Date   : Wed, 23 Oct 2002 22:59:27 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: BBC motherboard from a PC power supply

On Oct 23, 22:58, Richard Gellman wrote:

> > I have a spare Model B motherboard and keyboard and would like to know
> > if anyone has had any experience with fitting the motherboard into a
> > PC case and using a PC power supply to power it? Is there any info on
> > the web about this?
>
> Its possible, but has to be done in a certain way.
>
> IIRC, the BBC Micro requires 4 isolated supplies, 3 of +5v + 0v, and one
> of -5v (this is a purple wire in my Model B).
> The -5v line and at least one 5v/0v pair can be obtained from the main
power
> connector of the PSU. The other two you might have to gain from the drive
> power connectors.

Er, that's rubbish :-)  The supplies aren't isolated at all.  It's a simple
single +5V at about 2A and -5V at only a few mA, and that's only required
for the sound and the serial interface.  The three +5V/0V cable pairs are
commoned at the PSU output.  The reason Acorn had them built like that was
just to avoid voltage drops across the board.  In fact, the three +5V
supplies are also interconnected on the BBC circuit board.

Oh, and by the way, this technique of using multiple cables coming from the
same source is a common one.  In fact, PC power supplies do it for the
motherboard and dirve power leads -- they're all commoned.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York