I needed an EPROM board to complete this computer and i could not see any easy way to obtain an eighties era card, so i had to knock one together myself.
I started with an S-100 Prototype board designed by John Monahan. This has all the buffers etc that end up on nearly every S-100 card, so a lot of the hard work is done.
http://www.s100computers.com/My%20System%20Pages/Prototype%20Board/Prototype%20Board.htm
John’s board designs are available for anyone to produce (as far as i can tell) and they are easy to get in the States. Not so easy in Australia, but on this occasion a batch turned up in Australia so a bought a few.
I had what i thought was the boot ROM code on an 8″ floppy disk. I’d previously extracted the files so i could have a good look.
The boot code indicated that the boot ROM would be located at F800 and occupy 2k. The Jade DD floppy disk controller uses a 1k window at F000. CP/M is set up for 60k, so there is also 1k free at F000. The boot allocates this to a Micropolis boot ROM similar to the one in the Sorcerer/Micropolis System.
I allowed for an 8kB EPROM (2764). J1-3 sets which address lines are used for decoding. To get just the top 2k, J1=J2.
Switch 1 sets the address of the ROM: in this case 00F800: 0000 0000 0000 1111 1000 0000 0000.
The highest address line decoded is A18, so the ROM will reappear at 04F800 but I don’t expect to ever have that much memory. That allows room for 256k of RAM.
My design worked immediately … no, of course it didn’t! I made several mistakes and i had trouble driving the phantom line (which disable RAM at the EPROM address). I eventually settled on the following:
I programmed an EPROM with the boot code that i found on the floppy disk.
There’s plenty of space left on the prototype card for some I/O if i ever run out of things to do.