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Ancient Homebrew Z-80 System

2016/01/13

This would be my first computer, c. 1981. It's a Z-80 processor with 256 bytes of memory. I designed it myself and built it with wire-wrap methods. As I recall it took one or two evenings to assemble it and quite a bit longer than that to figure out why it didn't work.

The Z-80 - which is essentially what's in a TI-84 calculator - was the Pentium of the early 1980s and powered many high-end business systems. It could run at up to 4MHz.

This one is clocked by a 555 timer chip at a sizzling 5 KHz, so it's not exactly a powerhouse. The clock speed is adjustable and since the Z-80 is a "static" processor, meaning it'll clock right down to zero Hz and still operate, you could slow it down enough to watch the activity on the individual data bus lines as the machine did its thing.

Here's the wiring side of the board. Despite its appearance, these wire-wrap connections were actually quite reliable. I fired it up the other day and it still works, 35 years on.

It's programmed with the two banks of slide switches you see in the first picture. You set up the address in binary on the top row of switches and the machine language instruction on the bottom row. This has to be done for each instruction. Z-80 instructions consisted of up to three bytes each, so programming tended to be a leisurely process.

With judicious use of the Z-80 reference manual and a notebook of the pen-and-paper sort, such monumental computing tasks as integer addition and subtraction could be accomplished. I spent hours demonstrating it to my parents one evening, switching in a program that used a delay loop to generate a tone in a speaker. They were somewhat less than impressed but for me it was magic.

 

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