5 Nov
2005
5 Nov
'05
4:33 p.m.
What does it
do if you press 4 repeatedly? Does it flash the bottom 3 or
bottom 2 digits? If it's only updating the bottom 8 bits, you see, then
only 2 digits will flash from 0.
=
Indeed, pressing the "4" only flashes the last two digits!
How did you know that ?! Next, the response on the display from the keypad. I
always
started with pressing "CLR", then repeatedly the same key.
The rightmost (lsb) display is called "A", the one next to it
is called "B", the leftmost (msb) display is called "F".
=
button display reaction (1st time, 2nd, 3rd, etc)
--------------------------------------------------------
0 nothing to see
1 nothing to see
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.
[...]
Last 3 signals are the display values (3-bit) to the
7447 input
A, B, and C. On connector J1 pins 12, 11, and 10 respectively.
As always, the last thing to check should have been the first!
These 3 signals are continously 0 V. That explains why the display
always shows "000000". I can see a short positive pulse when a key
of the keypad is pressed.
=
This leads me to the following (early) conclusions.
1) the keypad is scanned. All "X" signals of the matrix (READ x H)
are OK, and all "Y" signals of the matrix (the 8881 outputs) are
all valid, and give the correct information when a key is pressed.
2) When a key is pressed it is momentarily shown on the display, but
then the value returns to all zero. I can not see the value, so
it could be any garbage ...
Since I haev 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
-tony