Does anyone have experience with the Zybo-Z7?
Many cheap scopes are based on a Zynq chipset. So building an LA on something like the Zybo Z7-20 seems a rather attractive approach. It provides important skills for hacking a a lot of low end T&M gear. It's ~$300 and has 40 PPIOs which appear to sample at 400-800 MS/S. I've not yet found whether it takes 1 or 2 clock cycles to store a bus state. In any case, one should be able to buffer a significant amount of state in the 1 GB DDR3 memory when logging.
https://store.digilentinc.com/zybo-z7-zynq-7000-arm-fpga-soc-development-board/ If you allocate most of the memory to a ring buffer, you can accept rather long bursts of high speed traffic on the bus. Large data transfers excepted. There are also a multitude of common bus transceivers implemented in hardware. In short, with minimal additional analog hardware, it should do everything one needs out of a medium speed LA
A 400-800 MS/S rate should provide good pulse edge resolution. And by only sending a timestamp and the changed bits, the average data rate should be tolerable. So logging to capacity of disk should be possible in the majority of cases at very fairly high effective sample rates and very long trace times.
There are lower cost boards, however, it seems to me that tor the effort of building a DIY, something like this offers significantly more performance for the effort. In addition to the 40 GPIOs, there are USB, uSD, HDMI and 10 /100/1000 Ethernet in addition to SPI. With a smattering of ADC that might be useful for detecting logic levels on unknown devices.
i suspect most FPGA boards in this price class offer similar features. It shouldn't be too hard to make something portable to multiple devices.