16 May
2009
16 May
'09
5:19 p.m.
With the service manual saying that that the FAULT
light being on is
most likely indicating a RAM problem, I started looking at the datasheet
for the MK4027 chips used in the 4907. I don't have any of these (or
equivalents) on hand, but I then looked at the datasheet for the MK4116
(a 16K x 1 DRAM), which, I happen to have a bunch of. The only
difference between the two devices is that the MK4116 replaces Pin 13
(~CS) on the MK4027 with A6. All of the other pinouts are identical,
and the voltages and general timing specifications are the same between
the two chips. What I'm wondering is if I could just pull all of the
MK4027's and replace them with MK4116's. In the schematic for the RAM
section of the 4907 controller shows the ~CS pin on all of the MK4027's
is wired to ground. This would have the effect, with a 4116 plugged in
place of the 4027, of forcing the A6 address line 0 at all times, which,
if my brain is working properly (there are days when I wonder), should
not have any effect on the 4116's ability to serve as a replacement for
the 4027s.
The problem comes when the CS/ pin of the 4027 is used for something. In
that case, if you replace the 4027s with 4116s, you are likely to end up
with a bus contention, since the 4116s will probably not be disabled at
the right time.
But if CS/ is grounded, the selection must be done in some other way
(gating the CAS/ line is the most obvious). In which case there is no
problem.
It should work fine.
-tony