I will post my progress on my homemade MMU and IOU here in this thread.
I just made my sources for the MMU publicly available here: https://github.com/frozen-signal/Apple_IIe_MMU_IOU
These sources are licensed under Creative Common Zero (public domain). I wanted to wait before releasing them until I did due diligence and identified the problem, but that may take a while so I made the repo public this morning.
The status is still the same as last week (I'm waiting for some 74HCT245s to arrive); the Apple IIe will boot and display the "Apple ][" logo; I can drop to the AppleSoft and write a program and it will run just fine. I can also drop to the monitor which also works and the soft switches works exactly as expected.
The problem is if I hit a key several times, the computer will eventually beep and drop in monitor. Trying to load a game will also fail; sometime it will drop to monitor, other times it will load a few sectors and then freeze and do nothing.
It's possible that it is a timing problem, but a 1MHz frequency should not really be a problem for the TTL and the FPGA. So, I haven't checked that possibility yet.
Another possibility is that the ton of wire and the added length introduce a delay or noise that crashes the computer. I could test this by using a similar breadboard with a similar wires setting, but use the real MMU instead of the FPGA to see if it works or not.
Also, my prototype is a bit of a sloppy work: there are floating pins on one of the 74LVC245 and there are no decoupling capacitors. I don't think it's the cause but it doesn't help for sure.
But my current hypothesis is that the output from the homemade MMU (3.3v) is too low for a logic HIGH. I have ordered a few 74HCT245 to level-shift them to 5v but, being in Canada, most of our nice things comes from the U.S. and takes forever to be cleared at the customs. I should receive them next week and then I will know if it's a voltage problem.
Finally, I could wire a real MMU and the homemade one side-by-side and look at the differences for each signal on an oscilloscope. That would be kind of a pain to do so, I will try doing this last.
In any cases, stay tuned for any progress.