Date   : Wed, 27 Jul 2005 19:23:05 +0100 (BST)
From   : Greg Cook <debounce@...>
Subject: Re: Null Modem Cable

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

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.

So why is there a 5 pin 180deg socket on the Econet?  In the Beeb's
heyday, those machines that had Econet were generally installed by
professionals, complete with cable... and no cassette.
Hence 8 pin offset sockets were an overspecification for an "Eco" net. 
Or it may have been an existing Acorn standard.

Greg




       
       
               
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