Physical Layer - CAN System Engineering - page 62

Fig. 2.18 Measurement set-up—dynamic behaviour of the transceiver output stage
As mentioned above, the dynamic characteristics of the transmitter part of the
transceiver are important. There, the transceiver drives the bus signal which will be
distributed over the network. The characteristics of the signal shape are not only rel-
evant if it is possible to distribute the information over the whole network but also
crucial for noise immunity and sensitivity against interference with (or of) other
network components. Furthermore, the signal shapes of dynamic processes have
a huge impact on EMC behaviour of the network. Important aspects for the con-
sideration of the signal flanks are the symmetry between the flanks of the CAN_H
signal and the CAN_L signal, the rounding of the flanks as well as the slew rate of
the signal flanks. Through the rounding and the limitation of the slew rate of the
signal flanks, high-frequency (HF) signal parts are limited to increase the EMC
robustness and immunity. The symmetry of the CAN bus signal, which describes
the line symmetry of the signals CAN_H and CAN_L, with the axis of symmetry
lying on the mid-level (typically 2.5 V) is important to avoid asymmetries. Asym-
metries are one cause for common-mode disturbances which enable the emissions
of electromagnetic interference.
In case of validating the dynamic processes of a sending transceiver, the domi-
nant to recessive state change is of particular importance because the transceiver ac-
tively drives the CAN bus with a differential voltage in the dominant state (provid-
ing energy into a system) and is turned off during the transmission to recessive level
(high ohmic output). Discharging processes may result, regarding the disabling of
an active energy source which may lead to disturbances. Such processes are anal-
ysed in detail further below.
Figure 2.18 shows an example of a measurement set-up to measure the flanks of
a transceiver.
On the receiver side are on the one hand the thresholds and on the other the
resistive and capacitive load against the bus from interest. The ISO 11898-2 stan-
dard defines the dominant state as a differential voltage greater than 0.9 V and
the recessive state as a differential voltage smaller than 0.5 V. This means that the
maximum possible threshold for the dominant state is equal to 0.9 V and the mini-
mum possible threshold for the recessive state is equal to 0.5 V. Consequentially, the