IBM 5161

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Apr 2 18:08:49 2003

> >But in the XT, one slot is different.
>
> Why?

There is a 74LS245 buffer chip that buffers the data lines to the
leftmost 7 slots and to other parts of the machine. Unfortunately, this
chip doesn't have enough drvie cpaability to handle all 8 slots (well,
not offiicaly).

Most manufactueres would have put all 8 slots on the same side of the
buffer and trusted to luck. IBM didn't -- slot 8 is on the 'wrong'
side of the buffer. A card in slot 8 must pull pin B8 low during a read
cycle, to control said buffer chip.

The IBM Async card can do this (if you put a jumper on the 2 pin header
on the card). A few other cards can too (the Microsoft Bus Mouse card is
one I think). And I've made cards that can do this.

-tony
Received on Wed Apr 02 2003 - 18:08:49 BST