Avionics Databus Tutorials

MIL-STD-1553 is a standard defining a local area network (LAN) originally developed for and widely used on military aircraft. This digital, command-response, time-division multiplexing network protocol is also used in many other military and commercial applications where fast, positive control is required. The standard defines the handshaking, data formats and timing requirements of the protocol as well as the electrical characteristics of the bus and the terminals' interface electronics.

One terminal is designated as the bus controller; all others are remote terminals each with a specific terminal address. All transmissions are initiated by the bus controller transmitting a command word. Encoded into the command word is a terminal address, a TR (transmit/receive) bit, a subaddress and a word count. Remote terminals monitor the bus and respond only to commands containing its own terminal address. The remote terminal transmits or receives the specified number of words from/to the specified subaddress. A status word that includes its terminal address is transmitted by a remote terminal before transmitting data words or to confirm reception of data words. Special messages are reserved for broadcast data and mode codes. Words are 16 bits long and are transmitted at one megabit per second. Messages can have up to 32 words. MACAIR is a variation of MIL-STD-1553.

The current version of the MIL-STD-1553 standard is MIL-STD-1553C.

View the Standards

  MIL-STD-1553C
  MIL-STD-1553B Notice 1-4
  MIL-HDBK-1553A Notice 2

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ARINC 429 is the industry standard that is used on virtually all commercial aircraft. It is a specification that defines a local area network for transfer of digital data between avionics system elements. The specification describes how an avionics system transmits information over a single twisted and shielded pair of wires (the databus) to all other system elements having need of that information (up to as many as 20 receivers). Bi-directional data flow on a given databus is not permitted.

There are two speeds (approximately 12.5 and 100 thousand bits per second). Words are 32 bits long including a label, parity bit and other fields. The label is an eight bit field that identifies the type of information contained in the word. The 429 specification defines the units, ranges, resolutions, refresh rates, number of significant bits, pad bits, etc. for the words transmitted by the different avionics system elements.

What is Williamsburg Protocol?

Williamsburg is a bit-oriented protocol used to transfer files between systems over ARINC 429 full duplex links. Each LRU has a transmit connection and a receive connection, and implements a control protocol to manage the data transfer. The original block transfer protocol was AIM (ISO Alphabet No. 5). The Williamsburg protocol, introduced in ARINC 429-12, has replaced the AIM protocol (now discontinued).

Files are transferred in blocks known as Link Data Units (LDU) each containing from 3 to 255 words. A system initiates a transfer by sending a Request To Send (RTS). Following the receipt of a Clear To Send (CTS), the source begins the LDU sequence with a Start Of Transmission (SOT) followed by up to 253 data words. The LDU transfer is terminated with an End Of Transmission (EOT) containing a 16-bit CRC value calculated from the contents of the LDU. On successful verification of the CRC value the sink system sends an Acknowledgment (ACK) message. The source may then repeat the process until the entire file is transferred.

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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.

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ARINC 708 describes the characteristics of an airborne pulse Doppler Weather Radar system intended for installation in commercial transport type aircraft. Display data and system mode/status information is transmitted over the Display Databus from the Transmitter-Receiver unit (T-R) to the Control/Display Unit (CDU). Data on the Display Databus consists of 1600 bit messages (words) that are preceded by and followed by a sync. Except for the following sync, bit count, and lack of parity, the signal characteristics on the Display Databus are very similar to MIL-STD-1553.

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ARINC 664 is a multipart specification that defines an Ethernet data network for aircraft installations. Part 7 of ARINC 664 defines a deterministic network, also known as Avionics Full Duplex Switched Ethernet (or AFDX®). AFDX® is a registered trademark of Airbus Deutschland GmbH. These extensions to IEEE 802.3 address the special requirements (quality of service, reliability, etc.) of an aircraft environment. With its much higher data rates than older databus protocols ARINC 664 is an attractive protocol for both new and retrofit aircraft systems.

Two commonly used databuses were used on the Space Shuttle. The Multiplex Interface Adapter (MIA) databus is a command response protocol that uses 24-bit words (plus sync and parity). Though some MIA characteristics are very similar to MIL-STD-1553 (e.g., biphase Manchester encoding, 1 megabit per second bit rate, sync, parity, and electrical), the two databuses differ in a number of ways. Both protocols have a terminal address in the command word, but other bits are used differently. Words on the MIA databus are 24 bits long and have 5.5 microsecond gaps between them, while words on the 1553 databus are 16 bits long and are contiguous. There are no status words in the MIA protocol.

The MDM (Multiplexer/Demultiplexer) databus provides serial point-to-point communications between a Space Shuttle Payload General Support Computer (PGSC) and various subsystems. The MDM interface consists of a serial databus and three discretes (Message In, Message Out, and Word). The discretes control the timing, direction, and number of words on the serial databus. The serial databus is bi-directional, but the discretes are always driven by the bus controller (the PGSC) and received by the remote terminal. The speed of the MDM bus is 1 megabits per second. Words have 16 bits, and messages can have up to 32 words.

ARINC 419 catalogs and compares many of the older avionics databus standards. Protocols for the transmission of digital data were originally defined as part of the standardization of individual equipment characteristics. Since there was no universal databus standard and needs varied, different digital transmission systems were called out in the equipment characteristics for:

• ARINC 561 Inertial Navigation System (INS)
• ARINC 568 Distance Measuring Equipment (DME)
• ARINC 571 Inertial Sensor System (ISS)
• ARINC 573 Aircraft Integrated Data System (AIDS)
• ARINC 575 Digital Air Data System (DADS)
• ARINC 579 VOR Receiver
• ARINC 582 Air Navigation System
• ARINC 585 Electronic Chronometer System (ECS)

Many of these equipment characteristics use more than one protocol to communicate to other components. ARINC 419 defines a system and then classifies these protocols according to their message, electrical, logical, and timing related elements. The lessons learned from these original protocols lead to the general purpose ARINC 429 protocol.

ARINC 453 was an aborted attempt to define a high speed databus that would improve on the 100K bits per second maximum data rate of ARINC 429. The idea was to make use of the 1M bit per second data rate and electrical characteristics of MIL-STD-1553 while preserving the 32 bit word structure of ARINC 429. Though there are some references to ARINC 453, the document was never approved so no equipment was developed using this protocol.

The ARINC 573 Expandable Flight Data Acquisition and Recording System (EFDARS) was the predecessor to the ARINC 717 Digital Expandable Flight Data Acquisition and Recording System (DEFDARS). The ARINC 717 databus was first defined in ARINC 573 as the data path between the Flight Data Acquisition Unit (FDAU) and the Digital Flight Data Recorder (DFDR). ARINC 573 is equivalent to the slow speed (64 words per second) version of 717.

ARINC 575 is an equipment characteristic for a Digital Air Data System (DADS) that provides essential air-data information for displays, autopilots, and other flight controls and instrumentation on commercial and transport type aircraft. ARINC 575 defines a digital databus to distribute information to displays and other systems. This databus was later described in ARINC 419 and became what is now known as ARINC 429. There are only minor differences between the digital databus of ARINC 575 and ARINC 429. The most significant difference is that ARINC 429 reserves bit 32 for parity, while ARINC 575 could use bit 32 for either parity (when BNR) or data (when BCD).

MACAIR is a nickname for a family of avionics databus specification that came out of McDonnell Douglas for Navy aircraft. These protocols are very similar to MIL-STD-1553 except that their transmitted waveforms are controlled to look more like sine waves than a trapezoid. The reason is that there are fewer high frequency components in a rounded waveform, and therefore it reduces the possibility of electromagnetic interference to sensitive aircraft systems. Generally, MACAIR and MIL-STD-1553 terminals can communicate with each other without difficulty. However, some purists chose to replace a terminal's MIL-STD-1553 transceiver with a pin compatible MACAIR version.