The ARINC 717 bus defines the communication between DFDAU(Digital Flight Data Acquisition Unit) and the DFDR(Digital Flight Data Recorder). The Data over the bus is a collection of all various data to be recorded.
The Data is organized in 4 subframes each 1 second long, which comprise one frame of 4 seconds. Each subframe contains 64 (or 128 or 256) words of 12-bit each. Each subframe starts with a unique synch word and these 4 words are known as Barker Code and are used to synchronize on new frame. The electrical signals of this bus are +/-5V differential pair Harvard Bi-Phase coded. The various data words are continuously transmitted in their chronological order and can be recognized by their predefined relative location in frame.