Image Processing Reference
In-Depth Information
be mentioned that, at present, several inexpensive communication technologies are available on the
market that allow bit rates in the order of tens or hundreds of Mbps).
Even though this may appear to be a very limiting factor, it will probably not have any relevant
impact in the near future for several application areas—including some automotive and process con-
trol applications—where a cheap and well-assessed technology is more important than performance.
However, there is no doubt that CAN will suffer in a couple of years from the higher bit rates of its
competitors, i.e., PROFIBUS-DP (up to Mbps), SERCOS (up to Mbps), FlexRay (up to Mbps),
and industrial Ethernet (up to Mbps). Such solutions, in fact, are able to provide a noticeably
higher data rate, which is necessary for systems made up of lots of devices or having very short cycle
times ( ms or less).
15.4.2.2 Determinism
Becauseofitsnondestructivebitwisearbitrationscheme,CANisabletoresolveinadeterministicway
any collision that might occur on the bus. As shown in Section . about schedulability analysis, this
implies that it is possible to design a CAN system so that messages are always delivered before their
deadlines (or, equivalently, to ensure that the transmission latencies experienced by every message are
upper-bounded). However, if nodes are allowed to produce asynchronous messages on their own—
this is the way event-driven systems usually operate—there is no way to know in advance the exact
time when a given message will be sent. his is because it is not possible to foresee the actual number
of collisions (i.e., interference) a node will experience with higher priority messages. This behavior
leads to potentially dangerous jitters, which in some kinds of applications—such as, for example,
thoseinvolvedintheautomotiveield—mightafectthecontrolalgorithmsinanegativewayand
worsen their precision. Moreover, if transmission errors due to electromagnetic interferences are
taken into account, it might happen that some messages miss their intended deadline.
Another problem related to determinism is that composability is not ensured in CAN networks.
This means, that when several subsystems are developed separately and then interconnected to the
same network, the overall system may fail to satisfy some timing requirements, even though each
subsystem was tested separately and proved to behave correctly and the overall network bandwidth
is theoretically sufficient to carry out all data exchanges. his is a severe limitation to the chance of
integrating subsystems made from different vendors, and hence makes the design and test tasks much
more difficult.
15.4.2.3 Dependability
The last drawback of CAN concerns dependability. Whenever safety-critical applications are con-
sidered, where a communication error may lead to damages to the equipment and even injuries to
humans (such as, for instance, in most automotive x-by-wire systems), a highly dependable network
hastobeadopted.
Reliable error detection should be carried out both in the value and in the time domain. In the
former case conventional techniques such as, for example, the use of a suitable CRC are adequate.
In the latter case, instead, a time-triggered approach is certainly more appropriate than the event-
driven communication scheme provided by CAN. In time-triggered systems all actions (including
message exchanges, sampling of sensors, actuation of commanded values, and task activations) are
knowninadvanceandmusttakeplaceatprecisepointsintime.Inthiscontext,eventhepresence
(or absence) of a message at a given instant provides significant information (i.e., it enables the dis-
covery of faults). For example, masquerading faults (a node that sends messages on behalf of another
one) can be avoided in time-triggered networks, and a fail silent behavior can be easily enforced
for nodes.
Also the so-called “babbling idiot” problem can affect CAN systems. In fact, a faulty node that
repeatedly keeps transmitting a very high priority message on the bus (usually because of a flaw in
