A fantastic feature of Lego Mindstorms is that they provided developer kits for both hardware and software, including schematics, datasheets, and source code. It was really fun to build a custom motor controller board which interfaced with Mindstorms, just because their open interface and information made it simple. Sad to see it go - hoping that their new offerings keeps the hacking alive.

https://education.lego.com/en-us/product-resources/mindstorm...