Established in 1929, Aeronautical Radio, Inc. (ARINC) is a private company acquired by Collins Aerospace in 2013. This organization was founded to create specification standards for avionic equipment used by aircraft worldwide. It was established by several airline companies as well as manufacturers of aviation equipment and parts.

The fundamental features for digital data communication between commercial avionic systems are outlined in ARINC-429. The specification of signal levels, timing, and protocol characteristics facilitates design implementation and data transmission. ARINC-429 is intended to provide Line Replaceable Units (LRUs) for commercial aircraft. Simply put, the ARINC-429 protocol aims to facilitate communication within the Local Area Network (LAN) of avionics.

HISTORY AND DEVELOPMENT

The original ARINC-419 Specification for digital communication in commercial aircraft laid the foundation for the development of the ARINC-429 Specification. First published in 1966, the ARINC-419 standard specified four wiring topologies, including a serial, twisted, shielded pair interface for use by the Digital Air Data System (DADS), known as the ARINC-575 and DAD-575 specifications.

The ARINC-429 Specification, initially published in April 1978 as ARINC-429/1 and now known as ARINC-429/15, evolved from this serial structure.

ARINC-429/15 was adopted by the AEEC in 1995 and consists of three parts:

ARINC-429, Part 1-15: Functional Description, Electrical Interface, Label Assignments, and Word Formats
ARINC-429, Part 2-15: Discrete Word Data Standards
ARINC-429, Part 3-15: File Data Transfer Techniques

Part 1 addresses the physical parameters of the data bus, label and address assignments, and word formats.

Part 2 defines the formats of words with discrete word bit assignments.

Part 3 describes the link-layer file data transfer protocol for data block and file transfers.

KEY FEATURES OF ARINC-429

1. DATA FRAME STRUCTURE

The "Data Frame Structure," a crucial component of ARINC-429, explains how data is organized and transmitted over a communication network. This structure defines the format of the data frame, ensuring reliable transmission between avionic devices.

Label

The label, a vital part of the data frame, serves as an identifier, enabling the receiving system to determine the type of data being transmitted. It typically consists of 8 bits and includes data such as altitude, airspeed, and other system-related measurements. Standardized labels ensure uniformity and interoperability among various avionic systems.

Data Field

The actual data being transmitted resides in the data field. This section carries numerical or status information related to the specified label and is typically 19 bits in size. The data format can be binary, BCD (Binary-Coded Decimal), or another representation, depending on the application.

Parity Bit

The parity bit enhances data integrity by providing a means to detect errors. A single bit is set to ensure the total number of bits in the data frame is either even or odd. The receiving system can identify and flag potential transmission errors using this parity check.

2. ELECTRİCAL CHARACTERISTICS

The "Electrical Characteristics" of ARINC-429 describe the specific electrical properties and signaling techniques used by the protocol to ensure reliable data delivery.

Differential Voltage Signal

Data is transmitted as a voltage difference between two wires, known as a twisted pair. Binary values are represented by voltage levels on these wires. This method offers benefits such as improved noise immunity and reduced susceptibility to electromagnetic interference.

Twisted Pair Wiring

ARINC-429's electrical design uses twisted pair wiring, which reduces electromagnetic interference by canceling out external signals. This configuration is vital for maintaining signal integrity over relatively long cable lengths in aircraft.

Voltage Levels and Binary Representation

Binary data is represented using specific voltage levels in ARINC-429. For example, a "0" may be represented by a positive voltage on one wire and an equal negative voltage on the other, while a "1" is represented by reversing the voltage polarity.

3. DATA RATE AND TRANSMISSION SPEED

ARINC-429's "Data Rate and Transmission Speed" is a critical component that determines the rate at which data is transferred between an aircraft's avionic systems.

Typical Data Rate

The standard data rate for ARINC-429 is 100 kilobits per second (kbps). This rate is suitable for many avionic applications, including monitoring, communication, and navigation.

Higher-Speed Versions

Higher-speed variants of ARINC-429, such as ARINC-429/P2 and ARINC-429/P3, offer faster data transmission rates, with ARINC-429/P3 supporting up to 12.5 megabits per second (Mbps).

Words Per Second (WPS)

The term "Words Per Second" (WPS) is often used to describe the transmission speed in ARINC-429. At 100 kbps, the standard ARINC-429 transmits approximately 3.125 words per second (100,000 bps / 32 bits).

4. LABEL SELECTION AND ASSIGNMENTS

ARINC-429 uses a standardized set of labels to define the type and significance of data transmitted between avionic systems.

Standardized Label Set

ARINC-429 employs a standardized set of labels, each representing a specific type of data. These labels ensure consistency and uniformity across different avionic systems and aircraft.

Data Identification

The label in an ARINC-429 data frame identifies the type of data in the accompanying data field. It acts as a descriptor, indicating the purpose of the transmitted information.

Flexibility in Assignments

While ARINC-429 provides a standard set of labels, users can assign specific meanings or units to these labels based on their avionic systems' unique requirements. This flexibility allows the protocol to adapt to various aviation industry applications.

ARINC-429 STANDARTS

The Mark 33 Digital Information Transfer System (DITS) bus is another name for the ARINC-429 specifications. These buses are primarily used in the avionics industry but are also employed in land vehicles, weapon systems, and other commercial and military equipment industries.

The standards outline the requirements for successful communication, including word structure, electrical component characteristics, and protocol. ARINC-429 uses the standard Mark 33 DITS bus, which is a simplex twisted shielded pair data bus. ARINC-429 specifies the hardware and data formats required for bus transmission.

A single twisted wire pair can connect up to 20 receivers or receiver hardware to a single transmitter or source. Bidirectional communication requires two channels or data buses, as simplex connections can only transmit data in one direction.

PHYSICAL LAYER AND ELECTRICAL CHARACTERISTICS

ARINC-429 uses bipolar return-to-zero (BRZ) differential voltage encoding. This encoding method enables long-distance data communication and provides noise immunity. Data is transmitted as voltage differences between two wires: high voltage (HI) and low voltage (LO).

The physical layer of the ARINC-429 protocol specifies the electrical characteristics required for devices to communicate correctly. It details impedance matching, timing characteristics, and voltage levels. Data is represented by the differential voltage between the HI and LO wires; a 2.5-volt difference represents a binary "1," while a -2.5-volt difference represents a binary "0."

The protocol imposes constraints on minimum resistance, maximum capacitance, and cable lengths to ensure proper electrical characteristics. Adhering to these guidelines guarantees reliable data transmission and reduces the likelihood of signal degradation or interruption.

DATA TYPES SPECIFIED IN ARINC-429

Binary-Coded Decimal (BCD): Each decimal digit is represented by a group of four binary bits. The data field's four bits are used to represent each decimal digit in BCD format.
Binary Number Representation (BNR): Binary numbers are used to store data in BNR encoding. The sign of the data is indicated by bit 29, with "1" representing a negative value.
Discrete Data: Discrete data can consist of ISO #5 bits, BNR bits, or a combination of BCD bits. Conditions such as Pass/Fail, Active/Inactive, and True/False are represented by preset bits in the word data field.
Maintenance Data/Acknowledgment: Maintenance details and acknowledgments require bidirectional communication between the source and receiver. Since ARINC-429 only supports simplex transmission, two ARINC-429 channels are needed for an LRU to send and receive data.
Williamsburg/Buckhorn Protocol: This bit-oriented protocol is used for file transfers over the ARINC-429 data bus. It involves a handshake between the source and receiver units to initiate file transfers.

ARINC-429 DATA TRANSMISSION

Data transmission in ARINC-429 uses three states: HIGH (1), NULL, and LOW (0). The transmission signal is considered HIGH when it goes from NULL to +10V and back to zero. Similarly, it is considered LOW when the signal drops below -10V and then rises. ARINC-429 specifies two data transmission rates: 100 kHz ±1% for high speed and 12.5 kHz with an acceptable range of 12 to 14.5 kHz for low speed.

1. ARINC-429 Bus Topology

Line Replaceable Units (LRUs) are typically configured in a bus-drop or star topology. Each LRU may contain several transmitters and receivers interacting over different data buses. The recommended bus connection topologies are shown in Figure 1. This simple architecture provides near node-to-node communication and highly efficient data transfer.

A transmitter is limited to communicating with up to 20 bus receivers on a single wire pair, where each receiver continuously monitors the relevant data but does not acknowledge receipt. A transmitter may request acknowledgment from a receiver after sending a significant amount of data. This handshake is performed in a verbal style rather than a wired one. When bidirectional communication is required, two twisted pairs or two channels are sufficient to transmit information back and forth.

The LRU source transmits 32-bit words, consisting of an 8-bit label describing the content and a 24-bit data payload. LRUs are identified by equipment identification numbers rather than addresses assigned by ARINC-429, enabling organized equipment management and file transfer systems.

Sequential words are separated by at least 4 bits of NULL or zero voltage. This NULL interval is used between sentences, eliminating the need for a separate clock signal. Therefore, this signal is called a self-clocking signal. Transmission speeds can be either a low speed of 12.5 kHz or a high speed of 100 kHz.

A 78 x twisted pair cable with shielding is used for the bus medium. The shield must be grounded at the bus ends and each junction point. The transmission source output impedance should be evenly shared between Line A and Line B, measured at 75Ω ±5Ω. The cable impedance and this balanced output must closely match.

The receiver sink must have a minimum effective input impedance of 8kΩ. There is no specified maximum length, as it depends on the number of sink receivers, sink drain, and source power. Most systems are designed for less than 150 feet but can be extended to 300 feet or more if conditions permit. ARINC-429 cable characteristics are shown in Figure 2.

2. ARINC-429 Word Formats

ARINC-429 assigns the first 8 bits as the word label, bits 9 and 10 as the Source-Destination Indicator (SDI), bits 11 to 28 as the data information, bits 29 to 31 as the Sign-Status Matrix (SSM), and bit 32 as the Parity Bit. ARINC-429 data words consist of 32-bit words with five main fields:

Parity (1 bit): Odd parity is used as an error check to ensure correct data reception. Bit 32 is set or cleared to make the total number of Logic 1s in the word odd.
Sign/Status Matrix (SSM) (2 bits): Depending on the Label word, the SSM field provides different information about the type of data being transmitted.
Data (19 bits): Bits 11 to 29 contain the word's data information. The format of the data pieces is highly flexible.
Source/Destination Identifier (SDI) (2 bits): Bits 9-10 are optional and can be used to identify the source or destination of the data.
Label (8 bits): The label identifies the type of data in the word and may include instructional content or reporting data.

ADVANTAGES AND LIMITATIONS of ARINC-429

1. Advantages

Standardization: ARINC-429 provides a widely used and standardized communication protocol, ensuring compatibility among various avionic equipment and manufacturers.
Reliability: The protocol's design ensures reliable data transmission, making it suitable for critical aviation systems.
Simplicity: ARINC-429's straightforward design and data frame structure make it easy to implement in aviation systems.

2. Limitations

Data Rate: The maximum transmission speed of 12.5 Mbps may be insufficient for high-speed applications requiring large amounts of data.
Limited Bandwidth: The single-channel architecture and limited bandwidth can be problematic when multiple systems need to exchange large amounts of data simultaneously.
Lack of Multicast Functionality: Without multicast functionality, ARINC-429 messages are sent to targeted receivers one at a time, potentially increasing communication overhead.

ARINC-429 Protocol