The ZX Spectrum Spills Its Secrets
This 48-channel wireless logic analyzer plugs directly into the expansion port of a ZX Spectrum to reveal every signal and CPU instruction.
Whether you have got some custom add-on hardware in mind, or you just want to better understand how the computers of the past worked, hooking one up to a logic analyzer is the first step. By using these tools, every electrical signal is laid bare, revealing exactly what is happening at the level of the CPU, memory, and other components, to make a sprite race across the screen.
When you factor in the address and data busses, and all of the control signals, there are a lot of lines that need to be monitored simultaneously to get a complete picture of the machine’s operation. And that makes for a huge tangle of wires that are easy to get mixed up with one another. But YouTuber Happy Little Diodes has come up with a nice and neat solution for the ZX Spectrum that makes it very easy to explore. It plugs into the expansion port, can monitor 48 channels simultaneously, and it is completely wireless.
The logic analyzer itself (or rather, the logic analyzers — there are two) was designed by Agustín Gimenez Bernad. It is powered by a Raspberry Pi Pico W and can capture 24 channels at 100Msps. Happy Little Diodes stacked two of these boards to support 48 channels, then created a custom ZX Spectrum interface PCB. This interface connects the logic analyzers directly to the signals exposed by the computer’s expansion port. This includes the address and data busses, as well as many other important signals.
Once the Picos have been flashed with their firmware, they can be wirelessly (or wired, if you want to use a pair of USB cables) connected to the LogicAnalyzer software that is designed to pair with the hardware. One of the best parts of this build is the use of a Z80 decoder library, which decodes the data on the data bus, and also the CPU instructions and their operands. Using this, it is far easier to make sense of all of the signals.
This is a pretty easy build to replicate if you are moderately handy with a soldering iron, so be sure to watch the video for some tips if you want to have some fun with your Speccy.