Civil Engineering Reference
In-Depth Information
Fig. 1.7
CAN overload frame
Flag
inside the
IFS
and will respond with their own
Overload Frames;
the
Over-
load Flags
will overlap each other. In this case, the start of a next frame is delayed
by the length of an
Overload Frame
. In both cases, the overload condition has no
effect on the error counters. Today's CAN controllers are fast enough; they are able
to handle all bus traffic without the additional delay of
Overload Frames
.
1.2.5
Timing Considerations
As described in Sect. 1.2.2, there are two different frame types, those with 11-bit
identifiers (also called
Standard Frame
) and those with 29-bit identifiers (also
called
Extended Frame
). Furthermore, each message may contain from 0 to 8 bytes
of data. Due to this, there are differences in effective data rates and minimum la-
tency times for high priority messages.
Under normal circumstances (in undisturbed operation), a minimum latency
time can be calculated only for the message with the highest priority in the network.
Delay times for lower priority messages are not deterministic; they can only be
determined statistically, using appropriate measuring equipment such as simulators,
emulators, or dedicated network analyzers. Alternatively, the time-triggered option
of CAN (TTCAN—time-triggered CAN) may be used, in which all messages are
sent in a predefined time schedule. TTCAN is described in Sect. 1.3.
The maximum delay time for the message with the highest priority depends on
the length of the longest possible message and on the bit rate. The length of the
longest message (with a
Data Field
length of 8 bytes) is shown in Table
1.1
; the
result on the left is for networks where only frames with 11-bit (short) identifiers are
used, while the result on the right is for networks where frames with 29-bit (long)
identifiers are used.
This means that in networks where only frames with 11-bit identifiers are used,
the message with highest priority has to wait at most 135-bit times frames for bus
access, 135 µs at a bit rate of 1 Mbit/s. If frames with 29-bit identifiers are used, the
maximum waiting time is 160 bit times, 160 µs at 1 Mbit/s.
The effective data rate of CAN frames is calculated from the ratio of data bits to
the length of the frame. The data rate depends on the length of the
Data Field
, the
length of the identifier and on the transmission rate. Table
1.2
gives an indication of
