Hi there, I am a newbie to this site and I'm trying to get my Apple 1 replica (NTI based) to work.
Video section did work immediately, except the character field seems to be a bit too much to the right, could be the monitor though.
DRAM does not work yet. The lower bank passes Mike Willegal's RAM test as long as the $EXXX bank has empty sockets.
It does not seem to be a problem with the !CS signals, and all the other signals on the DRAM are the same for both banks.
Board has been checked, no shorts or hair cracks, perfect soldering, bypass caps in every DRAM socket.
The 74123 one shot set to 480 ns.
I tried to get some more insight by searching this site with "Apple 1 DRAM", but found nothing that would help.
Maybe the timing of the whole machine is off due to some unknown issue with the 40+ years old chips.
Does anyone happen to have oscilloscope pictures how the timing of the Apple 1 should look ?
For the video issue, a snapshot of a horizontal line including the sync and some characters on the left hand side would help.
For the DRAM issue, CPU PHI2 vs. RAS, CAS would help. The row/column strobe to the 74S257 perhaps, too.
I think such scope traces taken from a known good Apple 1 replica could help me and other builders.
Any timing violations due to faulty TTLs must be ruled out before digging into the more elusive topics like reflections etc.
Anyone here knowing where to find such pictures with the correct timing or able to post them here ?
Maybe I'm asking too much, but I am no beginner with vintage computers and never saw such weird behavior of the RAMs in any other machine. The very same DRAM chips that work in the lower bank fail in the upper bank. Despite the !CS signals look OK (no clash). All signals of course look terrible as would be expected from this layout driven by 74S logic, lots of reflections seen, but none looks bad enough to cause the DRAMs to fail. I would hate to spoil the authenticity of the build by putting termination resistors in.
Lots of questions, I know, but any hint is much appreciated.