Civil Engineering Reference
In-Depth Information
Fig. 1.16
Calculation of
global time (Level II)
1.3.9
Drift Correction
At long basic cycles, slight differences in clock frequencies between individual
nodes may cause nodes to fall out of synch. Level II implementations provide a
drift-correction feature for compensation (Fig.
1.18
).
Therefore, each node compares the length of the basic cycle, transmitted by the
time transmitter with its own. The difference between the two results in a correction
factor, which adjusts the frequency divider in each node respectively through TUR
(see Sect. 1.2.3).
1.3.10
Extended Error Detection and Error Handling
Due to the cyclic character of TTCAN, there are additional diagnostic features on
top of the well-known error-handling techniques of CAN:
• Based on the reception tags mentioned above, a loss of an expected message in
exclusive time windows can be detected. The number of expected messages per
system cycle is known. The application will be signalled, if a specific number of
messages is lost.
• Further, the number of transmitted messages is counted. If for a specific partici-
pant the number of exclusive transmission windows exceeds the defined amount,
a further transmission of messages is abandoned until the start of the next system
cycle.
• Various watchdogs prevent the system from going into an uncontrolled state:
− A synchronization watchdog monitors the periodic transmission of the refe-
rence message. In the event synchronous operation mode, another watchdog
restarts communication in case of a loss of trigger.
