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its resolution, maximum and minimum transit interval, and for some labels,
the maximum transport delay.
Typically, messages are sent repetitively. For example, measured airspeed
is transmitted from the sensor to the instrument at intervals not less than
100 milliseconds or greater than 200 milliseconds. Messages may also be
sent in repetitive word sequences or frames. Messages from each fuel tank
level sensor are sent in sequence, and then the sequence is repeated after a
specified time. The specific data source to which the data applies is
determined either by the Label or the SDI.
Table 10 shows label 241, which is transmitted approximately once per
second. The sequence shown starts with the left main tank followed by the
right and then center. Once the 63-word sequence is completed, it repeats,
starting over with word 1. Most of the data is in BNR format, but some
words are in BCD.
Table 10. Message Sequence for Label 241
Word Signal Units Range Sig.
Digits Resolution Data
1 Left Main Tank #1 pF 319.922 12 .078125 BNR
2-13 Left Main Tanks #1 to #13 pF 319.922 12 .078125 BNR
14 Left Main Tank #14 pF 319.922 12 .078125 BNR
15 Left Main Bite Cap. No. 1 pF 319.922 12 .078125 BNR
16 Left Main Compensator pF 319.922 12 .078125 BNR
17 Load Select 10,000 Lb. 0-90000 1 10,000 BCD
18 Load Select 1,000 Lb. 0-9000 1 1,000 BCD
19 Load Select 100 Lb. 0-900 1 100 BCD
20 No Data Transmitted
21 Left Main Fuel Density Lb./Gal 8,000 12 .000977 BNR
22-42 Repeat Words 1-21 for Right Tanks
43-53 Repeat Words 1-21 for Center Tanks
54-58 No Data Transmitted
59 Load Select 10,000 Lb. 0-90000 1 10,000 BCD
60 Load Select 1,000 Lb. 0-9000 1 1,000 BCD
61 Load Select 100 Lb. 0-900 1 100 BCD
62 No Data Transmitted
63 Center Tank Density Lb./Gal 8,000 12 .000977 BNR
ARINC 429 Tutorial Bit Oriented Protocols
ARINC Protocol Tutorial 15
Bit Oriented Protocols
The Williamsburg, or "bit oriented", Protocol is a system for transferring
files between ARINC units. It was originally defined in ARINC
Specification 429-12 and expanded in Specifications 429-13 and 14 and is
further defined in Reference 5. It is currently under revision. It should be
used in lieu of the former AIM, file transfer, and maintenance formats used
in Specification 429-11. Normal ARINC data messages can be intermixed
with the bit-oriented messages of the Williamsburg Protocol.
A start up procedure is used to determine the proper protocol for
transferring data. When a system element wants to use the bit-oriented
system it transmits a message using the latest version of which it is capable.
A handshaking process adjusts the protocol to the lowest common
denominator that both sending and receiving systems can use. Currently,
two versions of the Protocol are available. Version 1 is defined in 429-12
and refined in 429-13. Version 2 is defined in 429-14 but was never used.
Reference 5 deletes Version 2 and it defines a new Version 3. It redefines
Version 1 to facilitate the communications of the ACARS Management
Unit (MU) and the Satellite Data Unit (SDU).
The source initiates communications by sending certain predefined codes.
If a bit-oriented transfer is desired, the initial code word will be an "ALO"
(for Aloha) signal to the potential recipient. The ALO word should be
sent by any system that supports the protocol just after the system powers
up, or performs a re-initialization for any reason. If any sink is capable of
receiving bit-oriented data it responds with an "ALR" code so that the
source knows that it can transmit to that unit. When a source wants to
transmit to a unit capable of handling the protocol, it sends a Request to
Send word (RTS), and waits to receive a Clear to Send (CTS). The RTS
includes a Destination Code and a Word Count, which are repeated in the
CTS for verification. If the CTS is correct, the source then initiates the file
transfer, following the sequence shown in Figure 8 for version 1. The
latter version provides the capability of sending a larger file (up to 7
LDUs), without needing to renew permission of the sink.
Files are transferred in blocks called Link Data Units (LDU) ranging in
size from 3 to 255 words. Following receipt of the CTS, the source
initiates a Version 1 transfer with a Start of Transmission word (SOT).
The SOT includes a file sequence number, a General Format Identifier
(GFI), and a LDU Sequence Number. The data words are then sent,
followed by the (up to) 255th word which is an End of Transmission
(EOT). Each LDU transfer (255 words or less) is terminated by an End of
Transmission Word (EOT). The EOT includes a CRC and identifies the
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