I am building a bunch of slave devices (Arduinos) that operate autonomously, but will report data back to a single raspberry pi and update their clocks from the pi's time.
I've only worked with one arduino using serial, but I would imagine that 20 arduinos needs a special solution. I don't know how to split serial, but I've also considered SPI and I2C. Would either of these be the way to go?
Given that there are 20 units, I don't think I should go with wifi as I fear there would be loss of connectivity often. I prefer a hardwired method.
EDIT: Also, these arduinos won't be close to each other. They will be spread out, no more than 30 feet from the pi.