Re: Baffling SCSI

[Wayne Stewart <waynes@intergate.bc.ca>]

David Empson wrote:

>
> 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.

I tried exchanging the socketed chips with a good card and everything
remained the same

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

That was the first thing I tried

> 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.

I was thinking towards a defective PAL, now mayhaps it is repairable