ARINC 429 Data Bus Specification and Architecture
ARINC 429
One of the most
widely used bus standards used in commercial aircraft is the ARINC 429 data
bus. The electrical and data properties and protocols are specified in the
ARINC 429 specification.
ARINC 429 uses
the Mark 33 Digital Information Transfer System, a unidirectional data bus
standard (DITS). 32-bit packets of messages are transmitted at a bit rate of
either 12.5 or 100 kilobits per second (referred to as low and high bitrate,
respectively). Due to the unidirectional nature of the bus, separate ports,
couplers, and cables are needed for an LRU to be able to transmit and receive
data. Be aware that in a plane with advanced avionics, a lot of bus connections
can be necessary.
A wide range of
commercial transport aircraft, including the Airbus A310/A320 and A330/A340;
the Boeing 737, 747, 757, and 767; and the McDonnell Douglas MD-11, have ARINC
429 installed on them. In order to lower the weight and size of the cabling and
to enable faster data speeds than are achievable with ARINC 429, more recent
aircraft, such as the Boeing 777 and Airbus A380, employ ARINC 629, which is a
greatly upgraded bus specification. The ARINC 429 standard has shown to be
quite stable, thus despite these shifts to faster, bidirectional bus standards,
it is expected to stay in use for many years to come.
ARINC 429 Specification
For avionics in
aircraft, ARINC 429 is a data transfer standard. It employs a
self-synchronizing, self-clocking data bus protocol (Tx and Rx are on separate
ports). Twisted pairs of wires used for the physical connection carry balanced
differential signaling. Most communications consist of a single data word,
which is 32 bits long. To other system elements that are watching the bus
messages, messages are sent at 12.5 or 100 kbit/s. The NULL state or 32-bit
data words are continuously transmitted by the transmitter (0 Volts). There can
be no more than 20 receivers and one transmitter on a single wire pair. The
protocol enables self-clocking at the receiver end, eliminating the requirement
for clocking data transmission. An alternative to MIL-STD-1553 is ARINC 429.
Electrical characteristics
A single
transmitter or source can be connected to a single receiver or sink using the
two-wire differential ARINC 429 bus. There are two selectable speeds: 12.5 kbps
and 100 kbps. 32-bit words are transmitted via the data bus using two signal
wires. Sequential words are transmitted at least four times in bits apart (zero
voltage). As a result, the system becomes selfclocking and no longer requires a
separate clock signal. The ARINC 429 electrical characteristics are summarized
below:
With either a
positive or negative polarity, the nominal transmission voltage is 10V1V
between wires (differential). Consequently, the range of each signal leg is +5
V to -5 V. Both conductors are at -5V if one is at +5V and vice versa. The 'A'
(or '+' or 'HI') conductor is one wire, while the 'B' (or '-' or 'LO') wire is
the other.
Bipolar return
to zero (BPRZ) modulation is the type of modulation used. When measured between
the conductors, the composite signal state may be at one of the following three
levels:
- HI, which should be between +7.25 and 11 volts (from point A to point B);
- NULL, which should be between +0.5 and -0.5 volts (from point A to point B);
- LO, which should fall between -7.25 V and -11 V (A to B).
The length of
the line and the quantity of receivers connected to the bus affect the received
voltage on a serial bus. A single bus with ARINC 429 shouldn't have more than
20 receivers linked to it. Each bus is unidirectional, therefore if a system
needs to respond to or send messages, it must have its own transmit bus.
Therefore, two distinct bus connections are required in order to provide
bidirectional data flow.
Protocol
A twisted wire
pair can only support one transmitter, hence ARINC 429 relies on a relatively
straightforward point-to-point protocol. The transmitter either stays in the
NULL state or continuously sends 32-bit data words. It should be noted that the
ARINC specification supports up to 20 receivers, even though there may only be
one receiver on a given bus wire.
Bit Timing and Slew Rate
The ARINC
waveform's rise and fall times are referred to as the slew rate. More
specifically, it describes the time required for the ARINC signal to increase
from the 10% to 90% voltage amplitude points on the leading and following edges
of a pulse. Both the high-speed and low-speed ARINC 429 systems are covered by
the data.
ARINC 429 data word format
An ARINC message
typically consists of one 32-bit data word. The type of data that is present in
the rest of the word is specified by the 8-bit label field. The five basic
fields of an ARINC data word—parity, SSM, data, SDI, and label—are all
frequently present. ARINC data words are always 32 bits long. The bits are
numbered by the ARINC standard from 1 (LSB) to 32. (MSB). There are numerous
conceivable data formats.
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