Civil Engineering Reference
In-Depth Information
Fig. 5.15
Controller area network (CAN) identifier
Function Code (bits 24…15)
Every application function is designated a unique
Function Code within its respective Message Type. In addition to describing the
next level of arbitration priority, the Function Code is used to transport logical data
without the use of the actual CAN data fields. In this case, the Data Length Code
(DLC) is 0, enabling efficient use of data bandwidth, particularly for R_PDOs and
R_SDOs which contain mostly status requests directed at smoke detectors and do
not carry any further information than the request itself.
Module ID (bits 14…5)
The Module ID field contains the unique network identi-
fication of the CAN node. This may also be a broadcast identification when a mes-
sage is directed at several nodes simultaneously. Two subfields Module Type and
Module Address split the Module ID into equipment classes and their individual
addresses. The entire Module Address space may be reused for every Module Type
on the network.
System ID (bits 4…0)
The System ID is used to tag the CAN identifier with a
unique system identification code. All smoke detectors and other fire protection
components are assigned a fixed value.
Data Frames
A Data Frame is generated by a transmitter to transfer application
data to one, or in the case of a broadcast, several receivers. Within the Data Frame,
the Data Field consisting of 1-8 bytes carries the application data. A Data Frame
may contain an empty Data Field (DLC = 0). In this case, data are carried through
the Function Code alone.
A smoke detector's 8-byte status Data Field is as defined in Table
5.3
with the
meaning of the data bits in Table
5.4
.
5.3.4.4
Network Management
It is of utmost importance that the system configuration and availability of re-
sources (smoke detectors) are known to the network master. Lack of configuration
control through the network master device would jeopardize safety and disqualify
the system. From a safety assessment point of view, the worst-case condition is an
undetected configuration error leading to an incorrect compartment designation in
case of fire; an alarm reported in the aircraft's forward cargo compartment while the
real fire occurrence is in the aft cargo compartment and vice versa. Such a case is
