Physical Layer - CAN System Engineering - page 59

Fig. 2.15 Limits of system extent according to ISO 11898-2
Fig. 2.16 Stub length in a network
on the location of the observation; it must be determined separately for each node
of the system. Even simulation-based analyses result in a very large amount of data.
In many cases, it is unfortunately not possible to establish the system as a bus
with short branch lines, as required by ISO 11898-2 (maximum bus cable length,
40 m; maximum stub length, 0.3 m) (see Fig. 2.15 ). Although this topology would
cause least disturbances by reflections, the maximum elongation from 40 m at
1 Mbit can hardly be achieved.
Due to the arrangement of the electronic control units (ECUs), for example, in
a motor vehicle, a multiple star configuration results quite often. Stub lengths of
several metres may be required as shown in the example stub “s” in Fig. 2.16 .
At the junction point of multiple parallel stubs, reflections increase, depending
on the number of branches. For symmetric structures, the returning reflected signals
will again come together and intensify. Symmetrical topologies are especially criti-
cal. For large networks with low symmetry, superposition of reflected signals may
erase each other. In spite of the increasing complexity of the network topology, the
resulting signals may be uncritical. The “golden rules” for the configuration can-
not be given, only trends and experiences. These are particularly problematic for
networks with a large number of optional ECUs. Problems can occur by adding or
omitting devices in very critical combinations—even if the system works initially
stable. Figure 2.17 shows such a case. To the system made up from the ECU 1 to
ECU 5, the sixth is added. The propagation delay is increased in this particular
case—by the ringing at the transition from dominant to recessive—from 280 to
422 ns.