PseudoDisk][ and IIGS

["Alex Freed" <alexf@mirrow.com>]

One other person confirmed my observations regarding the IIGS.
Bill Garber told me that he also noticed flaky behavior on a GS
"getting much worse at high speed".

To test the theory I had about putting the data on the bus too early
I have wired one extra signal - Q3 - from the slot and modified the
CPLD to use it. As a result the boot process became more reliable
but the other problem remains - NIB images work fine but the
ProDOS image does not. The message "Can't load PRODOS" is
all I get.

The logic analyzer shows that this happens as a result of a checksum
error reading a block. Not the same block every time. So some byte
gets corrupted in transit from the AVR to the IIGS bus. Furthermore
plugging the logic analyzer itself  into a slot makes the problem worse.
And this is a modern tool with rather low load capacitance. This also
indicates a marginal timing on the bus. Yet I don't see anything wrong
with a scope.
The NIB images work because DOS re-reads blocks in case of errors.
Apparently ProDOS doesn't always do it.

If anyone has any information on IIGS slot timing, please let me know.


-- 
-Alex.