Civil Engineering Reference
In-Depth Information
Fig. 2.26
Voltage pulse induced by common-mode chokes and GND shift
A self-induction describes the induction of a voltage by the inductance which
tries to hold the current and the magnetic current constant in case of a change in
the current flow. The proportionality factor L describes the intensity of the self-
induction as well. A high inductance results also in a high inducted voltage in
the case of a self-induction. Based on the above-described operating principle of
the CMC, it clearly shows that in differential mode only the stray inductance is
effective which is much smaller than the main inductance which works only in
common mode. After a node lost arbitration it switches off the transmitter stage
and the current in the CMC. An inducted voltage peak results, which is higher if
the main inductance is active - in case of common mode signal. A GND shift is a
source for common-mode currents which raises the effective working inductance
and with it the intensity of the self-induction which will be evoked by the loss of
arbitration.
As a consequence of this, it is possible to observe that CMCs induce voltage
peaks on the CAN bus after the above-described scenario. Measurement resulting
from these peaks is shown in Fig.
2.26
.
2.2.3.3
Limitations of the Cable Length in Consideration of the Bit Timing
In principle, two different factors limit the maximum network distribution. On the
one hand, the data rate, bit timing settings and oscillator tolerances influence the
maximum possible cable length. On the other hand, resistances and capacitances of
the cable and transceiver limit the possible cable length. The last condition refers to
