10 Oct
2010
10 Oct
'10
1:42 p.m.
On 10/10/2010 10:38 AM, Tony Duell wrote:
Ah, now that makes a lot of sense...
I appears that one signal (presumably either a 'bidrectional' one for
2-wire use or one half of the 4 wire set-up can be switched to either
pins 4,5 or 1,2. Pins 7.8 seem to be either the resistor I mentioend, or
not used, or another signal pair, I would guess at the other half of the
4-wire set-up. You don't happen to know which pair is Txand which is Rx,
do you?
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.
No, it's not that. There's a SPST reed realy (single form A, simple
on'off contact) that's wired to pins 3 and 6 on the RJ45. As far as I can
see those pins go nowhere else. So there must be some device that expects
a short between those pins under some circumstances.
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. 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.
Pity. While I am sure the ofifical standards would be interesting, I
doubt they would be worth the money for this...
Thanks.
-tony