Actually, if one counts up the loads ... it does require an extra buffer to
handle 8 slots, so it seems they've split the bus between two buffers. Of
course they're in series, which makes them skewed somewhat, but at 5 MHz ... who
cares? Apparently they're counting to a pretty conservative standard, however.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Friday, October 26, 2001 4:45 PM
Subject: Re: SLOT 8 (was: ISA cards for free..
> > > What's different about that slot?
> >
> > "Wrong" side of a buffer.
>
> From a quick glance at the schematics, there's the CPU data bus which is
> buffered by a '245. The far side of that goes to the first 7 slots, and
> to memory on the motherboard, etc. And to one side of another '245. The
> other side of that goes to the data lines on the motherboard I/O chips,
> the BIOS ROMs, and to the data lines on slot 8. The card in slot 8 has to
> pull a line low during reads to enable this last buffer.
>
> No idea why they did this, if indeed there is a good reason.
>
>
> > > I've never owned a "real" XT, so I've never
> > > had to wrestle with that. My first PC was a '186-based clone, and I've
never
> > > looked back. Was that "slot-8" compatibility creature a bug in the PC as
well?
> >
> > IBM had a LOT of serial cards that nobody wanted (on a 5 slot PC, the
>
> Although the IBM Async card does have the current loop feature which most
> clones missed off. Of course I'm about the only person to want something
> like that :-)
>
> > market wanted multifunction!)
> > Every XT from IBM came with a "FREE" serial card. It blocked slot 8 from
> > being used by anything else that MIGHT have a problem with it, and gave
> > the public image impression of a generous (they were more expensive then!)
> > freebie.
>
> IBM screwed up a bit on the design of the portablePC. This uses a normal
> XT motherboard (with the slot 8 problem). But the positioning of the disk
> drives means that an Async card will just not fit in slot 8 (it's
> litterally about 1/4" too small a space). So slot 8 is essentially
> wasted. IMHO that machine should have been a little deeper from front to
> back (nobody would have noticed) so that the async card would fit in slot 8.
>
> -tony
>
>
Received on Sat Oct 27 2001 - 02:36:37 BST