Fixed the 8088 board in Commodore 610

Dear all,

I would like to report a success in getting the 8088 board to work in
Commodore 610 :-)

As I mentioned in my posts a year ago, the primary reason that the board
was not working is that a memory is getting corrupt during IPC calls
from 8088 to 6509. The corruption pattern (every 128th byte) suggests
that the DRAM signals (most probably CAS and RAS) are getting wrong and
thus corrupting a whole DRAM page.

The RAS and CAS signals are supplied to the DRAM chips directly from the
PLA. During 8088 operation, the PLA drives these signals according to
the ECAS and ERAS signals obtained from the 8088 board. These arrive via
the P9 connector.

It is easy to observe that on the high-profile motherboard, the PLA chip
is located just near this connector, while on the low-profile
motherboard it is far way from it. That means that these signals need to
travel much longer way from 8088 board to PLA, and possibly suffer
interference from other signal traces running on the motherboard. Thus,
from time to time they get corrupted.

The solution is simply to solder additional wires between these points;
that is, from pin 40 of connector P9 to pin 26 of the PLA, and from pin
38 of connector P9 to pin 25 of the PLA. This gives the signals an
alternate way of going through, reducing interference. Probably these
wires should be well shielded to minimize problems; I just soldered two
thin ones and it works about 80% of the time. When time permits, I will
buy a good shielded wire.

Here is a nice boot screen after the fix is applied ;-)

http://a2.sphotos.ak.fbcdn.net/hphotos-ak-ash4/395995_10150482661642026_708927025_9004400_903114315_n.jpg

Therefore we now know why the 8088 board does not work in stock 610
models. It's not a power issue, not a PLA issue, not a chip speed issue,
not a signal termination issue. It's an issue of signal interference on
the motherboard. With two pieces of wire, these problems can be easily
overcome.

Regards,
Michau.

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