Date   : Sat, 02 Jun 2007 07:47:49 +0100
From   : info@... (Sprow)
Subject: PC/Beeb serial cable woe

In article <036601c7a444$c4e4aa40$3201a8c0@...>,
   David Hunt <dm.hunt@...> wrote:
> > Although these may work in the right circumstances, it seems 
> > less chancy to connect the 0V pins together to make a proper 
> > signal ground.
>
> The screen of the cables is ground. I can't see how an additional ground
> wire is going to make any difference?

The screen is often only ground at one end, and it may well be earth (ie.
metal touching the chassis which is earthed to the earth pin in the socket)
which is different to signal ground (via the power supply, ie. 0V, though an
isolated power supply).

Pin 5 on the 9 pin D is definitely needed, the screening is optional and
shouldn't be joined to pin 5 in any way in the cable.

This becomes more important over long distances, say between two buildings,
when there can be a significant potential between the one earth and one 0V
and the other, so joining them with a wire makes a nice big ground loop -
defeating the purpose of the screen,
Sprow.