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Category: Digital Projects

It's Alive: Z-80 Homebrew

It’s alive! Z-80 Computer hardware now complete.

I have just finished assembling the memory board of my Z-80 homebrew project. All components individually tested, I placed the test program on the ROM, inserted the Z-80 and it worked first time! There is still a little more to do on modifying the I/O board to run the monitor program (which also needs writing) as the board was designed to test the various different circuits. However, the computer successfully executes the following test routine: ;COMPUTER TEST ROUTINE ; ;OUTPUTS ON PORT 0 ANY INPUT ON PORT 0 WHILST CALLING AN EMPTY SUBROUTINE ; ; LD SP,9000H L1: IN A,(00H)…

Z-80 Update

Unfortunately, it has been a long time since I could work on my  Z-80 homebrew; it has been in storage for many months. However, my setup is active once more and I have built the processor board. It consists of a power supply, clock, reset logic and buffers for the data, address and control busses. Unlike the I/O board, whose schematic is only in my head, I have used KiCad to design the circuitry and I will upload that separately. On a separate board lives the I/O decoding and memory, on which I’ve made a start (see picture above). It’s…

CMOS Logic Computer

8-bit CMOS Logic Computer

This is my 8-bit computer built from CMOS logic gates, based on the SAP-1 in Albert Paul Malvino’s Digital Computer Electronics. It was hard to avoid creating a rats’ nest of wires, but there is (surprisingly) about 40m of wire in total. Its hard to gauge the amount of detail to describe this project in; there’s enough for a whole book’s worth describing it from the logic gates up, so I’ve provided a brief overview and a video that shows how it works in action! The computer is fully programmable and it can add and subtract numbers from 0 to 255. The computer…