Electrical connection
53
CAN BUS network
Guidelines for the topology of a CAN network
Technical principles
The CAN BUS comprises the cables CAN-High,
CAN-Low, GND and one +12 V supply cable for
BUS components without their own power supply.
The combined total load of all devices with 12 V and
24 V supply must not exceed 6 W.
Design CAN networks in a linear fashion and set a
terminator at each network termination. This is en-
sured by the termination of the end devices.
In the case of larger networks (covering several
buildings), problems can occur through electromag-
netic interference and potential differences.
To avoid or to the greatest extent manage such
problems, take the following measures:
Cable screening
The BUS cable screen must be connected well at every node to provide continuity. For larger
networks we recommend including the screen in the equipotential bonding, in line with the
examples shown.
Equipotential bonding
As low an ohm connection as possible to the earth potential is particularly important. Where
cables enter buildings, ensure that the cable entries are in the same location where possible
and that all are connected to the same equipotential bonding system (S
ingle
E
entry
P
oint
princi-
ple). The purpose is to create potentials that are as similar as possible, in order to achieve
the smallest possible potential difference to adjacent lines in case one line suffers a voltage
surge (lightning strike). Also ensure a corresponding clearance between the cable and light-
ning protection systems.
The equipotential bonding also has positive properties to counteract interferences emitted
from linked cables.
Avoiding earth loops
Where a BUS cable is routed between several buildings, ensure that earth loops are avoid-
ed. The reason for this is that buildings actually have different potentials compared to the
earth potential. An earth loop is created when connecting one cable screen in each building
directly
with the equipotential bonding system. In other words, a current flows from the high-
er to the lower potential.
For example, if lightning strikes near one of the buildings, the potential of that building will
briefly be raised by several kV.
In this case, the equalising current flows to earth via the BUS screen and causes an extreme
electromagnetic input which can result in the destruction of the BUS components.
CH1 – CAN-H
1V/Div.
CH2 – CAN-L
1V/Div.
Time/Div.
250µs
2.5V
CAN-H and CAN-L data signals
CH1 - CAN-H
CH2 - CAN-L
