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When printing from Eagle (a PCB CAD software), you can print only one layout per page. Usually I need two since when I print to transparency, I like to overlap two identical images to get a better "black" for photo-etching.

Using this method I can compose several layouts into a single page to be printed. This also results into less waste.
...continue reading "Printing from Eagle"


I got hold of an old Sinclair ZX81 in apparently good and working condition!

ZX81 was the first personal computer I owned. I was 13 years old. At that time, I copied its 8K ROM, byte by byte, into a notebook, and hand-disassembled it (a consequence of which I still suffer from: I still remember some Z80 opcodes), but I never opened it. Finally, now I can do what I missed 🙂

This post shows how it generates TV image and how its Z80 CPU boots. I instrumented it and captured scope and logic analyzer images as it was powering on.
...continue reading "Sinclair ZX81 lives on!"

This article is a follow-up to the "RS232 and Raspberry Pi" (link)

The same MAX board that I had also worked with Arduino at 5V although it was designed for 3.3V operation (capacitors on board were sized for 3.3V operation - page 10 of MAX3243 datasheet). Still, it seemed to work fine up to 28800 baud rate. Any faster and the software starts detecting data framing errors and the data becomes corrupted.
...continue reading "RS232 and Arduino"

Few months ago I had ordered a new PCI parallel port card. It turned out my computer did not have a parallel port and I wanted to try parallel port logic analyzer software described here:

...continue reading "Parallel Port Logic Analyzer"


I really wanted to find out exactly how much etchant would I need for a given board. That would be at least a theoretical minimum to etch all exposed copper, so after some calculation I made this table. In practice, you would probably want to double that amount to get it to etch faster, but that's a good starting value to measure.