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The interconnection of avionics devices through standard digital buses has paved the way for mod­ern avionic systems to evolve to the level of com­plexity and sophistication achieved in today`s com­mercial and military aircraft. They allow the entire complement of electronic functions employed on an aircraft to be partitioned both functionally and physi­cally on the aircraft platform. They also provide a well-defined boundary between avionics system components allowing design teams to develop these components independently while maintaining a high probability of successful integration into the complete system. Databuses allow true integration of avionics, which is the hallmark of emerging archi­tectures.
Weighing the pros and cons in selecting a bus standard is rarely an obvious choice. Issues of capacity, reliability and cost all weigh in different for each of these different buses. These issues of ca­
become particularly important to military transport designer who finds commercial databus standards migrating into his systems. 
 Fig.1 address the fundamental characteristics of these three buses.

ARINC 429 has seniority in the family of avionics data buses that are still being used on new platforms. Adopted by the industry in 1977, it is probably the most frequently used avionics databus found on commercial aircraft. ARINC 429 is well rooted into equipment designs, proven on many airframes and thus enjoys continued popularity based at least in part on its momentum. It also has found its way into military programs such as EUROFIGHTER aircraft. 
ARINC 429 is a simplex databus using one and only one transmitter and at least one but no more than twenty receivers for each bus implementation. The number of receivers is limited by impedance considerations rather than protocol limitations, that is, the combined parallel input impedance of 20 receiver loads (400 min.) is the heaviest load that any ARINC429 transmitter can be expected to drive. Bidirectional communication between devices (if required) is implemented by using two ARlNC 429 buses, one in each direction.   
Each ARINC 429 bus uses only two wires to transmit a return—to-zero bipolar 10V (5V relative to ground) trilevel (Hi, LO, and NULL) signal which ef­fectively encodes both the data and data clock in­formation (See Figure 2). 




The ARINC 429 specification defines a high data rate of 100Kbps 1% and a low data rate between 12.0 and 14.5 Kbps. Data is transferred on ARINC 429 buses serially in 32 bit words. Each word transmitted on an ARINC 429 bus is separated by an interword gap of four bit-times. Every word contains an odd parity bit and an eight bit system address label (SAL). ARINC 429 specifies several different protocols, each of which use the remaining bits in each word differently.   
 Three protocols are defined in ARINC 429 for Nu­meric Data, Discrete Data and File Data. Numeric Data in either BCD or binary format and Discrete Data use two bits which uniquely identify the data source (ie primary, secondary, tertiary) and another two of three bits for sign and status information in addition to the parity and SAL fields, leaving 18 or 19 bits of each 32 bit word available for transmission of data. Generally Numeric and Discrete data transfers are done using a unique SAL to identify the single piece of data contained in each word.
File Data transfers (also known as Bit Oriented Communication, Williamsburg, or Buckhorn proto­cols) are accomplished by a sequence of ARINC 429 words each having the same SAL.
 In addition to the SAL and parity bits,each word contains a 3 bit word type identifier. 
The word type may be a start—of—transmission (SOT), end—of—transmission (EOT), data word, or protocol word. A minimum message consists of an SOT, data word and EOT but may contain up to 253 data words each with 20 bits of data. The File Data protocols reflect the most recent enhancements to Specification ARINC 429 and have been developed to allow use of ARINC 429 for the transfer of data for Future Air Navigation System (FANS), Aircraft Com­munications Addressing and Reporting System (ACARS) and other OSI (Open System Intercon­nect) oriented applications.  





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