Civil Engineering Reference
In-Depth Information
• Safety
• Schedulability of timely behaviour
• Verifiability
• Fault tolerance
• Synchronous behaviour
CAN communication protocol implements a purely priority-driven bus-access tech-
nology which per se does not guarantee any specific latency time. CAN message
transfer is thus characterized by a typically big jitter in best-case and worst-case la-
tency times. Therefore, different methodologies had been considered over the years
in order to achieve a message transfer characterized by predefined latency times.
When inspecting and revising the CAN standard ISO 11898-1 periodically, the
experts checked whether the event-driven CAN protocol could be modified towards
a time-triggered version; and it turned out that this attempt could be achieved.
Hence, the time-triggered version of the CAN protocol was developed by a small
team of experts of Robert Bosch GmbH. The resulting TTCAN protocol then be-
came the ISO 11898-4 standard.
1.3.2
Constraints
• When developing TTCAN, one of the basic prerequisites was the compatibility
with the existing CAN standard features such as data frames, the proven error
detection and error handling mechanism. Therefore, CAN knowledge, experien-
ce and CAN development tools could easily be reused.
• Only in case of automatic retransmission on error detection of the CAN protocol
there is a small restriction: In exclusive time windows—refer to Sect. 1.3.4—er-
roneous messages are not retransmitted immediately; otherwise, the cyclic com-
munication would be corrupted. This message, anyhow, will be retransmitted
within the next cycle.
• In time-triggered systems, a condition that is absolutely needed is a time stable
basis, being available all over the whole system. A failure of an individual mo-
dule in a system must not lead to chaos nor cause a breakdown of the communi-
cation process of the remaining system.
• The communication process must be synchronized by external events in order to
synchronize further sub-networks, redundant channels, or events depending on
engine revolution speed.
• The system must be configurable over the bus. There, easy synchronous swit-
ching between the event-driven and time-triggered mode is required.
1.3.3
Time Triggered Basics
A common synchronized clock (a global clock) in all nodes participating in a com-
munication system is the basic principle in time-triggered systems .
