Re: Baffling SCSI

[dempson@actrix.gen.nz (David Empson)]

Wayne Stewart <waynes@intergate.bc.ca> wrote:

> I've come up with a baffling problem. I wanted to find out something
> for someone and dug out a Hi-Speed SCSI card. Problem is this card
> will not function on any IIe however it'll work fine on a IIgs. Other
> Hi-Speed SCSI cards will work fine with all these machines and hard
> drives.

Bang goes the "perhaps it isn't an enhanced IIe" theory.

The only thought which occurs to me would be an electrical or timing
problem, which allows it to work in a IIgs but not a IIe.  This is
probably a fault on the card, perhaps in the big ASIC.

Have you tried running it in non-DMA mode (controlled by switch 1)?

The High-speed SCSI card has some way of identifying whether it is on a
IIe or IIgs and adjusting its DMA behaviour: on the IIgs it can steal
all cycles, but on the IIe it only steals every second cycle, so that
the processor registers are refreshed (they could lose their contents
over a sustained DMA burst).

If this detection method involves a hardware pin (e.g. looking for the
muPSYNC signal in a IIe) then there could be a track fault, simple
component fault or broken pin, causing the card to think it is running
on a IIgs when it is actually in a IIe.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand