Date   : Wed, 27 Jul 2005 23:46:37 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Null Modem Cable

On Jul 27 2005, 19:23, Greg Cook wrote:
> On Tue, 26 Jul 2005 23:04:11 +0100, Rob <robert@...>
wrote:
>
> > Since the plugs can be inserted one of two ways into the Beeb, why
on
> > earth
> > did acorn not wire them so that doing so would act as a cross-over,
> > rather
> > than it just not working at all??
>
> A good idea, except the use of domino plugs is pretty much an
accident.
>  The BBC Micro being an educational machine, Acorn had to choose the
> DIN sockets so they wouldn't accept each other's plugs.
>
> The cassette socket has the audio industry standard, 5 way 180deg
> pinout plus an optional motor control circuit.  The RGB socket uses
TTL
> levels, so a 240deg socket was needed (6 way.)  The serial port is
> driven at +/- 6V,

+/-5V actually.

> and a 5 pin audio plug would fit an 8 pin offset
> socket, so that left 5 pin domino for the RS423.

There are other plugs that would have been suitable.

> As much as Acorn wanted to offer null modem and straight-through
> connection with one cable, they had to prevent two line drivers being
> connected together.  As it stands, the outputs connect to inputs
> whichever way round the plug is inserted.

I don't believe that's the reason.  You won't normally damage RS423
line drivers by connecting two together.  When I was at Acorn I was
told that it was intended to use the domino plug wired diagonally to
allow for reversing the signals, but it was laid out incorrectly and
there wasn't time to change it.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York