Physical Layer - CAN System Engineering - page 67

Fig. 2.23 CAN bus connecting circuit of a CAN node

be as close as the injection location which is normally at the bus connector pins of
the node. Usually, ESD protection components are realized with varistors which
are implemented between the bus signals and GND. Relevant for signal integrity
inspections are the parasitic capacitances which are normally depicted in the data
sheet of the ESD protection component.
To sum up all components of a CAN node bus connection, Fig. 2.23 implies,
depicted for signal integrity valuations, important equivalent circuit.

2.2.2.6 Isolation and GND Shift

In case of applications with huge topologies and high current consumptions, GND
shifts (or shifts on the supply line) may occur. In the following, an example case
explains which measures shall be taken into account to avoid influences of the ap-
plication's current load on the CAN communication. Further praxis-relevant solu-
tion approaches, related to industrial used CAN networks, could be found in the
DeviceNet specifications [ODVA94].
In this example, the CAN nodes have a huge distance to each other and thus
the cable length is relatively long. To illustrate, for example, a CAN control of in-
dustrial motors on a conveyor belt is used. GND line and supply line are installed
together with the CAN lines. In case of a common use of supply and GND lines
for application and CAN communication, the voltage drop over the GND cable
will be dependent on the cable resistance per unit length, the length of the cable as
well as the current load of the application (which could be huge in comparison to
the CAN communication current load). This voltage drop over the GND cable af-
fects as a GND shift which may influence the CAN communication and the related
application.