OT : RJ45 Telecom wiring

[Tony Duell]

> 
>   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, 
do you?

> > 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 
connector.

> 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