After a closer look at how to program the 4-Bit Nibbler CPU and how to cope with its intentional limitations it was time to go one step further and have a closer look at how the hardware works one layer further down. To see the signals and how they propagate through the system a logic analyzer is needed with as many inputs as possible to trace digital signals at different places. Unfortunately, most logic analyzers I found are not cheap, costing several hundred euros even for entry level models. Another alternative would have been to buy a cheap clone hardware from China and then use it with software from the original vendor. As I don't think that's fair I never considered that approach either. But thanks to the January 2016 edition of the Linux Voice magazine I stumbled across the Bitscope Micro, a 2 channel low cost oscilloscope and 8 channel logic analyzer that costs around 120 Euros, tax and shipping included. That's quite in the range of what I was willing to spend. In addition they offer their software for Windows, the Mac and also for Linux. In other words, a perfect match for my needs.
The logic analyzer can sample digital signals at a rate of up to 40 MSamples/s, enough to have a decent resolution for my Nibbler board running at 2.4 MHz. Any channel can be used as a trigger with rising and falling flanks or a high and low signal level so it's possible to capture signals at a specific moment. The picture and screenshot on the left shows the Bitscope Micro connected to the Nibbler and a commented screenshot that shows how instructions are read from the ROM and put into the instruction register two clock cycles later. For the screenshot I used the blinking LED program that just uses 5 instructions to switch the LED on and off again and then jumps back to the beginning of the program. In total that is exactly 10 clock cycles in which the instructions repeat over and over. This way it's easy to find the beginning and the end of the loop when looking at the signal levels. I spent many hours analyzing traces of signals from many different parts to confirm my theoretical knowledge of how the control unit, clock and phase make the system "come alive". A wonderful exercise during which I once again learnt a lot about what makes a computer really tick.