26 Oct
2001
26 Oct
'01
7:36 p.m.
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(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Friday, October 26, 2001 4:45 PM
Subject: Re: SLOT 8 (was: ISA cards for free..
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
What's different about that slot?
"Wrong" side of a buffer. > > 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