Printing from Eagle

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 in less waste.

Continue reading

Sinclair ZX81 lives on!

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 images and how its Z80 CPU boots. I instrumented it and captured scope and logic analyzer images as it was powering on.

What a beauty!

Continue reading

RS232 and Arduino

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 onboard were sized for 3.3V operation – page 10 of MAX3243 datasheet). Still, it seemed to work fine up to the 28800 baud rate. Any faster and the software starts detecting data framing errors and the data becomes corrupted.

However, that was purely a design limitation of that particular RS232 board that I had.

Continue reading

Software Scope using a Sound Card

I tried a simple software implementation of a digital oscilloscope using a PCI sound card and this software:

I connected its own signal generator (from the line out) into the line in and it worked — sort of. The input capacitors were skewing low frequencies. I de-soldered those caps but then hit a bigger problem of a floating DC component. Also, there was a lot of noise on the line.

Continue reading

Etching the PCB: Copper etchant calculator

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.