On Fri, 19 Jan 2007 22:09:53 +0000,it is alleged that John Rumm
spake thusly in uk.d-i-y:

Dave Plowman (News) wrote:

The most common layour seems to be the two central pins on the RJ11
connected to the two either side of the central pins (i.e. pins 2 and 5,
where often 1 and 6 are not fitted) on the BT plug.


Yes - pretty well all phone line connected stuff these days only uses two
wires. Even if it rings.
There doesn't seem to be a definite standard for the pins used on these
RJ11 connectors though when only two are used - I've got a modem which
uses different ones from a cordless phone. Since it's basically the US
telephone connector I wonder why?

Indeed, kind of why I said "most common". Lovely thing about standards,
if you don't like one, choose another!

Oh yeah, you can't have too many standards for the same thing g

You do raise an interesting point however... perhaps the US socket
wiring uses both left pins as A, and both left pins as B or some such
arrangement. (don't think I have ever seen a RJ11 with only the left or
right hand side pins wired!)

The RJ numbers are frequently misused, even in official documentation
that comes with modems/routers/telephones, they actually refer to the
service(s) supplied by the jack, but with the 6 position, 2 contact
(RJ11) or 4 contact (RJ14) connectors, in the US and Canada, pins 3+4
are for one line, and if present, pins 2+5 are for a second line.

There is also a 6 pin version with 3 lines present. (Line 3 being 1+6,
this is RJ25C)

Confusingly there's also an RJ12 which also has 4 contacts but the
outer 2 aren't for a second line, but for PBX off hook signalling to
busy out exchange lines etc or for data wires, much like 1+6 were used
on some systems using BT 631A plugs.

Oh yeah, and RJ45 isa very obsolete standard connection for modems,
not 10/100baseT ethernet, although networks use the same connector ;-)

--
"Programming today is a race between software engineers striving to build
bigger and better idiot-proof programs, and the Universe trying to produce
bigger and better idiots. So far, the Universe is winning." - Rich Cook.