Date   : Tue, 17 Mar 2009 00:00:12 +0000
From   : pete@... (Pete Turnbull)
Subject: Another set of dumb questions...lol

On 16/03/2009 22:43, Greg Cook wrote:
> 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.

They were indeed designed to be different to minimise the risk of 
connected the wrong device[1], but the domino plug for the RS423 was not 
the only possible choice.  I was told it was chosen precisely because it 
can be used two ways round[2], and it was intended that one way would be 
normal and the other would be reversed (null modem style). 
Unfortunately the connections on the PCB were made wrongly (the Tx and 
Rx should be diagonally opposite, ditto RTS and CTS), and by the time it 
was realised, it was too late.

[1] Not that it stops everyone.  I've seen all sorts of things plugged 
into the RGB sockets on Microvitec monitors, and it was once common to 
see the RGB plugs upside down, resulting in a rather red or magenta 
picture scrolling diagonally across the screen.

[2] Domino plugs were originally intended for headphone sockets on 
devices like TVs.  Plugged in one way, the loudspeaker is disconnected 
by a switch in the socket; the other way, the notch prevents this.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York