I played around to find a suitable bus for communication. Initially I wanted to use the 1-wire bus but as it turns out its not as suited for what I want to do after all. It seems its pretty much only used by Maxim and while there are client implementation for Arduino they don't seem to work on ARM which is unfortunate as I want to use a SAMD11 as the heart for each module.

It's a very in-expensive MCU, cheaper than the good old ATtiny85 and comes packed with a ton of peripherals, even USB! Which is great because it makes debugging a single module very easy if you plan to tinker around with your system or develop your own module.

Thanks to @Ted Yapo I found out about the SDI-12 bus which is also a bus that also only requires a single wire. It seems to be highly compatible with different platforms including the SAMD21 ARMs. The SAMD11 should be no exception as the library really only uses very basic Arduino functions for its implementation.

I did some basic tests with two Arduinos and will test it next with a SAMD09 test module once the ICs arrive. I will probably start with adding it to a fan module as that is the easiest electrical and mechanical wise.

If that goes well I will probably continue with making a few basic sensor modules like temperature and humidity measurement and maybe a height sensor. I don't think a height sensor will measure continues growth but alarm you at a threshold, the main reason I want such a sensor in the first place is to warn you when a plant grows too close to a lamp module as this can burn the plant which has happened quite a few times in my testing phase of #MoRaLiS: Modular Rail Lighting System . Plants can grow quite a bit faster than you would imagine if you give them enough light :)