TECHNICAL

ARINC 429 REVISITED

BY KIM WIOLLAND TECH-AID PRODUCTS

he ARINC 429 has been around now for over two de- Tcades and this article is intended to refresh us with its structure and defi- nition. In the early 1980s, the ARINC Committee decided to take knowledge gained from ARINC specification 419 that had become a collection of several digital data protocols such as ARINC 561(LRN), 568(DME), 573(FDR) and Label 001, Distance to Go (DTG); Label 100G, Selected Course (CRS); Label 114, Desired 575(DADS) to name a few. Track (DTK); Label 115, Waypoint Bearing (WPT) The systems evolving then were incorporating more computer first format uses a twoʼs complement components and requiring more pro- fractional binary notation (BNR) and comes to the forefront in any trouble- cessing functions. As a result of this the second format implements a Bina- shooting process, however it does growth, a demand was seen to pass ry Coded Decimal (BCD) notation. show the importance of the details em- this volume of information to other The large majority of assigned la- bedded within the software handling systems. The new standard became bels implement BNR notation because the data. The transmitter port should known as the “Mark 33 Digital Infor- it yields better resolution. This bus be able to handle up to 20 receiver mation Transfer System” (DITS). This operates open loop in that it requires ports without any degradation in sig- is a scheme where a source will trans- no feedback directly, however some nal. The amount of data or label con- mit information out of a designated equipment manufacturers through soft- tent on any particular transmitter port port over a single twisted and shielded ware will echo back some data from a will vary depending on the equipment wire pair to multiple receivers. receiver to confirm system integrity. manufacturer and the systems that are The committee realized a more The committee also recognized that interfaced. comprehensive digital information data validity would be important and The digital word is made up of 32 transfer system could evolve from this so the bus structure would also contain discrete bits with the least significant philosophy that would allow multiple a parity bit. bit sent first. If in any word all loca- label groups to be sent. A single direc- The bus employs a bi-polar, return tions are not used then they are filled tional asynchronous bus that could be to zero (RZ) form of logic that is typi- in with binary zeros unless BNR/BCD extremely flexible and expandable was cally driven by +/-10v supplies. The numeric data is present and in this case required. bit rate employed within the valid data bits are used. Labels that Bi-directional data flow on a given word can be in the range of 12.5-14.5 contain numeric data by nature require data bus is not permitted and, there- kilobits/sec (Low Speed) or 100 ki- more resolution and will typically use fore, separate transmit/receive ports lobits/sec (High Speed). The transfer all available data bits. A case in point would need to be assigned. This data rate or refresh rate that each label op- would be the previously mentioned format would also contain the clock erates at is determined by its critical- Label 204 where the required data res- within the bit stream and then be re- ity within the affected systems and so olution is 17 bits (2e17). generated at the receiving equipment. defined in the specification. Different LABEL There are five application groups for refresh rates can be seen, for example, this 32 bit word and they are: BNR in Label 204, Baro-Corrected Altitude, The Label or first eight bits of the data, BCD data, discrete data, main- where the minimum transfer rate is 31 32 bit word serve to define the content tenance data and acknowledgement. msec vs. Label 012, which represents and meaning of the data. Labels are This new philosophy could then in- Ground Speed where its minimum typically represented in octal numbers tegrate all systems on the aircraft by transfer rate is 250 msec. and these first eight bits are transmit- using different encoding formats. The This specific detail is one that never ted in reverse order with the MSB

42 AVIONICS NEWS • NOVEMBER 2005 TECHNICAL

Table 1 BIT POSITION BIT 1 BIT 2 BIT 3 BIT 4 BIT 5 BIT 6 BIT 7 BIT 8 BIT WEIGHT MSB LSB OCTAL WEIGHT 2 1 4 2 1 4 2 1 DATA 0 0 0 0 1 0 1 0

transmitted first. In all cases the label pad bits or discrete bits as mentioned represents VHF Comm Frequency is always transmitted ahead of the data previously. The pad bit would be used (BCD). This label will identify a logic so the receiving equipment can identify to fill out the field if that bit location “1” at bits 30 and 31 as a “Squelch and decode or just disregard the data. was not required. A discrete bit might Disable” condition. There are some differences among be imbedded through association such the labels as you go from the original as the Marker light bits in VOR Label PARITY ARINC 429P1-15 specification to the 222 where they occupy bits positions The Parity bit which occupies the GAMA ARINC 429 specification and 11, 12 and 13. The bit weighting will last bit in the data stream (32) is in- most equipment configuration routines be in the form of BCD or BNR as de- tended for error detection and correc- will reflect this. Label 371 is used to scribed in the 429 specification. Table tion. This bit will constantly change as identify the equipment by manufac- 3 and Table 4 show the differences in the label and data change in order to turer. This label contains a hexadeci- the data fields between BCD and BNR provide “Odd Parity.” What this means mal equipment ID code (bits 11-18) encoding. is that at any time the 32 bit word will followed by a binary company ID code always have an odd number of 1s con- (bits 19-24). This identification infor- SSM FIELD tained within it. mation is important if specific software The Sign and Status Matrix field can sub-routines need to be implemented. represent many different conditions TROUBLESHOOTING Shown in Table 1 is a breakdown for based on whether the data is encoded There are some simple checks that label 012 which represents Ground in BCD or BNR form. The BCD form can be made to increase oneʼs con- Speed. utilizes only bits 30 and 31 as shown fidence in the operation of the bus. above where BNR encoding also em- Digital Storage oscilloscopes like the SDI ploys bit 29. Table 5, Table 6 and Agilent 54622D model can be a tech- Bits 9 and 10 in the data contain Table 7 on page 44 depict these two nicianʼs best friend. Storage scopes what is called the Sign/Status Matrix or encoding forms and how they differ. will allow you to freeze data in memo- SSM. This two bit field contains equip- There all always exceptions to rules, ry and then search through memory in ment condition, operating mode (Sys- a case in point is for Label 030G which Continued on following page tem 1, 2, 3 or All Call) or data validity. When you configure a GPS or FMS as LRN1 or LRN2 for example you are Table 2 setting the SDI bit pattern. BIT POSITION BIT 9 BIT 10 IDENTIFICATION A situation where bits 9 and 10 are 0 0 ALL CALL required for part of the data field would 1 0 SYSTEM #1 be the BCD encoding of present posi- 0 1 SYSTEM #2 tion latitude/longitude as in Label 010 1 1 SYSTEM #3 (latitude) and Label 011 (longitude). When the receiving equipment is con- Table 3

figured as System #1 then it will only v v BIT 32 31 30 29 11 want to decode and process data that is identified for System #1 and other data PARITY SSM BCD DATA FIELD/PAD/DISCRETES will be ignored. Table 2 depicts the MSB LSB SDI bit weighting. Table 4 DATA FIELD BIT 32 31 v 29 28 v 11 10 9 The data field will contain the infor- PARITY SSM BNR DATA FIELD/PAD/DISCRETES mation so designated by the label pre- MSB LSB ceding it. This field also may contain

AVIONICS NEWS • NOVEMBER 2005 43 Table 5 BCD ENCODING BIT 31 BIT 30 IDENTIFIED AS: 0 0 PLUS, NORTH, EAST, RIGHT, TO, ABOVE 0 1 NO COMPUTED DATA (NCD) 1 0 FUNCTIONAL TEST 1 1 MINUS, SOUTH, WEST, LEFT, FROM, BELOW

Table 6 BNR ENCODING (SYSTEM STATUS) BIT 31 BIT 30 IDENTIFIED AS: 0 0 FAILURE WARNING 0 1 NO COMPUTED DATA (NCD) 1 0 FUNCTIONAL TEST 1 1 NORMAL OPERATION

Table 7 BNR ENCODING (DATA SIGN) BIT 29 IDENTIFIED AS: 0 PLUS, NORTH, EAST, RIGHT, TO, ABOVE 1 MINUS, SOUTH, WEST, LEFT, FROM, BELOW

ARINC REVISITED support team for the equipment man- Continued from page 43 ufacturers can always be helpful to time to analyze that data. This simple isolate a problem however sometimes check will allow the technician to con- only the software engineer may know firm the signal integrity of the trans- the answer to your problem. mit port. Normally if data is present In todayʼs environment where tech- on both the TX-A and TX-B lines the nical data is becoming more restricted, source is considered operational, just the systems more complex and inte- make sure the lines are not reversed. gration more software dependent, the Selecting between High Speed and technician needs to constantly sharpen Low Speed bus operation is another his/her troubleshooting and insight simple check that will help confirm skills as well as be provided the tools bus operation. to perform the task. Persistence and The transmitted data does come out knowledge are certainly important in a complimentary fashion and this is tools to have in this ever changing clearly seen by comparing the TX-A world. line to the TX-B line. If data is pres- With respect to general knowledge, ent and changing, then specific Label a good tool to put in your shop library verification is the next step. There are would be the Bendix/King training many bus analyzers out there from document entitled “Common Digital portable models to computer based Circuitry.” This publication has prov- cards. If your shop has invested in an en very helpful over the years in not analyzer then verifying just one label only maintaining their equipment but is all the confidence one needs to prove also just providing a good knowledge bus integrity. base. Reference documents from both Next itʼs on to Installation and/or ARINC and GAMA are also available Software Bulletins that might be ap- on the 429 standard discussed in this plicable to your symptoms. The field article. ❑

44 AVIONICS NEWS • NOVEMBER 2005