19 Jun
2012
19 Jun
'12
6:17 a.m.
On Mon, Jun 18, 2012 at 7:58 PM, Camiel Vanderhoeven
<iamcamiel at gmail.com> wrote:
So, it looks like the memory chip that controls bit 10
for addresses
ending in 01 (binary) is at fault here. I'm going to do some wire
tracing to find out which chip this might be...
Did my wire tracing, came up with the following picture of the board
(use a fixed-width font to view):
+-----------------------------------------------------------+
| 15 14 bit 1 0 |
| +u+ +u+ ................................. +u+ +u+ |
| | | | | | | | | |
| 0xxxx00 | | | | | | | | |
| +-+ +-+ +-+ +-+ |
| |
| +u+ +u+ +u+ +u+ |
| | | | | | | | | |
| 1xxxx00 | | | | | | | | |
| +-+ +-+ +-+ +-+ |
| |
| : : |
| : : |
| word : : |
| : : |
| : : |
| |
| +u+ +u+ +u+ +u+ |
| | | | | | | | | |
| 0xxxx11 | | | | | | | | |
| +-+ +-+ +-+ +-+ |
| |
| +u+ +u+ +u+ +u+ |
| | | | | | | | | |
| 1xxxx11 | | | | | | | | |
| +-+ +-+ ................................. +-+ +-+ |
| |
| |
| |
+-+ +---+ +-+
+-------------------------+ +-------------------------+
So at least I can now trace an error bit to a chip. However, I must
have damaged something in the process (believe me, I've been careful),
because for all addresses ending in xxxx10, the data reads back as
000000 now. I hooked up my logic analyzer, and found that data dis
written to the memory chips in this row correctly. The data read from
these chips is also correct. The data is then fed to a few 74153
4-to-1 MUXes. I checked the selection inputs to the MUXes, and these
are wrong for row 2. Now I need to see where those inputs came from...
To be continued...
Camiel.