Military Communications and Information Technology: A Trusted ...

Chapter 1: Concepts and Solutions for Communications and Information Systems
97
Data bus CAN [4, 6-9 and 16] for its simplicity, very effective diagnostic tools and
because of the fact that many microcontrollers have a CAN driver integrated was used
for mutual communication of individual modules. The bus CAN has specification
for avionic systems by CANaerospace protocol that operates at the application layer
of ISO/OSI reference model. The protocol is not already widely used, as is evidenced
by only small number of projects in the Czech Republic and abroad (Ae270, SATS,
V220 and V300 aircraft engines, SOFIA [6]), so this area is still under development.
It is important for avionic systems that individual messages are sent and received
in precisely defined instants, so that time-triggered method was chosen [7, 9
and 16]. The basis of the method is a time schedule, which is defined by so called
Cycle matrix [6, 7]. Complete designed on-board aircraft electronic system consists
of the following parts.
The NEC station as needed (asynchronously) sends messages with CAN ID
selected from high-priority area of the CANaerospace protocol service messages,
thus makes an interface between aircraft and ground station. There is a message,
which makes master block to assign the communication schedule. There are also
messages that tell the master unit to start or stop communication, i.e. reference
message transmission. It also allows sending other messages of the node service
protocol, which generally accelerate and make more precise the communication
of individual modules (e.g. message for system baud rate reconfiguration).
The MASTER receives important messages that the NEC station transmits
and adequately reacts on them. It stops communication according previous Cycle
matrix after the communication schedule assignment command reception, and
ensures assignment messages transmission that include flight data messages CAN
ID allocation in the new Cycle matrix. The number of assignment messages corresponds
to the number of elements in the Cycle matrix. Subsequently it starts to
periodically transmit reference message for the time synchronization of communication,
which period determines the value set in the comparative timer register
of its microcontroller, based on the number of Cycle matrix columns (time slots).
It stops the synchronization message transmitting after communication end message
reception. It resumes communication reference message transmission by communication
start message reception.
Individual modules fill predefined Cycle matrix with specific messages that
provide to the system after assignment message reception. The nodules start to
provide data to the system by synchronization messages reception. The time interval
during which the module must send the message is defined by so called time slot.
1
tS
nD d
(2)
bitrate
where t S is time slot length, n D is bit number of the message (maximum number
of stuff bits is supposed, thus bit number of the message is 135), d is time delimiter,
the message to be send in order.