Date   : Mon, 16 Mar 2009 22:43:57 +0000 (GMT)
From   : debounce@... (Greg Cook)
Subject: Another set of dumb questions...lol

On Monday, 16 March, 2009, 9:43 PM, Alex Taylor <zeem.uk@...>
wrote:

> I've got one Beeb serial lead (which I took to Byte Back
> just in case,
> but never needed). I've always wondered why they chose a
> socket design
> which can go in either way round. Is it so you can reverse
> the Rx/Tx
> wiring by rotating the plug, or does it only work one way
> round?

I have a theory that this was to prevent the three standard DIN sockets 
in the back accepting each other's plugs.  3/180 and 5/180 were standard 
audio connections then and so 7/270 was adopted for the cassette port; six 
pins were wanted for the RGB output, bagging 6/240, and as RS 423 needed 
at least five conductors, the 'domino' plug was the only remaining option.
 This left nothing for Econet which thus kept on using cheap 5/180 plugs
but as Econet was mostly found in managed environments this wasn't such a
problem.

Unfortunately the same RS 423 cable cannot be both a straight connection 
and a null modem; Acorn wisely wired the serial socket so that output always
goes to input to avoid harmful contention if inserted upside down.  The only
effect is that the data outputs are connected to the semaphore inputs and
vice versa.

Greg Cook
debounce@...
http://homepages.tesco.net/rainstorm/