This is probably one for Paul but if anyone else knows that will be fine.
I am needing to make up a phone patch cord from a US (RJ11?) phone cable but with a British Telecom plug on the other end (computer modem conectivity). I seem to recall that the UK uses the outer two pins for line 1 as opposed to the centre two. I am not certain about the polarity though on a UK telephone connector.
I see that unlike 8 pin cat 5 computer plugs, each end of a telephone lead is wired as a mirror image of the other.
If you place a BT431 plug with the contacts uppermost and the latch on the right, then the A-wire (tip) is on the left and the B-wire (ring) on the right. These correspond to terminals 5 and 2 respectively on a BT socket. Pin 3 is used for ringing and is connected to pin 2 via a 1.8uF capacitor in the master jack, so if you're using a 4-wire cord make sure the other end doesn't connect to anything which will be affected by this.
By the way, thanks to a major lack of communication, when these plugs were introduced in 1981 we ended up with the official standard numbering the pins on the plug the opposite way around to the terminals on the socket!
According to eircom's technical requirements they're not necessarily polarised in Ireland and it shouldn't matter to a normal phone which way they're connected. Particularly as balanced ringing is in use.. i.e. 40V out of phase on both legs.. i.e. you get 80 ish volts between the two.
However, that being said, almost all phone wiring is correctly polarised.
Green: Tip line 1 Red: Ring line 1
Black: Tip line 2 Yellow: Ring line 2
Turn an RJ 11 plug upside down and look to see normal polarisation of the centre two.. should be green & red.
they're always the centre pair though.
[This message has been edited by djk (edited 01-16-2004).]
If you place a 6-way modular plug with its contacts up and the locking tab underneath, cord coming toward you, then the contacts are numbered from 1 on the left to 6 on the right. 4-way plugs have only 2 thru 5, of course.
As Dave said though, most modern equipment isn't at all fussy about polarity.
Ringing on the common System X exchanges here isn't balanced. Normal idle conditions have -50V battery on the ring/B-wire, but on an incoming call battery is transferred to the tip/A-wire and AC ringing voltage is applied to the ring/B-wire.
I would have thought that TBR21 (european harmonisation) requirements would have been all that's required these days.
Btw, according to a France Télécom document I was reading TBR21 requirements include a reference to the use of standard RJ11 connectors on the phone/modem/fax end and are supposed to make it illegal to use non-standard wiring i.e. the phones should be wired to carry the line on the middle pair... the BT way's not acceptable
You may be right. To be honest, the rules and directives pouring forth in recent years are so numerous that I've given up trying to keep track of them. Most new telephone equipment sold here now comes with the CE 0168 mark on it.
BABT approval was never strictly enforced from a practical point of view. When the system was introduced in the early 1980s the requirement was that approved equipment must carry a sticker with a green dot and the appropriate "approved for connection" wording.
It was still perfectly legal to sell unapproved equipment; the only requirement was a sticker with a red triangle stating "Prohibited from connection..." (It could legitimately be used on a private system, for example.)
I certainly used all manner of home-built and imported Hayes modems back then which were illegal to connect to the PSTN. Nobody ever bothered me.