TELECOM Digest OnLine - Sorted: Re: I Have a Telephony Mystery

Re: I Have a Telephony Mystery

Paul Coxwell (
Fri, 6 Jul 2007 02:42:14 -0700 (PDT)

> The 'splitter' in question is here:

> Now, they call it a telephone/model double adaptor. Presumably the
> non-BT style socket (the 'top' one) is meant to be for a modem.
> Here's the thing -- after looking at documentation, I can't tell why.
> As I said, it seems that only two connectors are needed for a fully
> functional phone line. They're called the 'A' and 'B' connections,
> according to this site:

That's a standard adapter which will pass the four connections of the
BT431 plug straight through and tap the A and B wires (tip & ring
respectively) for the RJ11 jack. The line appears on the OUTSIDE
contacts of the BT plug (terminals 2 and 5).

As Pat suggested, if you had a similar adapter before which had four
contacts on the modem jack, then the outer contacts were almost
certainly just spare.

There is nothing in the adapter/splitter itself which specifically
makes the RJ11 a modem jack. It's just the standard configuration
used in the U.S. for a single line, and as modems sold in the U.K. now
also use an RJ11 configuration this adapter means that you can connect
them with a straight modular cable.

> The extra two are apparently superfluous, and are usually connected
> to look 'neat and tidy'... ?

In a simple domestic system white/orange (terminal 4) is superfluous,
but orange/white (terminal 3) may be needed and should be connected.

When this jack system was introduced in the early 1980s, British
phones were wired to accept ringing on that third wire (by way of the
capacitor at the master jack). In more recent years, many phones
(intended for world markets) have appeared which don't actually rely
upon that third wire and will work with a basic two-wire connection.

By contrast, in the U.S. the standard connection is just two-wire and
every phone has its own internal bell capacitor, so the third-wire
issue doesn't arise.

If I now understand your problem correctly, you have the Maplin
adapter plugged into your master jack and an extension connected from
the BT outlet on that adapter to a jack upstairs. And a phone
connected to that upstairs jack is not giving your dialtone, correct?

If that's the case, then plug the phone directly into the BT jack on
the adapter downstairs and check that it works. If it does, then you
have a problem in your extension wiring.

> Trouble with getting such splitters as RJ14s (4 pins) instead of
> RJ11s (2 pins) is they're damn-near impossible to find.

Just to clarify this, RJ14 does NOT refer to the fact that there might
be four connections in the jack itself. RJ14 is specifically a
designation used (in the U.S.) to specify TWO phone lines to the jack.

> [TELECOM Digest Editor's Note: I have rebuilt a two-pin plug so that
> the two _required_ pins terminated in the center (openings) but that
> is a terrible pain to do;

Ain't that the truth!


[TELECOM Digest Editor's Note: Well, if you can extract the wires from
a phone cord you will find a little tiny strip of metal on the end;
this little strip of metal can be _carefully_ inserted into the
correct hole on the other half of the socket, and then bent over to
make it stay in place. It is quite a hassle, but can be done if no
other way is possible. PAT]

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