Date   : Sat, 02 Jun 2007 12:04:03 +0100
From   : dm.hunt@... (David Hunt)
Subject: PC/Beeb serial cable woe

> > > 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.
> 

Yes, you're right. I just assumed (yikes!) since the Beeb had no chassis
ground connection and I knew the PC was earthed, I forwent the chassis
ground altogether and used the screen as the signal return. In retrospect,
that is probably bad and I should have an extra wire for signal ground and
connect the screen to chassis ground at the PC end. I guess if it didn't
work, I put in the extra effort to eliminate the hum.

Dave ;)