Thanks for the kind words!
Way cool, Wickey!
Ever since I read an article about diode matrix rom I've wanted to do this. Good on you for beating me to it.
Thanks Rev! Hope I don't discourage you from making your own though, It's always fun when you can get something like this working.
This is awsome! I wonder what a 1K board with a mini-game would look like
If the diodes are the ROM, what's in the IC's?
1K would be a ridiculous amount to wire, but it would definitely be cool to see! I considered going higher to fit a small game, but as you can see the boards I use just barely fit 64 bytes. It would have been cool to do 256 bytes so I could fit my 256 byte pong clone
The IC's are used for address demultiplexing and buffering mostly. Since the data bus is shared among many other chips, it is necessary to disable the cartridges ability to control it by placing it in 'high impedance' mode. You need a tri-state buffer for this. The address demultiplexing is necessary to convert the 6 address lines into 64 individual select lines, one for each byte. I also use a buffer on the address lines to strengthen the signal. Not sure if this is necessary, but I didn't want to find out the hard way
The other two chips are inverters, which I used to cut down on the diode count. I figured out that if I used a diode to signify a '1' rather than '0' it would use far less diodes (173 vs 339). They basically just invert the state of the data lines before they go to the tristate buffer.
Individual transistors. I can see it now..
That could get interesting... Maybe a 16 byte ROM made of discrete transistors, resistors, and diodes. I'll look into it!