep 4 20:23 UTC 1994

Yes, TS, I should have called them "pairs", not "lines". Boy, these
purists.... And I know about the ca. 50 volts, and green-tip-(+) and
the red-ring-(-), and the 90 V AC ringer zappo. So, I have 5 pairs
on a single drop, which go to various jacks around the house. I want
to find out which is which, without unwiring the block (yet). If there
is a magnetic field probe I could clip to one wire of a pair, I could
go around the house taking phones off-hook, or plugging in a dummy
load, to see which jacks are serviced by that pair. But is there such
a probe? (I think my alternative is to wire all the pairs to terminal
blocks now, open them one at a time to find the corresponding jacks,
and jumper them later for two incoming drops. But this is no plebian...)

response 8 of 99:

Sep 5 07:04 UTC 1994

Heh! Greg, if I can detect the *current* in a pair, I can separate them
without disconnecting, since this doesn't flow into the other pairs. How
small a current in a wire could a Hall Effect transistor be used to
detect?

response 10 of 99:

Sep 5 19:23 UTC 1994

In the long run, I'd have lot of uses for a flux probe. In this case, there
would be a lot of wiring and unwiring, as each pair is traced, and with the
probe, my daughter could watch the meter in the basement, and let me know
by radio when I find the associated jacks. But Greg does have a point: I
could have run around a lot, and gotten it done, even though the telephones
would be out of service for a while, in the time I've spent discussing how
I can do it simply, here. Of course, though, if we found there were such
a probe, everyone else (reading this) would be able to save lots of time.
Look to the future (or, "never do today what you can put off for tomorrow").

response 12 of 99:

Sep 6 10:30 UTC 1994

Ummm, what I said, correctly, is "when the voltmeter is connected,
red/black (most often), the red lead is tip and the blacklead is ring
 (and the v-meter reads -48)."  Most VOLTMETERs use red/black for
polarity identification and since the tip is negative wrt ring, and
tip is above ring on a punchdown block, the VOLTMETER leads, correctly
connected (red/black) will produce a -48 VDC on the VOLTMETER.
  
There is NO red/green pair coming INTO the demarcation point. There is
a red/green/black/yellow two-pair INSIDE WIRING  - AFTER the demarc,
unless 3-pr/6-pr/12-pr/25-pr is run instead, in which case there STILL
is NO red/green wiring until the outlet is installed, and the red/green
goes from the block to the rj-11/rj-14, etc jack.
  
I've punched 300-pr cables and their ain't no red/green anywhere.
  
Further along the trivia line, the reason the polarity was reversed
in the first place, and tip became (-), was because the operators
would take their phone plugs and run the tip along the patch panel
to see which one "popped" intheir headsets - thus indicating the hole
to plug into. Too often, before the polarity as reversed, the  tip <at
+48V> would scrape across ground (!!!) and zap fuses and sprks would
fly, and all sorts of mean, nasty, ugly things (apologies to Arlo)
would result.
 
since the operators were only looking for a "pop" from the panel, the
panel stayed at ground potential (same as "tip") and the ring was
put at +48. And ring was Insulated from the panel. 
  
So, when the operators scraped across all the ring contacts they
would get their requisite "pops,' but if the tip scratched across
the metal panel itself ...... no potential diference, no sparks,
no blown fuses ... !
  
And linemen had to reverse their either their probes (RED/BLACK) when
teh checked for battery, ... or ...  get used to -48v when there
was battery voltage. The latter became the standard for historical
reasons.
 
And until the late 60's early 70's, polarity was critical for your
phone to work. Eventhe early TouchTone (r) phones were polarity
sensitive. 
  
If you couldn't "break the dialtone," the fix was to reverse the
leads in the wall jack. 
  
Oh., rcurl, there is a toner and probe gizmo-combo that traces
wires perfectly. You can pull a hot pair out of a thousand-pr
cable in about 5 seconds. Yes, I have the gizmo-combo, and it
is on loan at the moment.
  
I'l try to get it back, rsn, I need it too.

response 14 of 99:

Sep 6 15:49 UTC 1994

Going in reverse: I also used to know the "ringback" number in some other
exchanges, but don't know it here. The idea to detect the ring is to
be considered, if I could find a clamp-on AC probe. Its not quite as
"passive", or quiet, as looking for the off-hook DC current, though.

TS, your information differs from my sources, which say that tip is
positive and on the green wire, and ring is negative and on the red
wire. Maybe the confusion comes from the tip (green) being at *ground*
in the on-hook state (but still positive w.r.t. ring (red)). My sources
are the ARRL Handbook, and Sokolowski's _The Talking Telephone_ (Tab
Books, 1990 - not a particularly good source, but useful). We better
get this straightened out before we carry this discussion further.

response 16 of 99:

Sep 7 05:50 UTC 1994

I presume the toner puts a signal on a line, and the probe detects it.
However my five pairs are connected in parallel, so they could not be
distinguished. (Unless I disconnect them, which is what I wanted to delay.)

Re polarities, here is what it says in 1991 ARRL Handbook, p. 28-23:

"The equipment leads are called "tip" and "ring", or just T and R. For the
idle or on-hook state, the tip (positive) lead (green) is grounded. The
CO ringing generator and the -48 V dc supply connect to the ring lead (red)."

Say, were you born with reverse polarity, TS? ;-)

response 18 of 99:

Sep 7 11:13 UTC 1994

There are TWO errors, then, in the AARL Handbook. NEITHER lead is
grounded - phone lines are a floating, balanced pair, equally  isolated
from ground (earth). 
  
If you want to blow up your telephone service, ground one wire
of your pair and try to use that line.  There should be an
available ground connection available on the block. You can 
either unearth the facts with direct experimentation, or pay 
Bell $60/hr to stop by and unearth it for you. 
  
Ummm, all five of your +drops+ are connected in parallel ???? Or,
are there 5 in-house pairs connected to one Bell drop-wire ?
The latter sounds more plausible, actually.
  
And yes, if the second hookup is true, then you do have to
disconnect the in-house wiring at the block to isolate a
specific in-house pair running to a specific jack. No biggie,
standard practice. However if the black/yellow wires are NOT
connected to anything at the block, THEN you use this spare
pair for trace identification, no biggie again, and no dis-
connection necessary either.
  

response 20 of 99:

Sep 7 15:01 UTC 1994

I have taken TS's challenge, and measured the voltages on the drop to my
house: with everything "on hook", one side is *exactly* at ground
potential (and has the green wires), and the other side is at -50 V (and
has the red wires). ARRL is sustained! 

However, off hook, the difference is about 7 V, and both sides are
negative, at ca. -31 and -37 w.r.t ground. This fits with the *source*
impedance being approximately balanced. 

That was a good suggestion, to follow the black/yellow pairs! That is, if
they are connected at the jacks. In fact, it may solve the whole problem.
Now, why didn't I think of that? I was looking at the hanging black/yellow
wires, but it just didn't occur to me. 

response 22 of 99:

Sep 8 11:25 UTC 1994

Sorry to slip that one in on you, rcurl, ..... but wrt measuring the
voltages: yes, one of the wires shows NO potential difference to ground.
  
However, that does nOT mean that they are the +same point+, ahem.
 
What it means is that (way back there when I explained WHY the
poliarities were reversed) the tip striking ground will NOT blow
a fuse, there is no potential difference. 
  
If they are the same point, then an ohmic measurement is the validity
check. I think that it takes a megger to measure that though. Phone
pairs are balanced, isolated and floating - not unbalanced to ground.
 
Besides, my test was not to measure teh potential differnce, but
rather to ground whichever wire and try to make a phone call, you
changed the test and got an accurate measurement with a VERY misleading
conclusion therefrom. 
  
With the phone off-hook, the battery drops to about minus 5 to 9 volts, and
your difference of (-37) - (-31) = -6 VDC, which is just fine, tip to ring.
  
Since a phone pair is a loop (thus, "loop current" of 20-100 mA) and
the phone instrument is in series with that loop, you can see that
the source impedance is rather high, proportionately. 
  
It is through padding the source impedance that telco adjusts loop
current to compensate for 500 or 25,000 feet of series copper wire
between you adn the switch.
  
ARRL has not yet been vindicated. Great organization, I was a member
for quite a while, but that particular section .... well, they're human.

~p
  
Oh, it doesn't matter whether the bk/yl is connected in the jack,
teh wires are there! Sure, you might have to open a jack in some
room womewhere and connect something to bk/yl in order to trace,
but, it'll work with no problem (unless either bk or yl have been
used to "replace" a goofy rd or gn wire at some earlier time)
  
Keep usinformed please.

response 24 of 99:

Sep 8 22:59 UTC 1994

Without question, the " 'tip-ring' thing goes *way* back," let me 
emphasize, way, WAY, W A Y back! Mu guess (guess) is that by the
time of hte third installation, the polarity was reversed. 
  
Thank you, "the jacks are/were insulated from the panel." 
  
Please recognize that there are TWO types of ground - I'll try
a freehand ascii representation, but who knows .......?
                    |  
signal ground       -
                    _
  
That supposed to be the three decreasing width horizontal lines 

                       |
                      __ 
and   earth ground   /// 
  

As to the 3 & 4-wire plugs, perhaps both grounds are included, signal
and earth. However, tip is still negative wrt ring, by 48 VDC, and the
phone pair is floating, isolated, balanced, ABOVE earth ground.
  
Wrt "the CO *battery* positive electrically grounded" that is correct
ONLY in respect to +signal ground+, not +earth+ ground, (see previous
test suggestions not taken) even though the potential difference, measured,
between the two IS  zero volts, dc, the tip does float above ground
in ohmic measurements (no answer yet, nor tests made - probably no megger
available). 
  
You are not goint to blow Bells line by grounding one or the other
of the balanced pair - they are used to that happenning accidently
and nothing untoward will result, trust me, but you won't be able
to complete a call if you earth-ground either signal wire.
  
We can agree that tip (signal ground) "is at ground potential," 
even as it is ohmically    W A Y   above earth ground.  And we
can agree that "the local circuit "floats" on the CO
 balanced line impedances," but we don't agree, yet, that tip is
negative wrt ring, or that proper polarity on the VOLTMETER shows
a -48VDC CO battery.  
  
Ummmmm, you introduced a new acronym, CO, which means Central 
Office (CO) and is interchangable with "the switch"in most situations.
in most situations.