LIBRARYWhat is a bus?

A bus is a set of shared wires plus the rules for using them, and it is how the chips on a board pass data to each other. The rules are the protocol: which wire carries what, when a bit counts as valid, and who is allowed to talk. Get the bus right and two parts that have never met can trade bytes reliably.

Serial or parallel?

A serial bus sends bits one at a time down a couple of wires. A parallel bus sends a whole byte at once across eight or more. Parallel moves more per clock tick, but it spends a pin on every bit and every trace has to stay length-matched. On a small board, pins and space are the scarce thing, so almost every on-board bus is serial.

What a protocol fixes

A protocol is the agreement both sides keep. It fixes the framing (where a byte starts and stops), the timing (how fast bits move, and whether a separate clock marks each one), and the roles (which chip drives the exchange). Two chips that follow the same protocol interoperate even when different companies built them.

Controller and peripheral

Most buses have one part that starts each transfer, the controller, and one or more that answer, the peripherals. The controller decides when a transfer happens and, on a clocked bus, supplies the clock. Older datasheets call these two roles master and slave; the idea is the same.

On a One Thousand Drones board every on-board data link is serial: USB back to your computer, and a serial bus out to each sensor. That is what keeps the pin count low and the board small.