ARINC 429
is a data format for aircraft avionics. It provides the basic description of the
functions and the supporting physical and electrical interfaces for the digital
information system on an airplane. ARINC 429 is the predominant avionics data
bus for most higher-end aircraft today. Heim system
data recorders can record and play back this data format in its entirety.
All original data is written to the media in the Heim DATaRec proprietary format
and replayed in real time or at a faster speed if required.
Technical Description
ARINC 429 is a two-wire data bus that is application-specific for commercial
and transport aircraft. The connection wires are twisted pairs. Words are 32
bits in length and most messages consist of a single data word. The
specification defines the electrical and data characteristics and protocols.
ARINC 429 uses a unidirectional data bus standard (Tx and Rx are on separate
ports) known as the Mark 33 Digital Information Transfer System (DITS). Messages
are transmitted at either 12.5 or 100 kbit/s to other system elements that are
monitoring the bus messages. The transmitter is always transmitting either
32-bit data words or the NULL state. No more than 20 receivers can be connected
to a single bus (wire pair) and no more than one transmitter.
ARINC 429 Word Format
Each ARINC word is a 32-bit value that contains five fields:
Bit 32 is the parity bit, and is used to verify that the word was not
damaged or garbled during transmission.
Bits 30 to 31 is the Sign/Status Matrix, or SSM, and often indicates
whether the data in the word is valid.
OP (Operational) - Indicates the data in this word is considered to
be correct data.
TEST - Indicates that the data is being provided by a test source.
FAIL - Indicates a hardware failure which causes the data to be
missing.
NCD (No Computed Data) - Indicates that the data is missing or
inaccurate for some reason other than hardware failure. For example,
autopilot commands will show as NCD when the autopilot is not turned on.
The SSM can also indicate the Sign (+/-) of the data or some
information related to it like an orientation
(North/South/East/West).
Bits 11 to 29 contain the data. Bit-field,
Binary Coded Decimal (BCD), and
two's complement binary encoding (BNR) are common ARINC 429 data
formats. Data formats can also be mixed.
Bits 9 and 10 are Source/Destination Identifiers (SDI) and indicate for
which receiver the data is intended or more frequently which subsystem
transmitted the data.
Bits 1 to 8 contain a label (label words), expressed in
octal,
identifying the data type.
Labels
Label guidelines are provided as part of the ARINC 429 specification, for
various equipment types. Each aircraft will contain a number of different
systems, such as Flight Management Computers, Inertial Reference Systems, Air
Data Computers, Radio Altimeters, Radios, and GPS Sensors. For each type of
equipment, a set of standard parameters is defined, which is common across all
manufacturers and models. For example, any Air Data Computer will provide the
barometric altitude of the aircraft as label 204. This allows some degree of
interchangeability of parts, as all Air Data Computers behave, for the most
part, in the same way. There are only a limited number of labels, though, and so
label 204 may have some completely different meaning if sent by a GPS sensor,
for example. Many very commonly-needed aircraft parameters, however, use the
same label regardless of source. Also, as with any specification, each
manufacturer has slight differences from the formal specification, such as by
providing extra data above and beyond the specification, leaving out some data
recommended by the specification, or other various changes.
