Date   : Thu, 24 Oct 2002 07:14:50 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: BBC motherboard from a PC power supply

On Oct 24, 12:01, Michael Foot wrote:
> Quoting Richard Gellman <r.gellman@...>:
>
> >
> > >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.
>
> OK. So far so good.
>
> >
> > 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.
>
> Is it not possible to have all 3 sets of 5v/0v leads coming from the main
> power connector? ie. connect 3 wires to the 5v output and 3 wires to the
0v
> output.

Yes, as I wrote in another reply, that's exactly what a Beeb PSU does.

> > There's certain enough power in an AT PSU to drive a Model B at any
> > rate.
>
> More than enough I'd imagine - depending on the wattage of the PSU.

Beeb takes about 2A (depending on how many ROMs and other add-ons it has)
and a 150W AT power supply will typically provide up to 12A, so no problem
there.  The only thing to watch out for is that a Beeb PSU will happily run
with nothing connected to it, whereas some AT PSUs need a small load and
will be damaged if they are run with nothing at all connected.

> > Failing all that, you can always build an isolation grid with 1:1
> > isolator
> > transformers.....
>
> Um, probably way beyond my technical skills...

And most other people's :-)  Transformers won't operate on DC :-)

> The other consideration is the keyboard. I'm thinking I might put it in a
> custom box and make up a long lead to run to the AT box. How long can I
have
> this cable before it becomes unreliable?

Not very long.  The old Viglen separate keyboard/box setup managed about 2
feet.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York