Applications - CAN System Engineering - page 270

Fig. 5.13 Smoke detection system using proprietary detector supply and communication loop

surpass the existing architecture while keeping development and purchasing costs
at a comparative level.
The latter was feasible by reusing the existing smoke detector core and fitting it
with an altered communication and power interface (see Figs. 5.13 and 5.14 ).
The communication medium had to meet a number of requirements for eligibil-
ity in a safety-critical application:

• Advanced data integrity and error detection features,
• Deterministic behaviour,
• Operability in challenging EMC environments and
• High degree of flexibility in choice of network size and topology.

Considering the 30-year design life of a modern passenger aircraft, the long-term
availability of electronic components was scrutinized in order to minimize the risk
of equipment redesign resulting from component obsolescence throughout the life
cycle of the aircraft.
The CAN bus was deemed the most suitable communication medium capable of
fulfilling the above requirements.

5.3.4.3 Protocol

The CAN protocol, as defined in ISO 11898 [ISO11898], covers layers 1 and 2 of
the OSI communication model. The remaining layers, up to layer 7, have to be man-
aged by additional services up to the application. Various standardised higher layer
protocols such as CANopen are available and widely used in industrial applications.
Instead of selecting a generic high layer protocol, a specific to-type application
layer protocol was developed and documented in a System Interface Document
[SCHMID] in order to ease compliance with RTCA/DO-178 [DO178B] guidelines.