Aerospace and Electronic Systems - September 2018 - 23

Di Perna and Rodrigues

CI + CI2 + 12 S FC 2CD,0CD,2W 2

(12)

S FC CD,0 ρ S

where ρ is the air density, CD,0 is the coefficient of profile drag, CD,2 is
the coefficient of induced drag, and CI is the FMS cost index corresponding to the ratio of the time related cost and fuel cost. This index
is determined by the pilot who inputs it into the FMS.

THE ARINC 429 COMMUNICATION PROTOCOL
The ARINC 429 Specification [20], [21] describes how
avionics systems should interact with one another, i.e.
the electrical characteristics and the bit structure. The
avionics devices (such as a global positioning system
(GPS)) are designated as line replaceable units (LRUs).
The LRUs are connected using a shielded twisted pair
wire and only support unidirectional communication.
Each transmitter supports up to 20 receivers on a single
twisted pair wire. For bidirectional communication,
such as serial receive and transmit, a second twisted
pair wire is required. Figure 7 presents the different
topologies found in ARINC 429, with the "Star" and
"Bus-Drop" topologies being the most commonly used.
A message in ARINC 429 commonly consists of
a single 32-bit word, although multiple 32-bit words
are possible in a single message (see Figure 8). The label and the parity bit are the only necessary overhead
bits for each 32-bit word, while the sign/status matrix
(SSM) and source/destination identifier (SDI) bits can
be used to provide extra precision. The use of the SSM
and SDI bits are specific to the label being used and the
documentation (see [20]) should be referenced for the
use of the labels. The data word can be coded in binary,
binary coded decimal, or alphanumeric
data according to the International Organization for Standardization Alphabet No. 5 [20]. The label is decoded into
octal format to match the standard.

Marinvent Corporation has supplied a STAP program, Stapify,
to which the software FMS can be connected. Using Stapify, one
is able to request data from the avionics (longitude, latitude, altitude, etc.), as well as send data (desired roll and pitch commands,
and a target speed). The process of requesting data from the avionics functions works as a subscription service. By subscribing to
a particular label, the software FMS will receive the desired data
periodically. Figure 9 describes the syntax to request a subscription
where the receiver number is the ID of the avionics component
from which one would like to request the specified label.

Figure 7.

ARINC 429 connection topologies [21].

THE ARINC 834 STAP
COMMUNICATION PROTOCOL
The ARINC 834 Specification [22] describes how to interface with an aircraft
network using commercially available
technology (i.e. laptop, tablet, etc.).
The ARINC 834 Specification provides three aircraft interface services:
Generic Aircraft Parameter Service,
Simple Text Avionics Protocol (STAP),
and Avionics Data Broadcast Protocol.
Currently only STAP provides both
read and write access to avionics devices and is therefore the only viable
choice for the software FMS.
SEPTEMBER 2018

Figure 8.

ARINC 429 word format [21].

Figure 9.

ARINC 834 syntax to add a subscription [22].

IEEE A&E SYSTEMS MAGAZINE

Aerospace and Electronic Systems - September 2018

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - September 2018