VAX Cluster on Ebay in OK (was Re: More DEC equipment spottedin

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed May 5 18:30:44 2004

> > Incidentally, 'There is no such thing as groud' is one of Voanda's
> > laws and has 2 meanings. The first is the one I am using there, that
> > voltmeters have 2 leads, and it's arbitrary where you connect the
> > 'refernce' lead to.
>
> I was speaking in an electrician's sense, not as in an engineer's sense
> of the word.

What's the difference? Even when working on power-distribution systems,
voltmeters have 2 leads. And the output of a transformer is isolated from
ground, so you can connect any single point in the circuit to the ground
rod, and call it your reference point. Sure it's conventional to use the
start point of a star (Y) 3-phase supply or the centre-tap of a
centre-tapped supply, or whatever, but you're not required to do so.

You may have guessed I have little time for 'electricians' who follow the
rules blindly and have no real clue as to what they're doing or why.
Actaully, I have little time for anyone who follows the rules blindly
without a clue as to why!

>
> > The second is that any real connection has impedance, and that the
> > 'ground' in a high-speed digital circuit may not be quite the same
> > voltage everywhere. In fact 'ground bounce' -- mostly due to the
> > inductance of the ground connection -- is a problem in laying out high
> > speed circuitry.
>
> That doesn't apply as much to AC power wiring, though you still have to

Ere, watch out. RF interferece can, and does, pass through common earth
wires in some cases. The impedance to ground can be signfiicant.

And it certainly matters on the classic computers we all know and love!

-tony
Received on Wed May 05 2004 - 18:30:44 BST