Foward to the past

OK, I admit it I am a bit old when it comes to the technology business.  I built my first "computer" such that it was in 1977 when I was thirteen years old.  I still have it in a box someplace too.

So this started out as an exercise to show my kids (whom I love dearly, but see computers simply as commodities) just what is involved.  Before you all think I am a complete sadist one is studying mechatronic engineering, and another wants to work in IT when he finishes school.  (There's also an architect and a three year old who wants to be a doctor, but I digress)

The plan was pretty simple.  Build a functional Z80 system from scratch.  Simple in principle, and hopefully execution as well.  When I mapped out the task I came up with the following:

  1. Create a simple design, based on datasheets.  Processor, RAM, ROM, Serial and Parallel I/O
  2. Get all the parts together
  3. Assemble it on a set of breadboards
  4. Write a simple monitor ROM
  5. Find something meaningful for it to do when I'm finished
After about a week of mucking about I have come up with a 12-chip design that will do everything I wanted.  I could cut it back to 8 by eliminating the buffers for the CPU, but then I may want to connect it to something bigger next time, so better to start as we mean to go on.

The biggest complication so far was that 64Kx8 SRAM is almost impossible to  find these days, so I decided to use a 128Kx8 (AS6C1008) and just tie A16 down.  There's no attempt to do bank select or anything so esoteric at this stage.

I also allowed for a 32Kx8 EPROM and have built in full address decoding and switching so I can take this out of the address space if I want to try and implement CP/M or similar in the future.  The MicroBee did this years ago, and while I didn't copy their schematics I hope the designers don't mind me borrowing the design.

I will post the schematic here shortly, once I export it from my design package.  Feel free to use some or all of it for any non-commercial purpose.


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