# Avionics Data Bus

## Summary


A bus is a data highway that links one LRU (Line Replaceable Unit) to another. In earlier avionic systems, communication cables are used to transmit the data. To transfer a bit of data at least one pair of wires is needed for each signal. These wires transmit 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). A data bus connects all the internal components of the computer to the CPU and main memory. When referred to as an aircraft, it is the data highway that links one computer to another aircraft like the FMC (Flight Management Computer) and the ADC (Air Data Computer).

## Discussion

### What are the types of avionics data bus?

Based on the type of connection data buses are classified into two types. They are serial bus and parallel bus.

    + **Serial Bus**: As the mentioned in the name, each bit of the data word is transferred via the same wire. Only one bit of word is sent at a time. Examples of serial data bus are PCIe, USB, ARINC, I2C, etc.
    + **Parallel Bus**: In this type, each bit of the data word is transferred via a specific wire. It requires a lot of wiring. Examples of parallel bus are conventional PCI.

### Which type of data bus is better, serial or parallel?

Parallel buses should have been faster than serial data buses. But, there are many issues in using parallel data buses. Such as, parallel buses suffer from clock skew. It means a bit of data can reach the destination before or after other bits. It causes lack of synchronization in parallel data buses. Also, the no of wires used in the parallel data bus is more than the serial data bus. Also, parallel data buses are not synchronized as fast as the serial data buses results in low data rate. 


### What are the types of serial digital data transmission?

Common serial digital data transmissions are two types. They are single source-single sink and multiple source-multiple sink.

    + **Single Source-Single Sink**: It is the earliest application that contains a dedicated link. One LRU (line replaceable unit) communicates with a single LRU. Example: Sea Harrier and Tornado aircraft.
    + **Single Source-Multiple Sink**: In this system, one LRU transmits data to multiple LRUs at a time. ARINC 429 is the example for this type.
    + **Multiple Source-Multiple Sink**: In this system, multiple computers transmit data to multiple LRUs at a time. This is also known as full duplex and is widely used by military users. Example: MIL-STD-1553B and ARINC 629.

### Why avionics data buses are created in such a way?

In 1950s and 1960s, communication between two stations is done through a lot of wires without a bus. It makes very difficult for the installation and also for the modification. The parts of these systems are elctro-mechanical which are heavy and requires large space. After that digital systems are used to transmit the data. They are also heavy, slow and difficult to reprogram. Each component of the system contains its own computer and memory. They use a bus between components limiting the usage of more wires. 


### What are the major avionics data buses?

Many commercial and military aircraft utilize many avionics data bus technologies. The major avionics data buses are MIL-STD-1553, ARINC 429, ARINC 629. 

    + ARINC 429 defines how avionics equipments and other systems communicate on the aircraft. It is a unidirectional broadcast bus (simplex). The highest speed of data transmission is 100kbps and the least is 12 to 14.5kbps. It is a low weight and low cost with high reliability.
    + MIL-STD-1553B is a serial time division command response data bus. It has a bit rate of 1Mbps. It is a military half-duplex protocol.
    + ARINC 629 is used in Boeing 777. It is a multi-transmitter data bus. It is half-duplex with a bit rate of 2Mbps. The data is transferred using electromagnetic induction between two or more systems.

### What are the problems a data bus solves?

Data buses solve many problems. They are: 

    + **ARINC 429**: It is developed to promote interoperability between devices on civil aircraft. Designed to connect a single transmitter to 10 receivers. Messages are expected to be transmitted between once and up to 50 times a second. Receivers receive their data simultaneously which accomplishes the synchronization.
    + **MIL-STD-1553B** Designed to connect up to 32 devices with the ability to transmit 16-bit words of data in a message. Simultaneous transmission by multiple devices is prevented. Synchronization is accomplished by using a Synchronization Mode Code.
    + **ARINC 629**: Published in 1990. It is intended to work in concert with ARINC 429. It is an attempt to reduce the wiring complexity inherent in the point-to-point ARINC 429 architecture. Facilitates the transfer of non-periodic data such as databases or video. Designed to connect up to 120 devices and be able to transmit strings of up to 256 sixteen-bit words with a speed of 2MHz. Simultaneous transmission by multiple devices is prevented by a set of timers in each device along with the ability to detect transmission on the bus.


## Milestones

1929

Four major airlines found ARINC to offer a single radio communications licensee and coordinator outside the government. 

1970

ARINC 429 is widely used in commercial aircraft. 

1970

Single Source-Single Sink used in Tornado and Sea Harrier avionics system. 

1973

US air force first publishes the MIL-STD-1553B standard data bus to use in the F-16 Falcon fighter aircraft. 

1975

Many improvements are done in ARINC 424 to introduce new record types and field values and revision of coding rules are done as required by new technologies and classes of equipment such as GPS and Flight Management Computers. 

1978

ARINC introduces a datalink system known as ACARS (Aircraft Communications Addressing and Reporting Systems). It uploads and downloads data via an onboard Communications Management Unit (CMU). 

1980

Boeing develops a digital data bus known as Digital Autonomous Terminal Access Communication (DATAC) is the digital data bus which later becomes an ARINC standard as ARINC629. 

1995

The ARINC 629 computer bus introduced to use on the Boeing 777. 

2013

ARINC Inc., sold a small subsidiary and is looking to sell another in the wake of its $1.4 billion purchase of the aviation engineering and communications firm. 

2014

SAE International bought the ARINC Industry Activities (IA) Division. It now operates under the SAE Industry Technologies Consortia (SAE ITC). 

2018

MIL-STD-1553C is the latest version of MIL-STD-1553 made in February.