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01F Fuel Quantity Totalizer 041 Satellite Data Unit
020 DFS System (720) 046 CTU
023 Ground Proximity Warning Sys (723) 047 Digital Flight Data Recorder
024 ACARS (724) ---- additional items
025 Electronic Flt. Instruments (725) ---- “
026 Flight Warning Computer (726) ---- “
027 Microwave Landing System (727) 241 High Power Amplifier
The transmitting and receiving circuits must be designed for reliably
sending and detecting the null transition between high and low states. The
parameters vary with the type of operation as defined in Reference 2. The
ARINC 429 Tutorial ARINC 429 Electrical Characteristics
ARINC Protocol Tutorial 5
slew rates and tolerances are shown in Figure 1 for both 100K and 12.5K
data rates.
Figure 1. ARINC 429 Bit Encoding Example
Table 2 summarizes ARINC 429 characteristics.
Table 2. ARINC 429 Characteristic Summary
Electrical Characteristic Value
Voltage Levels, each leg with respect to ground +5V, 0V, -5V
Voltage Levels, Leg A with respect to Leg B +10V, 0V, -10V
Bit Encoding Bipolar Return to Zero
Word size 32 bits
Bit Rates 100K or 12.5K bits per second
High Speed Slew Rate 1.5 +/- 0.5 μsec
Low Speed Slew Rate 10 +/- 5 μsec
Protocol
ARINC 429 is a very simple, point-to-point protocol. There can be only
one transmitter on a wire pair. The transmitter is always transmitting either
32-bit data words or the NULL state. There is at least one receiver on a
wire pair; there may be up to 20.
In most cases, an ARINC message consists of a single data word. The
label field of the word defines the type of data that is contained in the rest
of the word.
ARINC 429 Electrical Characteristics ARINC 429 Tutorial
6 ARINC Protocol Tutorial
Bit Timing and Slew Rate
The slew rate refers to the rise and fall time of the ARINC waveform.
Specifically, it refers to the amount of time it takes the ARINC signal to
rise from the 10% to the 90% voltage amplitude points on the leading and
trailing edges of the pulse. See Figure 2.
Table 3. ARINC Bit Characteristics
Parameter High Speed Low Speed
Bit Rate 100K bits/second 12.5K-14.5K bits/second
Time Y (one bit) 10 μsec ± 2.5% 1÷(bit rate) μsec ± 2.5%
Time X 5 μsec ± 5% Y/2 μsec ± 5%
Pulse Rise Time 1.5 ± 0.5 μsec 10 ± 5 μsec
Pulse Fall Time 1.5 ± 0.5 μsec 10 ± 5 μsec
Figure 2. Slew Rates and Bit Timing Diagram
ARINC 429 Tutorial ARINC 429 Word Format
ARINC Protocol Tutorial 7
ARINC 429 Word Format
ARINC data words are always 32 bits and typically use the format shown
in Figure 3 which includes five primary fields, namely Parity, SSM, Data,
SDI, and Label. ARINC convention numbers the bits from 1 (LSB) to 32
(MSB).
32 31 30 29 11 10 9 8 1
P SSM DATA PAD DISCRETES
MSB LSB
SDI LABEL
Figure 3. Generalized ARINC Word Format
Parity
The MSB is always the parity bit for ARINC 429. Parity is normally set to
odd except for certain tests. Odd parity means that there must be an odd
number of “1” bits in the 32-bit word that is insured by either setting or
clearing the parity bit. For example if bits 1-31 contain an even number of
“1” bits, bit 32 must be set to create ODD parity. On the other hand, if bits
1-31 contain an odd number of “1” bits, the parity bit must be clear.
SSM
Bits 31 and 30 contain the Sign/Status Matrix or SSM. This field contains
hardware equipment condition, operational mode, or validity of data
content. Applicable codes are shown in Table 4and
Table 5.
Table 4. SSM Codes for BCD data
Bit Meaning
31 30
0 0 Plus, North, East, Right, To, Above
0 1 No Computed Data
1 0 Functional Test
1 1 Minus, South, West, Left, From, Below
Table 5. SSM Codes for BNR data
Bit Meaning
31 30
ARINC 429 Word Format ARINC 429 Tutorial
8 ARINC Protocol Tutorial
0 0 Failure Warning
0 1 No Computed Data
1 0 Functional Test
1 1 Normal Operation
Data
Bits 29 through 11 contain the data, which may be in a number of different
formats. Some examples are provided later in the tutorial. There are also
many non-standard formats that have been implemented by various
manufacturers. In some cases, the data field overlaps down into the SDI
bits. In this case, the SDI field is not used.
SDI
Bits 10 and 9 provide a Source/Destination Identifier or SDI. This is used
for multiple receivers to identify the receiver for which the data is destined.
It can also be used in the case of multiple systems to identify the source of
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