11 Feb
2006
11 Feb
'06
12:56 p.m.
I discovered tonight that the right-most ISA slot (the one closest to
the power supply) in my newly-unearthed 5160 won't take the Silicon
Valley Computer ADP50L IDE controller I'm playing with. It will take
other (very short!) cards, but when I put the IDE controller into it,
either the option ROM is invisible (ie. 55AA not found) or won't
initialize for some reason, because I don't get functionality out of it
(least of all, the boot "banner" produced by the card is missing).
I can only conclude that:
1. The IDE board is slightly awry
2. The right-most ISA slot is slightly awry
3. There is some technical limitation I'm not aware of
#1 and #2 are easy to accept, but not #3. Is there such a limitation?
Yes, it's more (3) than (2). There is no fault with your particular board.
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.
-tony