11 Feb
2006
11 Feb
'06
2:23 p.m.
On Sat, 11 Feb 2006, Tony Duell wrote:
For some reason, the data lines on slot 8 are wired to
the other side of
a bus buffer chip to the data lines on the other 7 slots. A card in slot
8, therefore, has to assert a special signal on the ISA connector to
enable this buffer -- my memory says it's that it must pull pin B8 low
during a read access to any device on that card.
The IBM Async card could do this (fit an unmarked jumper on the board).
Some Microsoft mouse interface cards did this too. I homebuilt one card
that did it (I needed it to go in slot 8 of an XT). AFAIK no other cards do.
We don't know what IBM ORIGINALLY intended to do with that slot.
Whatever it was, might not have ever happened.
When the XT came out, IBM did not want people to plug other boards in
there. They also had a horrendous surplus of some boards, such as the
Asynch (RS232 & 20mA), due to the popularity of "multifunction" boards.
So the XT was sold with a "FREE" Asynch board already installed in slot 8.
--
Grumpy Ol' Fred cisin at xenosoft.com