Civil Engineering Reference
In-Depth Information
terminated connection) or by the so-called daisy chaining. The advantage of this
topology is the avoidance of reflections on the bus lines, which may make the
EMC get worse.
A passive bus implies a reduced flexibility in a certain way. That is why the
passive star is another preferred bus topology, especially for cars. This topology
is characterized by all nodes being connected in a star-shaped form, or even a
combination of multiple stars is applied. However, the termination of all seg-
ments with low reflection is not feasible. In order to achieve an optimum between
flexibility and reflection characteristics, two distributed terminations are applied,
which are located at the ends of the two segments with the largest geometric dis-
tance. Another termination method is implemented in the star node itself. In this
method, the termination of the longer lines as well as the HF decoupling of the
lines are applied. All these result in optimized EMC characteristics of the CAN
system as a whole.
In general, there is no simple relation between bus topology and related EMC
characteristics. However, there is a unique dependency between signal integrity—
that is, the characteristics of CAN_H and CAN_L bus signals—and the EMC char-
acteristics of the system. In comparison with an optimal bus layout, a decline of
signal quality coming, e.g. from reflections from the line ends or attenuation by pas-
sive components results in a reduction of system safety margin, which, in conjunc-
tion with external noise, may cause system failure. Furthermore, it can be stated that
shortening the length of the bus system normally improves EMC due to a reduced
effective antenna surface, and thus less noise is emitted or immitted.
2.4.2.2
Bus Lines
For CAN bus cables, twin lines with or without shielding are used. Nominal dif-
ferential line impedance shall be 120 Ω. Typically, the value is in the range of
90-130 Ω. Shielded twin lines typically are applied in industrial control applica-
tions and avionics. In current cars only UTP cables are used with various twist
pitches between 20 and 50 mm.
When using shielded bus lines, immission and emission of noise are widely re-
duced or even eliminated. The efficiency of shielding depends on the cable charac-
teristics itself as well as on the applied concept of shielding (cable, connector and
module box). High shielding effect in the relevant frequency range (f = 150 kHz up
to a couple of GHz) can only be achieved if the shielding concept is strictly applied
to all components.
These twisted pair bus lines do not have any shielding. The advantage of twisted
pair cables is that external noise (e.g. HF electromagnetic fields) affects the system
as common-mode interference. Therefore, the differential-mode CAN signals are
not disturbed. In that respect, the same is true for interference fields emitted by the
bus lines. At a high-level symmetry of the bus lines, the HF interference parts com-
pensate each other and thus they are not radiated as common-mode signals.
