CS/A65 RAMDisk

This is the current set of errors. The spurious counter pulses seem gone:

1) I wrote some test programs (not yet very sophisticated, but working The programs work (so far) in the lowest 64k of the RAMDisk. What I found out that there are only two addresses in a page, that give problems - $45 and $65. And that only on some of the pages, pages $1x, $5x, $9x, $dx. Interestingly the two addresses have different problems:

1. when writing the byte offset in the page (or its invers) into the byte, both addresses show read mismatches. A printout of the values shows that mostly the value is converted from $45->$65 for the $45 case. $65 results in high nibble 2,4,6,a,e and low nibble 1,4,5,7,d in combinations. When I write the invers of the value, interestingly $45 (xor $ff) still gets $65 (xor $ff), while $65 is inconclusive. For a further analysis I guess I have to save that on disk and run an analyzer program on the PC on it, if necessary.
2. when writing the page address into the byte (or its invers), only $65 shows mismatch
3. when I write the 256 pages once, and read them each 256 times, $45 consistently has an error count that is a multiple of 256, which indicates a write error, while $65 has a error count where the low byte is not 0, which proves a read problem.
4. all other addresses are rock solid.
5. same behaviour appears when using a different RAM module, or a different bank in the same module.

I don't yet know what to make of that, though. The bit pattern don't make sense to me, esp. address bits 12/13 (that seem to have to be fixed to 1/0 to give problems) and bit 5 (that seems to give problems) are handled in different ICs.

2) I scoped the supply lines for the ICs and the DRAM modules. I found a jitter of up to .3 Volts on each line (GND and VCC). So I added a .1uF multilayer capacitor to the 'LS257 that generate the multiplexed RAM address lines, and a 1u MLC in parallel to the already existing capacitors in the DRAM supply lines. The jitter did not improve much, although I had one effect: before adding the caps the problems mentioned above in 1) were also seen in addresses $47 and $67, and sometimes $44, $48, $64 and $68 - which is fixed now.