Soldering (so much soldering!), oscilloscopes and logic analysers! My experience building an RC2014 Z80 based computer kit.
The kit consists of a backplane, Z80 CPU board, RAM, ROM, Serial I/O and a Compact Flash storage card. Currently it's running MS Basic, but later I'll get CP/M running and explore that.
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https://www.youtube.com/watch?v=2HVt1FPGzpA