ARINC 429
Protocol Tutorial
ReAdapter
Tel: (913)735-3784
www.readapter.com
service@readapter.com
Copyright 2015 ReAdapter, LLC
Version
Date
1.1
August 10, 2015
1.0
March 16, 2015
Author
Aaron Taylor
Andrew Langford
Comment
Initial Creation
Additional content, formatting
ARINC 429 Protocol Tutorial
Page 2
Table of Contents
Purpose ..................................................................................................................................... 4
Specification Overview and Use ................................................................................................. 4
Overview ................................................................................................................................ 4
Use......................................................................................................................................... 4
Physical Characteristics ............................................................................................................. 5
Label Format .............................................................................................................................. 5
Overview ................................................................................................................................ 5
Label Field – Bits 1-8 .............................................................................................................. 6
Source/Destination Identifier (SDI) – Bits 9-10........................................................................ 7
Sign/Status Matrix (SSM) – Bits 30-31.................................................................................... 7
Binary Labels (BNR) ........................................................................................................... 8
Discrete Labels ................................................................................................................... 8
Binary-Coded Decimal (BCD) ............................................................................................. 8
Data – Bits 11-29 .................................................................................................................... 9
Binary Data (BNR) .............................................................................................................. 9
Discrete Data .....................................................................................................................11
Binary Coded Decimal (BCD) Data ....................................................................................12
Parity .....................................................................................................................................12
Quick Reference .......................................................................................................................13
About ReAdapter ......................................................................................................................14
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ARINC 429 Protocol Tutorial
Purpose
This manual is designed to provide an overview of the ARINC 429 Specification along with
typical characteristics and application. This is intended as a quick primer or reference guide,
but not a complete document. The ARINC 429 Specification is a copy written document owned
and distributed by Aeronautical Radio, Inc. A copy of the complete specification can be
purchased at www.arinc.com.
Disclaimer: All information below is provided as a guide, ReAdapter, LLC provides no
guarantee to the accuracy or use of the information.
Specification Overview and Use
Overview
The ARINC 429 Specification defines a protocol used to transfer data between systems on
commercial aircraft. Electrical, timing, signal levels and other characteristics are defined to
allow for wide interoperability across avionics subsystems.
Manufacturers are encouraged to follow the standard data sets and definitions in the ARINC
429 Specification, though there is no requirement to do so. Following the data sets in the
standard allows for greater interoperability across manufacturers and more standard
interactions between equipment from different manufacturers.
Use
The current implementation of the ARINC 429 Specification, last updated in 1983, is composed
of three parts:
Specification Part
Description
ARINC Specification 429, Part 2-15: Discrete
Word Data Standards
Describes how discrete data is stored with bit
fields
ARINC Specification 429, Part 1-15:
Functional Description, Electrical Interface,
Label Assignments and Word Formats
ARINC Specification 429, Part 3-15: File Data
Transfer Techniques
Describes physical and electrical components
as well as labels, addresses, and word formats
Describes the link layer protocol used for data
and file transfer
Standard formats are described in greater detail later in this tutorial.
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ARINC 429 Protocol Tutorial
Physical Characteristics
The ARINC 429 Specification describes how avionic subsystems communicate and transfer
data on commercial aircraft. ARINC 429 can be wired with only one transmitter per network, but
supports up to 20 receivers. Data is half-duplex, meaning data only flows in one direction, from
transmitter to receiver(s). Two channels are required for bi-direction communication. Each
subsystem may contain any number of transmitters or receivers, and it is not uncommon to
have a 'data concentrator' which can receive streams of data from multiple ARINC 429
transmitters.
ARINC 429 data payloads are referred to as labels. Each ARINC 429 label is composed of 32
bits of data, each with a specific meaning. The standard requires at least 4 bit-times of zero
voltage between sequential labels.
There are two supported data rates
1. Low speed – 12.5 kilobits per second (tolerant in the range 12.0-14.5 kbps)
2. High speed – 100 kilobits per second ±1%
Both speeds cannot be used on the same data bus, so LRUs typically pick one or the other,
depending on bandwidth or latency requirements.
Label Format
Overview
Each ARINC 429 label is made of 32-bits, with each bit having a specific meaning. Generally
each label is composed of the following fields:
 8-bit label ID
 2-bit Source Data Indicator (SDI)
 19-bit Data Section
 2-bit Sign Status Matrix (SSM)
 1-bit Parity
3
2
P
3
1
3
0
SSM
2
9
2
8
MSB
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
Data
1
9
1
8
1
7
1
6
1
5
1
4
1
3
1
2
1
1
LSB
1
0
SDI
9
8
7
6
5
4
Label ID
3
2
Table 1-ARINC 429 Label Description
Unlike most other protocols, ARINC describes each bit starting with 1. Most references to bit
numbers in other protocols or computer science literature starts counting with bit 0. ARINC 429
is very consistent in counting from 1 to 32.
Bit 32 is the most significant bit, Bit 1 is the least significant bit. The fields described show the
typical uses of each bit, but the only required fields are the Parity and the Label fields. All other
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ARINC 429 Protocol Tutorial
1
fields can be defined by the equipment manufacturer. While there is some variation, the vast
majority of labels conform to this standard.
Label Field – Bits 1-8
Labels are used to determine how the data in the label can be interpreted. In a data stream,
each label is typically used to denote a specific type of data with a specific definition. For
example, label 150 is commonly used to represent time of day. Each label on a bus for a
specific device has a predefined meaning.
The eight label bits are interpreted as ‘reverse octal.’ The reverse means that the MSB and
LSB are reversed from the rest of the data in the label. The octal means that eight bits are
interpreted as octal. The data is described in the following table.
LSB
Bit
8
Octal Digit
7
Ones
6
5
4
Tens
Table 2-Label Field Description
3
MSB
2
1
Hundreds
For example, Label 273 would be written with the following bits
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ARINC 429 Protocol Tutorial
LSB
Bit
8
Octal Digit
Binary 273
1
7
3
1
6
5
0
1
4
7
1
Table 3-Label Field Example
3
2
1
0
2
MSB
1
1
Another way to translate the data is to give each bit a value and add the values








Bit 1 = 200
Bit 2 = 100
Bit 3 = 40
Bit 4 = 20
Bit 5 = 10
Bit 6 = 4
Bit 7 = 2
Bit 8 = 1
So, using the same example as above,
Label 273 = (200) + (40 + 20 + 10) + (2 + 1)
Using this encoding, labels can be in the range from 0 to 377, with no 8’s or 9’s.
Source/Destination Identifier (SDI) – Bits 9-10
The SDI (Source Destination Identifier) is represented through bits 9 and 10 on a standard label.
This field is optional. These bits are typically used when there is more than one instance of the
same data on a data bus. For example, the same device may have two sensors for
temperature, temperature_1 and temperature_2. The SDI bits could be used to distinguish
which sensor was the source for a label by setting bits 9 and 10 to
 binary 1 (‘01’) for temperature_1
 binary 2 (’10’) for temperature_2
This standard scheme limits the use of SDI to 4 values. This limitation has led some
manufacturers to use an ‘extended SDI’ scheme where bits from the Data field are used to
extend the available bits in the SDI field. Adding a bit, for example bit 11, doubles the number
of available SDI sources.
The SDI field can also be used to extend resolution for the data field. For higher precision,
these bits can become part of the data field.
Sign/Status Matrix (SSM) – Bits 30-31
The meaning of the Sign/Status Matrix (SSM) varies with the type of data the label contains.
Generally this field contains the validity or state of the data. Each SSM may be set to
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ARINC 429 Protocol Tutorial




a valid value, indicating the data encoded is good
a bad value, indicating the data cannot be used or the unit has failed
No Computed Data (NCD), indicating the data cannot be computed currently, though the
unit has not failed.
Functional Test, indicating the unit is performing a test, and the data is representative of
the test, not the normal functioning of the unit.
There are three types of SSM, each with a slightly different encoding scheme, as described in
this section.
Binary Labels (BNR)
Binary Labels use the SSM to describe the state of the data, as described by the table below.
31 30 Interpretation
Failure 0 0 Data or unit has failed
No Computed Data (NCD) 0 1 Data cannot be computed
Functional Test 1 0 Data is from a test and is not normal data
Valid 1 1 Data is valid, normal operation
Discrete Labels
Table 4-Binary (BNR) SSM
Discrete Labels use the SSM to describe the state of the data, similar to Binary Labels, but with
inverted states.
31 30 Interpretation
Valid 0 0 Data is valid, normal operation
No Computed Data (NCD) 0 1 Data cannot be computed
Functional Test 1 0 Data is from a test and is not normal data
Failure 1 1 Data or unit has failed
Binary-Coded Decimal (BCD)
Table 5-Discrete SSM
Binary-Coded Decimal Labels (BCD) use the SSM to indicate the sign or direction of the BCD
data in the data field as well as the status of the data. The meaning of the value fluctuates with
the type of data encoded.
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ARINC 429 Protocol Tutorial
Plus
No Computed Data (NCD)
Test
Minus
Data – Bits 11-29
31 30 Interpretation
0 0 Positive, North, East, Right, To, Above
0 1 Data cannot be computed
1 0 Data is from a test and is not normal data
1 1 Negative, South, West, Left, From, Below
Table 6-Binary-Coded Decimal (BCD) SSM
Data is not transmitted as a series of integers or floating-point numbers as is more common with
other data transmission protocols. This is often a source of confusion. Instead, ARINC 429
data is packed in one of three ways.
Binary Data (BNR)
Binary data starts are bit 29. This bit can be treated as the sign (±) bit, with a zero indicating
positive and 1 indicating negative. Unsigned data typically leaves bit 29 as zero. The data is
encoded as 2’s compliment data, not sign-magnitude. An easy way to interpret the data is to
treat bit 29 as negative twice the value of bit 28. Examples to illustrate this are below.
The next bit, bit 28, is then the Most Significant Bit (MSB) of the data. Typically this is picked to
be a round number for convenience, though this is not required. Each bit from then on
represents a value half that of the bit before it. This continues until an adequate level of
precision is achieved. Remaining bits up to the SDI field, bit 10, are then pad bits unless
otherwise defined.
BNR labels typically encode both their Data and SSM fields using BNR formatting.
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ARINC 429 Protocol Tutorial
Examples
The value of the data in this example is
512 + 1 + 0.5 + 0.25 = 513.75
1
8
9
10
1
Label ID
8
9
SDI
11
pad
0
13
pad
0
12
14
15
pad
pad
pad
0
0
18
0.5
1
19
20
21
22
1
1
4
0
2
8
23
16
25
64
24
32
26
128
28
512
27
29
30
31
32
1
256
+/-
SSM
Parity
0
15
16
17
Sign
pad
pad
pad
0.703125
26
MSB
pad
21
25
0
pad
0.175781
24
0
pad
19
23
0
0
pad
22
0
1
13
20
0
0
0
18
0
SDI
pad
14
LSB
Label ID
11
12
0
0.125
0.25
10
0
16
17
The value of the data in this example is
-(2*90) + 90 + 45 = -45
27
28
29
30
31
32
Table 7-Binary Label Examples
0.351563
1.40625
2.8125
5.625
11.25
22.5
45
90
+/-
0
0
0
0
0
0
0
0
LSB
0
0
0
0
0
0
1
1
1
MSB
Sign
SSM
Parity
ARINC 429 Protocol Tutorial
Page 10
Discrete Data
In a Discrete label, each bit in the Data field represents a separate True/False condition.
Typically bit 29 is not used in a discrete label, though its use is acceptable. The data in a
discrete label is often used to represent information such as the status of a system, a fault, or a
physical switch state. Labels in the 270s are typically used for discrete data.
Discrete labels typically encode their Data and SSM fields using Discrete formatting.
An example is below showing bit 28, “Receiver Fault,” set to True (1).
1
8
9
10
Label ID
SDI
11
pad
0
13
pad
0
12
14
15
16
17
18
19
20
21
pad
0
pad
0
pad
0
pad
0
pad
0
pad
0
pad
0
pad
0
pad
0
22
Reserved 2
0
24
Weight on Wheels
0
26
Analog Fault
0
28
Receiver Fault
1
23
25
27
29
30
31
32
Reserved 1
Power Valid
Digital Fault
+/-
LSB
0
0
0
0
MSB
Sign (unused)
SSM
Parity
Table 8-Discrete Label Example
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ARINC 429 Protocol Tutorial
Binary Coded Decimal (BCD) Data
Binary Coded Decimal (BCD) labels are designed to provide a fixed precision up to a specific
number of decimal digits. Where Binary labels can provide more precision, it is frequently only
possible to represent a value within the resolution of the label and not an exact decimal value.
BCD labels can provide an exact decimal representation at the expense of resolution.
A BCD label splits the Data field up into 4-digit segments, each with a fixed precision.
3
2
P
3
1
3
0
SSM
2
9
2
8
Digit 1
2
7
2
6
2
5
2
4
Digit 2
2
3
2
2
2
1
2
0
Digit 3
1
9
1
8
1
7
1
6
Digit 4
1
5
1
4
1
3
1
2
Digit 5
1
1
Table 9-BCD Label Description
1
0
SDI
9
8
7
6
5
4
Label ID
3
2
1
A BCD label defines the value of each digit and how many digits are used. There is no
standard. Each digit to the left is the next decimal digit, i.e. ten times the previous digit. Each
four-bit digit can be in the range from 0 to 9, with values greater than 9 being invalid.
BCD labels typically encode their Data and SSM fields using BCD formatting, meaning the SSM
is used to describe the sign of the data.
Examples
In the example below, all five digits are used, with a resolution of 0.01. The value being
encoded is 704.93.
3
2
P
3
1
3
0
SSM
0 0
+
2
9
2
8
2
7
Digit 1
1 1 1
7
2
6
2
5
2
4
2
3
Digit 2
0 0 0 0
0
2
2
2
1
2
0
1
9
Digit 3
0 1 0 0
4
1
8
1
7
1
6
1
5
Digit 4
1 0 0 1
9
1
4
1
3
1
2
1
1
Digit 5
0 0 1 1
3
1
0
SDI
9
8
7
6
5
4
Label ID
3
2
1
2
1
Table 10-BCD Label Example 1
In the example below, only four digits of precision are used, with a resolution of 1. The value
encoded is -2800.
3
2
P
3
1
3
0
SSM
1 1
-
Parity
2
9
2
8
2
7
pad
0 0 0
n/a
2
6
2
5
2
4
2
3
Digit 1
0 0 1 0
2
2
2
2
1
2
0
1
9
Digit 2
1 0 0 0
8
1
8
1
7
1
6
1
5
Digit 3
0 0 0 0
0
1
4
1
3
1
2
1
1
Digit 4
0 0 0 0
0
1
0
SDI
9
8
7
6
5
4
Label ID
3
Table 11-BCD Label Example 2
ARINC 429 labels use the most significant bit, bit 32, as a parity check for error detection.
Labels use an odd parity, meaning bit 32 is always set or cleared to ensure the total number of
bits at logic 1 in the label is an odd number. This check provides for very simple, efficient error
detection.
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ARINC 429 Protocol Tutorial
Quick Reference
This section is intended to provide the most used reference data for quick access. Print and pin
this up to refer to.
3
2
P
3
1
3
0
SSM
2
9
2
8
MSB
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
Data
1
9
1
8
1
7
1
6
1
5
1
4
1
3
1
2
1
1
LSB
BCD
31 30 BNR Dis
Valid Pos, N, E, Right, To
0 0 Fail
NCD
0 1
Test
1 0
Neg, S, W, Left, From
1 1 Valid Fail
1
0
SDI
9
8
7
6
5
4
Label ID
3
2
1
Bit 8 7 6 5 4 3 2
1
Octal Digit
Ones
Tens
Hundreds
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ARINC 429 Protocol Tutorial
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Tel: (913)735-3784
www.readapter.com
service@readapter.com
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ARINC 429 Protocol Tutorial