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

Re: I Have a Telephony Mystery :-)

Jeremy Morton (
Wed, 04 Jul 2007 20:36:20 GMT

Adam Sampson wrote:

> Jeremy Morton <> writes:

>> I'm in the UK. [...] The extension plug is an RJ14, but the
>> splitter's socket is an RJ11 (2 pins instead of 4). [...] But
>> here's the weird thing -- the ADSL connection works fine when the
>> modem's plugged into the upstairs socket.

> Are you sure that's actually a splitter, not a microfilter? It sounds
> from your description like the upstairs socket was installed for ADSL
> use with a microfilter on the master socket -- an arrangement that was
> pretty common in the early days of BT ADSL. Some models of microfilter
> do look very similar to regular splitters, except that they have an
> RJ11 for the ADSL modem rather than a normal BS6312 phone socket.

I am sure that it's actually a splitter, not a microfilter. I bought it
myself, as a replacement for the last part (that looked identical), which
physically broke. :-)

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:

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

However, I distinctly remember the last splitter (looked identical to
the one I linked above except it was broken) had four pins in the
'top' connector, whereas, as you can see from the picture, this one
only has 2. Shouldn't make a difference, but I came to you guys to
check. If the extra two do, why do they? It's definitely a splitter,
not a microfilter... and it's connected directly to the master socket.

Trouble with getting such splitters as RJ14s (4 pins) instead of RJ11s (2
pins) is they're damn-near impossible to find. In the form-factor I need,
anyway. And I don't want to buy one unless someone can explain to me why
the 4 pins are needed. Any help appreciated.

Best regards,
Jeremy Morton (Jez)

[TELECOM Digest Editor's Note: I personally do not think all four
pins are needed; just two pins, but their placement in the little
slots are critical. Again, to use the USA illustration, I see a lot
of answering machines and other devices available with just two pins,
but the two pins provided are the two _center_ pins. You'd think that
as inexpensive as phone cords and sockets are; the manufacturers would
just make them all with four pins, but 'they' say they are afraid the
end user will plug it in 'incorrectly' or get the second phone line
activated 'in error', etc. So, they prefer to sell those devices to
an unsuspecting public and include a note saying 'this will only work
on the first line', or words to that effect. By the way, I do not
recall you saying you did or did not hear 'battery' on the upstairs
phone or not, as though it was only the filter blocking out the dial
tone from reaching you, or if the upstairs phone was totally DEAD
(which means the pin outs are out of order up there.

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;
believe me, I would just go to Radio Shack and buy a new 4-pin piece
in the future. That is why I suggested to you, based on what I knew
at the time, to simply buy a new one and plug it in instead. PAT]

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