Telephone wiring
Telephone connectors and adapters
RJ45/RJ12/RJ11
For the most part I follow the CuTEL wiring standards. On the RJ side, these are wired as follows:
| RJ45 pin | RJ12 pin | RJ11 pin | Function |
|---|---|---|---|
| 1 | Unused, optional Line 3 – A | ||
| 2 | 1 | Unused, optional Line 3 – B | |
| 3 | 2 | 1 | Line 2 – A (Tip, 0V) |
| 4 | 3 | 2 | Line 1 – B (Ring, -48V) |
| 5 | 4 | 3 | Line 1 – A (Tip, 0V) |
| 6 | 5 | 4 | Line 2 – B (Ring, -48V) |
| 7 | 6 | Unused, optional Line 4 – A | |
| 8 | Unused, optional Line 4 – B |
The common RJ12 telephone plug (US style) follows the same schema, but only Line 1 is connected.
BT phone plugs
BT phone plugs are wired as follows. The pins are numbered from left to right, with the retaining latch on the right:
| BT pin | Function |
|---|---|
| 1 | Not used |
| 2 | 'B' wire (-48V to earth, “ring”) |
| 3 | Bell wire |
| 4 | Earth (PABX only) |
| 5 | 'A' wire (0V to earth, “tip”) |
| 6 | Not used |
Adapters
RJ plug to BT socket
I use two types of adapter for this:
The Excel adapter converts from RJ45 to a BT socket, while the TUK converts from an RJ12 to a BT socket. Functionally they're mostly identical, in that both include a ring capacitor for the BT side.
Unfortunately they both connect the ring capacitor back to the RJ port, so they're only suitable for use on RJ connectors which have only the centre two pins connected. If they're plugged into a port which has other pairs connected, the adapters will connect the ring capacitor and earth to some of those other pairs (which pairs depends on the adapter).
This is particularly pernicious on the TUK adapter, which – if connected to a CuTEL trunk port – will short lines 1 and 2 together.
Modem and telephone connecting cables (BT plug to RJ plug)
Annoyingly there are a bunch of different “wiring standards” for these cables, and nobody seems to do it the same way.
- 2-wire “correct”: BT pin 2 to RJ pin 3, BT pin 5 to RJ pin 2. This connects “A” to “A” and “B” to “B” on both ends.
- 2-wire “direct wired”: BT pin 2 to RJ pin 1 or 4, BT pin 5 to RJ pin 4 or 1. This is a piece of 4-core flat telephone cable shoved into an RJ jack with two wires nipped off. I assume it exists because it's cheaper to manufacture than the proper BT cable.
- 4-wire rollover and crossed-pairs: Usually used for telephones
Most modems don't need the ringing capacitor, but also connect various pins on the modem RJ jack together. I assume this is so that they'll work with a straight-wired BT-RJ adap
The most reliable cable I've found for connecting modems to BT jacks is the 2-wire BT-to-RJ cable. This completely eliminates problems caused by the modem's RJ jack having the outer pins connected to the inner ones, by not connecting the outer pins to anything.
Alternatively, if the source connector is an RJ11, RJ12 or RJ45 with the phone line on the centre pins, then an RJ12-to-RJ12 cable with two wires connected will work fine. The wiring for these is typically the “Rollover” style, but modems aren't usually sensitive to line polarity.
Some telephones may need a 4-wire RJ cable, if they use the “ring” line. In my experience this is uncommon.