I would like to record internal behavior of a Canon Pixma MX340 multi-function inkjet, specifically the paper feed roller position encoder and two photo interrupter sensors. The sensors are binary on/off affairs that change relatively infrequently, so they should not be a challenge. The interesting element would be the quadrature encoder, reporting how far and how fast the paper feed roller motor is turning that shaft.

A rough calculation told me I need something that can sample two encoder outputs at a rate of at least 20 kHz. My Saleae Logic 8 hardware is advertised to sample at speeds up to 100 MHz. Maximum sampling rate drops if it needs to cover more channels, but 2 channels should be good for 50MHz for plenty of headroom. Unfortunately Saleae's Logic 2 software does not perform quadrature decoding. I don't know why I had assumed it would be part of the analysis toolkit, but I did, and so I was surprised to find it absent. A quick online search confirmed quadrature decoding is still on their "user requested feature" list for some undefined time in the future. And as I established earlier, Saleae's analyzer extension framework doesn't support writing my own decoder across multiple channels.

During this search for quadrature decoding support in logic analyzers, I came across mentions it was a capability in sigrok, forwarding to a terse page in sigrok documentation. I understand sigrok to be (very) roughly analogous to Saleae's Logic 2 software, with support for lots of different hardware performing a wide variety of tasks. Perusing sigrok hardware support page I saw two names I recognized. Bus Pirate is listed as supported, but very limited due to its basic hardware. GreatFET One is also on the list, and it has a much more robust list of capabilities. I consider this a good reason to add GreatFET One to my investigation list for future purchase. If quadrature decoding analysis becomes a recurring need, it would be enough to motivate me to switch from Saleae's proprietary solution to an open-source alternative.

Another advantage of sigrok's open source nature is that, unlike Saleae's proprietary solution, the software is not tied to the hardware. Not even officially supported hardware. It has support for documented interface protocols like SUMP, so any hardware can theoretically act as signal acquisition hardware for sigrok. However, this is apparently easier said than done.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.