OT : RJ45 Telecom wiring
ard at p850ug1.demon.co.uk
Sun Oct 10 15:42:13 CDT 2010
> 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.
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,
> > 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
Again, that makes sense. Such a resistor can be connected by a relay,
presumably for testing leased-line modems. I assume (and will know more
when I get this thing working) it will check the transmit level is what
it should be given that resistor value.
> 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.
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.
> > 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.
Indeed. Henace my comment that the twisted-pair ethernet socket is not
strictly an RJ45. It's even mechancially different.
However, I suspect this device, being a telecoms tester, with RJ45-like
sockets (with the extra groove) will simulate some standard uses of 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.
Pity. While I am sure the ofifical standards would be interesting, I
doubt they would be worth the money for this...
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