So I've got far enough with my Halcyon clone project to get the CPU working properly. I wrote a small test program to test RAM I/O and also that the second EPROM (U32) can be read properly. Right off the bat, my test program reported a RAM test failure.
After doing some digging, I isolated the problem to pin 20 of the RAM chip not having the proper timing. This pin is driven by pin 8 of U18, as shown here:
A logic analyzer capture of a looping RAM read/write operation revealed the following on my clone PCB:
Pin 8 was starting and ending late. You can compare this to the original Halcyon where pin 8 much more responsive:
Being utterly confused, I decided to hook up my scope to get a better view of the problem and saw this:
Finally, I looked again at the schematic and noticed that there is a capacitor attached to this line. Doh!
The problem ended up being that C39 needs to be a 0.001uF cap, not a 0.01uF cap. Once I replaced the cap with the proper value, things started working.
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