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the transmission. In some cases, these bits are used for data. ARINC 429
can have only one transmitter on a pair of wires, but up to 20 receivers.
Label
Bits 8 through 1 contain a label identifying the data type and the
parameters associated with it. The label is an important part of the message
and is described in more detail below. It is used to determine the data type
of the remainder of the word and, therefore, the method of data translation
to use. The various data types are described in detail below. Labels are
typically represented as octal numbers.
Transmission Order
The least significant bit of each byte except the label is transmitted first,
and the label is transmitted ahead of the data in each case. The order of the
bits transmitted on the ARINC bus is as follows:
8, 7, 6, 5, 4, 3, 2, 1, 9, 10, 11, 12, 13 … 32.
When a 32-bit ARINC word is transmitted on the bus, in the case of the
label, the most significant bit is transmitted first. This reverse order is in
contrast to the transmission order of the other bits in the ARINC word.
Note:
ARINC 429 Tutorial ARINC 429 Data Types
ARINC Protocol Tutorial 9
ARINC 429 Data Types
All ARINC data is transmitted in 32 bit words. The data type may be
Binary Coded Decimal (BCD), two’s complement binary notation (BNR),
Discrete Data, Maintenance Data and Acknowledgment, and ISO Alphabet
#5 character data. In the newest versions, bit oriented packets of messages
can be used to transmit files.
BCD Data Encoding
BCD, or binary-coded-decimal, is a common data format found in ARINC
429 and many other engineering applications. In this format, four bits are
allocated to each decimal digit. A generalized BCD message is shown in
Figure 4. Its data fields contain up to five sub-fields. The most significant
sub-field contains only the bits, so that its maximum decimal value can be
7. If the maximum decimal value is greater than 7, bits 29 through 27 are
padded with zeros and the second sub-field becomes the most significant.
The example message in Figure 5 conveys the data that the DME distance
is 25786 and has a positive sign. The specific equipment, numeric scale,
and location of the decimal point are a function of the label and are
discussed later.
32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 1
P SSM CHAR 1 CHAR 2 CHAR 3 CHAR 4 CHAR 5 SDI LABEL
Figure 4. Generalized BCD Word Format
32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 1
P SSM 0 1 0 0 1 0 1 0 1 1 1 1 0 0 0 0 1 1 0 SDI LABEL
0 0 2 5 7 8 6
Figure 5. BCD Word Format Example
BNR Data Encoding
BNR or “binary” encoding is also a very common ARINC data format.
This type of encoding simply stores the data as a binary number, much in
the same format that is used on virtually every modern-day computer.
Figure 6 shows the general BNR format. Bit 29 is the sign bit and bit 28 is
the most significant bit of the data field, which represents one half of the
maximum value of the parameter being defined. Successive bits represent
the increments of a binary fraction series.
ARINC 429 Data Types ARINC 429 Tutorial
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32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 1
P SSM Data Pad SDI LABEL
Figure 6. Generalized BNR Word Format
Negative numbers are encoded as the two’s complement of positive values.
If bit 29 is a ‘1’ then the number is negative (or South, West, Left, From, or
Below). Otherwise, it is positive (or North, East, Right, To, or Above).
Figure 7 shows an example of BNR encoding. The particular message uses
label 103, which is Selected Airspeed. By referencing the ARINC 429
specification, we know that the scale is 512, and 11 bits are used (29
through 19). A zero in bit 29 shows that this is a positive value. The
numeric value is obtained by multiplying the scale factor, determined from
data type associated with the label, by the ratio indicated by each
successive bit and adding them together. Bit 28 is ½ of the scale factor
(256 in this case), bit 27 is ¼ of the scale factor, bit 26 is 1/8 of the scale
factor, bit 23 is 1/64, bit 22 is 1/128, etc. Thus, in this example, Selected
Airspeed = 268 Knots (256 + 8 + 4).
32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 1
P SSM Data Pad SDI LABEL
0 1 1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 103
Figure 7. Example BNR Encoding
This may appear to be more complex than it really is. The underlying
principle is conventional binary mathematics as performed by any modern
day computer. A computer programmer can shift the BNR data and sign
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