6 Nov
2005
6 Nov
'05
9:06 a.m.
Allison wrote:
I'd consider that the 8008 is getting bad input
data from a bad scan line
which it will output or that it's outputing correctly and the data is
corrupted after output due to bad latch or other hardware.
One likely possibility is not ram as a prblem but a I/O port that has a
stuck bit. Or the control for that port is stuck. This is most likely
if the basic fuctions are there (8008 program runs) but some parts of
the data used to control the 11/34 or display it's status is not correct.
FYI the 8008 can be single stepped if you pull the "READY" line each
cycle and just look at the bus with a simple logic probe or clip.
Thanks for the good input Allison!
I hooked a logic analyzer to the address lines A0-A7 on one of the program
ROMs, and simply sampled the data without any trigger. With the display in
octal mode, I can see the executed address sequence. The 8008 executes the
main loop and calls SHFT1 *and* returns to the correct PC. That means that
the ROM, RAM (to store the return address) and associated circuits must be OK.
Thanks for the tip, but I hope (after studying the LA usage) that cycle stepping
will not be needed. Now that I have the LA set up (first time use!), and checked
the program execution, I will refocus on the scan matrix. With the 'scope, I had 2
time traces at best, with the LA I can have 16, thus *all* signals for the keyboard
and the display (because scan keyboard drive == common display drive).
I will post any progress!
- Henk, PA8PDP.
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