Well, I've been busy playing with my logic chips and an 8bitbaby (which is a generic retro interface card) and the Plus/4. I'm currently trying to map a small range of I/O addresses ($FE00-$FE7F) to a LED.
That is, whenever this address range is accessed, the LED should light up - in theory anyway.
After fixing it several times - mainly because the IC diagrams were confusing, particually as I was wiring the thing upside down! I'm getting more and more respect for folk that have been doing this for years, it takes a really steady hand and serious dedication as you have to quadruple check things to make sure its all working! (not to mention follow messy wires all over the place)
This is my first real attempt at actually building a whole circut that I've designed, away from a breadboard and its taken ages! I think what I really need to do is build an interface that I can hook-up to a breadboard; things will go a lot faster if Im not soldering all the damn time!
However, all that said, it does actually work! If the program hits locations $FE00 to $FE7F, the LED comes on!! How cool is that!
Oh...and I also see why people use GAL's everywhere now; they are ideal for this kind of thing! I have a couple ordered, but no sign yet..... and although my EPROM programmer can write to them, I have no software that builds the binary file TO write to it?!?!?! So I've not a clue how to physically use them yet! This image shows the logic I've used... which is also why its a bit ugly. I guess I could use an EPROM instead of a GAL for now....
Edit: Oh...and looking at that diagram, yes...I really need to get some basic AND and OR gates!