15 Jun
2011
15 Jun
'11
1:09 p.m.
On 2011 Jun 15, at 11:14 AM, Tony Duell wrote:
I thought I understood that he'd put the ROM he removed from socket 4
(to
prevent the Pac auto-booting) into the location 3 and found he could
read that chip correctly. Which would seem to indicate that the
selection
circuitry is fine and that the original ROM #3 is defective.
Right, that's checked then. I think I saw the addressess with x000 in
them and thought he was using the lowest chip location for the test.
I removed the number 4 chip, plugged in the ROM-PAC
and booted.
Success, I was in the monitor. I then dumped the memory locations
corresponding to the ROMS. The two lower ICs (C000-CFFF) showed all
different hex numbers. In other words it looked like it contained
good code. However, the third IC (D000-D777) contained all zeros!!
I then swapped over the D800-DFFF chip I'd removed into the socket
which originally contained the D000-D777 and checked that number 4
chip. The code looked good and I could even see the bootstrap code
at
the end at DFFD and DFFA (of course it was at location D7FD and D7FA
as I had this number 4 chip in socket 3).
Conclusion. It's the number 3 IC which is faulty.
Next task. Unpack the EPROM burner and try to figure out how to use
it.
Great work!
Just a suggestion, one more thing you might check, as confirmation, is
to place the suspect ROM chip in another location and look at the
contents from there, to confirm that it is the ROM chip and not the
ROM-selection circuitry or something particular to that chip position
causing the problem (although it probably would show all-FFs if it was
the selection circuitry).