BusIO is a multi-user serial data bus/protocol working over a single twisted pair + optional supply wire. Similar to I2C bus in operation but unlike I2C it allows direct connection to standard RS-232 devices i.e. using a terminal on a PC for diagnostics and programming. Sensors using this bus often have an added third wire for power supply.

A BusIO message is two-byte message (16 bits) using standard RS232 little-endian format. Transmission speed 1200 bps default. The first byte is called BusByteL second is BusByteH. The BusByteL usually contains data or dontcare. The BusByteH high’s nibble is always a command. There are up to 16 possible command slots available. There are 16 unique device addresses available.

; two 8 bit bytes
; last first
; | |
; 76543210 76543210 captured as BusByteL/H
;
; 1010AAAA DDDDDDDD program . address / data 8 bit
;
; 0101AAAA DDDDDDDD A-status . address