THE WORLD of AVIONICS IN YOUR HAND
PROTOCOL: Harvard Bi-phase Bit Encoding
Other standards include ARINC 573, a Flight Data Recorder output format. This device sends a continuous data stream of Harvard Bi-Phase encoded a 12 bit word which is encoded in frames. The data in a frame consists of a snapshot of the many avionics subsystems on the aircraft.
Each frame contains the same data at a different snapshot in time.
Each frame is broken into four sub-frames. At the start of each sub-frame is a unique sync word that is used by the receiver to synchronize with the incoming data.
Logic 1 : UP GOING EDGE IN THE MIDDLE OF THE SIGNAL
Logic 0 : DOWN GOING EDGE IN THE MIDDLE OF THE SIGNAL
ARINC 717 supersedes ARINC 573 and is used to perform the same function. It adds a number of different bit rates and frame sizes. It also provides for an alternate output data stream that is identical to the primary, Harvard Bi-phase encoded stream, except that it is encoded in BPRZ format (the same as ARINC 429)
ARINC 717 provides design guidelines for a Digital Expandable Flight Data Acquisition and Recording System (DEFDARS) or more simply a Flight Data Recorder (FDR). Primarily used on airlines, the FDR provides capability to meet the FAA mandatory flight data recording requirements plus other data acquisition needs. The DEFDARS consists of several components. The Digital Flight Data Acquisition Unit (DFDAU) samples, conditions, and digitizes or reformats the flight data. The Digital Flight Data Recorder (DFDR) is the actual recorder with a crash protected medium. Other components may provide input data or be used to control the system.
The DFDAU time division multiplexes the data and passes it to the DFDR over a special continuous time division multiplexed serial databus. Parameters are differentiated by their position (time slot address) in the data frame. It is this databus that is implied when reference is made to an ARINC 717 databus. Words are 12 bits long and are nominally transmitted at 64 or 256 words per second, though the specification also permits 128 and 512 words per second. Some of the characteristics of ARINC 717, including the DFDR databus, were derived from its predecessor, the ARINC 573 Mark 2 Aircraft Integrated Data System. An ARINC 717 system also makes use of the ARINC 429 databuses.