loop-start

That describes the commonly used telephone line.
The term "loop-start" is as opposed to a "ground-start" line.
Typical telephone sets use a loop-start line because they are electrically isolated from ground.
PBX's often use ground-start lines (rather than real trunks) to connect to the Public Switched Telephone Network (PSTN). Ground-start allows easier "anti-glare" implementation, which prevents the line from being seized at exactly the same time by both ends (which is an unnecessary feature for a regular telephone set).
Reply to
Floyd L. Davidson
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They are fully interchangable.
But note that it 1) was not always that way, and 2) it is only true for loop-start when it is properly used.
It's been a few decades since there has been anything from the telephone industry that required the "right" polarity. The original touchtone telephones would not work if the voltage was reversed, but shortly after that the touch tone pad was fed through a diode bridge that would provide the correct voltage regardless of the tip/ring turnovers.
Seems to me that the last modem I saw which cared one way or another was from Telebit, but that's at least 15 years ago and my mind is foggy...
Today I don't think any telco actually pays attention to polarity for loop start phones, and you might well get a line from the telco that is turned over.
Reply to
Floyd L. Davidson
I have. Older Western Electric, ITT, and other phones made prior to the late 80's are often polarity sensitive where TouchTone models are concerned.
If you're hooking up an older phone, mind the polarity or the dial won't work.
Keep the peace(es).
Reply to
Dr. Anton T. Squeegee
[snip]
I see wrong polarity from SBC Pac Hell on occasion, which is all to often if you ask me. Those splice jockeys in the manholes apparently can't tell the diff between Tip and Ring on those old pulp pairs. :-/
Reply to
Watson A.Name - "Watt Sun, th
Thanks for the explanation. So for a loop-start are tip and ring fully interchangable or is there a preferred polarity?
Reply to
Rick Merrill
Mostly.
According to tradition, there is a "right" and "wrong" polarity, but I can't recall ever seeing a bit of gear that didn't work with the "wrong" polarity.
Reply to
Grant Edwards
That is because most consumer gear is more tolerant of reversed polarity than gear of a certain vintage. My 1973 vintage 2702B needs to have the polarity in the right order for the DTMF pad to work. But my $29.99 900Mhz cordless phone could care less what the polarity is.
Reply to
Tony P.
The symptom of crossed tip and ring with a phone is that you get dial tone, but you can't "break" it by touching a key. IOW dialing doesn't make a call.
Reply to
Al Dykes
The pair of wires from your telephone to the central office is known as the local loop. Normally this is an open circuit to DC. When you go off hook a DC path is created and current flows in this loop, this flow is detected in the CO and it then returns dial tone. This is loop start.
Another is ground start where one of the wires is momentarially grounded forming a current path through the wire and ground.
DC polarity used to mean a lot in analog telephones but not so much anymore as far as operation. But, a good tech still keeps it correct. Telco always makes it right and a wrong polarity somewhere could mean somebody has been messing with the wiring. Meanwhile a lot of modern circuits are polarity sensitive and keeping polarity straight is a good habit to get in to.
TerryS
Reply to
TerryS
There are the old touchtone phones that required proper polarity. Didn't payphones use polarity reversal to signal something associated with billing?
Reply to
dold
Just for interest, in regards to polarity, you can remember it this way:
TIP = Tip Is Positive RING = Ring Is NeGative
Also, the tip is positive ground. So RING can have some other meanings:
RING = Ring Is Not Ground and RING = Ring Is Not Green Ring is Red, in the old green/red wiring.
Or at least that's the way it's spose to be.
Reply to
Charles
Tip, Ring, and *Sleeve* (which might not be at ground).
The Tip is indeed at ground! Noise depends on *where* it is grounded.

Tip >----------------------------+ +---------------< 200 Ohm Windings } || { +----+ } || { | -|---+ } || { | / | | } || { GND | / | | } || { +------|- | | } || { | | | +------+ || +------+--------> ----- | | | 1 mfd || | 1 mfd --- | | ----- || ----- - | | ----- || ----- | | | || | | | +------+ || +------+--------> +48VDC ----------------------------+ +---------------< Repeat Coil
A simplified schematic of the interface between a customer loop (tip and ring) and a line card. Virtually every interface that uses any type of loop supervision is some variation on that circuit. The "A Relay" senses loop current. (Basic circuit copied from "Notes On The Network", 1980 edition, AT&T, page 43, with added labels for clarity.)
Reply to
Floyd L. Davidson
(much snippage...)
Years ago, when I was just a pup, an old timer, a splicer, told me you only needed to know three things to be a good splicer: Tip, Ring and Payday!
(with all respect due to cable splicars!)
Al
Reply to
Al Gillis
Floyd...
It must be finger-numbing cold up there already! Your +48VDC indications should be -48 volts, of course. (and don't you love how proportional fonts butcher cool ASCII drawings?
Al
Reply to
Al Gillis
Ah ! History. There are (can be) three terminals, TIP, RING and GROUND. Named from the old three terminal switchboard plug. TIP is positive battery, yes, but not GROUND. If your TIP is grounded it will be pretty noisy.
TerryS
Reply to
TerryS
^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It will indeed.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
That's exactly what I said.
Now, anyway you want to look at it, the Tip is at ground potential. That's not a "may be grounded", that's an absolute. Have you *ever* seen a CO with a floating battery used for loop current?
Reply to
Floyd L. Davidson
Opps. Yes, that is indeed -48VDC, not plus. Sorry 'bout that.
Why would anyone read Usenet with proportional fonts?
>"Floyd L. Davids>> +48VDC
Reply to
Floyd L. Davidson

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