6 Nov
2005
6 Nov
'05
12:20 a.m.
Tony wrote:
Interesting, the LSB is being lost somewhere (that
would explain why
1 does nothing, why 5 behaves as 4, etc).
What sort of RAM is on this board (I can't be bothered to find the
prints tonight ;-)), do you know if it's good. I've found old SRAM
to be notoriously unreliable.
Interesting, I had not thought of that, may be a stuck bit.
I must have a better look at the program code listing. Checking out
the RAM is a bit difficult. It consists of two 4-bit chips with
separate data IN and data OUT pins, type 86586 (?).
If the 8008 uses the RAM for storage of variables, I guess that I can
safely say that the RAM (plus associated circuitry to read/write) must
be OK.
Since I have a logic analyser 'to hand', I
would start looking at those
3 inputs to the '47. Where do they come from on the M7859? An output
port on the 8008? Do we know that port is latching correctly? Or more
likely, the 8008 is writing the wrong values to it.
Page KY-4 has all the decode logic. There is a second post to this list
on its way, posted Friday morning ... it's now Sunday morning ...
The signal KY4 READ INPUT 5L from the 74154 is used on page KY-3 to
enable 8093 AND gates to read the scan matrix lines.
The signal KY4 LD REG 0H via a 7404 from the 74154 is used on page KY-6
to enable a 74175 latch of which the outputs drive the scan matrix.
Likewise, KY4 LD REG 1H drives the 3 NUM lines for the 7447.
Before I go on writing long stories here, I think it is best to wait
for the e-mail from last Friday to appear here.
Thanks for working with me Tony, it helps keeping focussed ...
- Henk, PA8PDP.
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