Philex Technical

NEVER connect un-approved devices which take power from the mains to the telephone system. Even if you survive the mains on the cabling the poor wireman sitting in a muddy hole some way away trying to find a fault may not.

Although pin 4 has no function in a domestic installation it is usually connected for the sake of neatness.

Which way around they are connected usually doesn't matter but some modems (especially older USA sourced ones) and some answering machines are fussy about polarity, so it's wise if possible to check the voltage on the line and connect -48V to the B leg (J2) and 0V to the A leg (J5).

The colour code shown above is the one which would normally be used by BT. However it isn't always adhered to, especially if internal wiring in a new house has been installed by an electrician or it's been done or modified by previous occupants. You should therefore never rely solely upon the colour code - always check both ends of the cable.

The standard plugs encountered are the small RJ11 found on line cords at the telephone or modem end and the BT431A plug at the socket end. On RJ11 it is usual for the centre two pins to carry the signal, on BT431A it is the outer two.

BSI and BT managed to number the 431A plug in the British Standard as a mirror image of the socket, so when inserted pin 1 on the plug goes to pin 6 on the socket, pin 2 to pin 5 and so on.

Many packets of plugs in the DIY sheds come with a little diagram on the back which often follows the BS numbering. If you do the logical thing and put a plug on assuming the numbers are as on the socket the phone may not ring!

Looking at the plug with the contacts uppermost the BT numbering is shown above with some common colour codes. That on the left is the older BT colour code, that in the centre is the one more commonly seen now, and that on the right is often found on modems, imported phones and Sky/Free-View Digital boxes with only two connectors.

A simple way of remembering the correct orientation for the BT plugs if you like mnemonics is Ring on the R ight. When you are holding the plug as shown in the diagram with the latch on the right the ring terminal (4 on the plug, 3 on the socket) is always just right of centre.

Note that with the two rightmost 2 wire examples in the diagram going from RJ11 to BT will involve the colour code going "wrong" at one end or the other unless a junction box or joint is used. If the lead has a BT adapter at one end the colour swap is usually made there, USR tended to swap it in the RJ11 plug, others do their own thing. NORMALLY the center two pins on the RJ11 are connected to the outer two (2 and 5) at the BT end. No matter what coloured wire is used the two wires always go to pins 2 and 5 at the BT end however it isn't safe to assume they ALWAYS go to the center two at the RJ11 end.

This is the easy bit. Just remember to connect all the numbers together, so 2 to 2, 3 to 3, 4 to 4 and 5 to 5. Connectors 1 and 6 have nothing to do on a plain telephone circuit.

Make sure you use telecoms cable (single solid core) and not alarm cable (different colour code and stranded wire), Alarm cable is not twisted in pairs and has quite poor noise immunity compared with telecoms cable. It also won't reliably fit in IDC connectors. Cat 5e Networking cable comes in twisted pairs, has in fact superior noise immunity, and works fine for telephone. Plus it gives you spare wires for your normal network wiring!

It is important (especially for modem use) to ensure you keep pairs together. Telephone cables rely on having pairs twisted together to reduce noise so pins 2 and 5 should always be on matching colours (ideally orange/white, acceptably blue/white) NEVER blue on one and orange on the other.

DON'T cross any wires - crossing 2 and 5 for instance (so 2 in one box goes to 5 in another) creates a situation where either socket will work by itself but once a phone is plugged into both they will either continuously ring or not work.

A common complaint is that one phone will work in an extension yet another plugged into the same socket won't. Usually this is because either the extension has been wired up with only terminals 2 and 5 connected or 3 has come adrift. Some phones originally designed for the overseas market will accept a ringing signal on 2 and 5 only (they usually only have those two wires connected) and so will work, others designed to use pin 3 for the ringing circuit won't ring but they will dial out and you can speak on them.

Some modems use only two wires, others need three and won't auto answer without the third. Wire all three correctly and you can't go wrong. Many modem problems are simply caused by incorrect wiring of extension sockets.

Nearly all modems these days have a removable lead with a BT plug on one end and an RJ11 on the other. The most common configuration is from the center two pins on the RJ11 to the outer two on the BT plug, However, this isn't universally true by any means. If the lead you are using isn't the one that came with the modem you can't altogether trust it. The fact it works on another (different manufacturers) modem means nothing.

Running a modem on a long extension lead isn't going to degrade the signal significantly however trailing leads easily get damaged so ideally wire up a proper extension as explained above.

Dial Tone detection circuitry on some modems is also quite fussy. If you have the BT answering service (Call Minder) which gives an interupted or modified dial tone when you pick up the receiver then often modems connected to that line will give "no dial tone". The cure is to remove call minder or set the modem initialisation string to ignore dial tone detection.

If your calls drop make sure you don't have call waiting enabled, the call waiting tone often causes the modem to drop the line. Similarly if you have other extensions on the line then even the briefest lifting of another phone will cause the connection to drop. In this case replace sockets with privacy sockets or use privacy adapters, these stop other extensions affecting the one engaged in a call.

Phone ringing continuously.-
Terminals 2 and 5 swapped (2 at one socket connected to 5 on another and vice versa)

Very poor speech quality, possibly poor bell. -
Terminal 3 and 2 or 3 and 5 transposed

Ringing but no speech (or very poor speech) and can't dial out. -
Wire between terminals 2 or 5 broken.

You have an installation where the main socket works and the remote in the garage roof doesn't. You need to test the continuity of the circuit. So you can use a very long lead with and a test meter - or cheat.

Firstly, disconnect the BT line completely.
In the remote end bridge any two terminals (make a note of which two).
Measure continuity between these two wires at the master socket end - should be no more than a few ohms.
Repeat for the second pair of wires.
If either show a fault swap the combinations - so if you tried 2 and 4 and that was OK, and then 3 and 5 and that failed, you know 2 and 4 are both good so trying 2 and 5 and 2 (or 4) and 3 will show you the faulty wire.

OK - so you have tested the cables and there is 150 yds of cable buried in the newly decorated wall and only 2 wires have continuity - what do I do??

The second master gives you back your ring signal so connect the two working wires to terminals 2 and 5 on the new master and hope for the best!

REN measures the load a device places on the line when ringing. A normal BT line will support a REN of at least 4, in other words at least a total of 4 phones/fax/modems should work on any line so long as their REN figures added together don't exceed 4.

The REN is normally found on a label at the base of the machine
(near the green approval symbol).

In practice you can quite often exceed this number because devices with a REN of 1 may actually have a real REN of only a fraction of 1.This is an anomaly of the test procedure used. Moreover many lines can drive a REN of more than 4.
Note that some elderly fax and answering machines can have very high REN's (and they really are high!). If some or all of your phones fail to ring or some ring very anaemically then its possible you have exceeded the REN. Try unplugging devices until they work.
You can get REN Boosters which will increase the ringing capacity of a line if desired, although if you get to this stage you should probably be thinking of installing a small PABX.