Date   : Thu, 18 Jul 2002 17:26:50 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: BBC to Mac Link Lead

On Jul 18,  5:22, RWAPSoftware@... wrote:

> Yes - I was expecting one with two notches, [...]

> > It will push into the BBC in 2 orientations. I have just double checked
> > with
> > another the same as the one I sent you. There is one notch on the plug
> > which
> > will fit into either of the two notches in the socket. As discussed
before,
> > your best bet is to pick an orientation and stick to it. Mark TOP on
the
> > top
> > you have decided upon. Do it on the metal casing that protrudes from
the
> > plastic housing. I usually use indellible marker, but it might be
better to
> > engrave it :-)

> Does this mean that it is now impossible to get the BBC plugs with two
> notches on I wonder??

They were *never* made with two notches.  You're referring to the "dent" in
the side of the metal part, right?   This type of "domino" pattern DIN plug
was originally designed for headphones, and was deliberately designed to
fit into the corresponding socket in two ways.  There *should* also be a
notch cut out of the metal part, on one side only, 90 degrees ariund from
the dent.  The reason is that a DIN headphone socket incorporated a set of
switch contacts; with the plug in one orientation, the notch allowed the
contacts to remain connected, leaving the internal loadspeaker of your TV
or radio switched on, while in the opposdite orientation, the contacts
would be pushed apart and the loadspeaker would cut off when the headphones
were plugged in.

Acorn originally intended that the domino plug would allow them to wire the
BBC serial socket in such a way that reversing the plug would swap TxD for
RxD and vice-versa, and similarly swap RTS for CTS.  By the time someone
realised that the board layout was wrong for that purpose, it was deemed an
established feature and it was too late to change it.

The convention used to be to have the cutiout at the top, but I wouldn't
worry about that much.  Just mark "TOP" in indelible marker!

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York