10 Oct
2010
10 Oct
'10
12:01 p.m.
On 10/10/2010 10:38 AM, Tony Duell wrote:
What pins would I expect to find a normal
'switched' telephone line on (4
and 5, I think?)
Yes, the middle two.
What about a 2-wire leased line ('private
circuit')?
Also the middle two.
Or a 4-wire one?
Not sure about voice service.
For a T1 digital line, the pairs would be
1/2 and 7/8.
Why would there be a resistor of about 866 ohms
connected between pins 7
and 8?
That's to program the transmit power level of the device plugged into
the socket. Typically used on leased lines. On a real installation of
that type, the resistor value is chosen by the installer to compensate
for loop length.
Why would pins 3 anf 6 be shorted together?
I'm not sure specifically about about 3 and 6, but on some jacks there
are one or two shorting bars to preserve continuity of a loop when there
is no device plugged into the jack. This is used in RJ41X service,
typically for alarms, because the alarm system can seize the loop
(disconnect it from the downstream phones). When nothing is plugged
into the jack, the shorting bar provides loop continuity to the phones.
In case anyone's wondering, I've bought a
non-working telephone line
simulator, and am trying to make sense of the numerous relays connected
to the telephone connectors, which are RJ45s (genuine RJ45s, with the
extra polarisation notch).
Note that RJxx is a USOC (Uniform Service Ordering
Code), and defines
the type of service as well as what jack is used: 4-position,
6-position, 8-position, with or without shorting bars, how it is wired, etc.
While an RJ45 service uses an 8-position jack, an 8-position jack is not
in and of itself an RJ45, despite everyone misusing the USOC code in
that way. An 8-position jack could also be used for RJ31X service,
RJ41X, RJ48C, RJ48S, RJ48X, RJ61X, etc.
Similarly, a 6-position jack may be used by RJ11 service, RJ14 service,
or various others.
Books on US telcoms practicce are few and far between
over here...
The same seems to be true here in the US. Oh, there are plenty of
books
that will tell you how a basic POTS service works, but little beyond that.
The real info was in the "Bell System Practice" documents, which were
not available to customers, and in Bell standards, which were. After
the divestiture, the standards went to Bellcore, which is now Telcordia,
and the standards cost a LOT of money.
Eric