Civil Engineering Reference
In-Depth Information
These considerations apply to a high extent to automotive electronics as in au-
tomotive electronics mostly unshielded bus cabling is applied other than in other
application areas such as industrial control or avionics where typically shielded
cables are used. These characteristics result in much higher immission and emission
of electromagnetic stray fields and transient interfering pulses. These very harsh
EMC conditions must be compensated by correspondingly increased constraints
for bus interface design. In order to ensure this, automotive manufacturers are re-
quired to coordinate any of the EMC activities and design constraints as well as
to specify the related qualification criteria. The layout of bus topology, the choice
of protection circuitry for the bus interface and the applied transceiver have great
impact on the EMC characteristics of the whole system. Therefore, characterization
of these semiconductor devices requires EMC measurement already in an early de-
velopment stage of cars. This task is performed by vehicle manufacturers in various
ways. German car manufacturers have agreed upon a common procedure in 2009.
This comprises the specification of the basic structure of the bus interface as well as
its passive and active components.
2.4.1
EMC Requirements, Specifications and Guidelines
For CAN systems, there is no explicit EMC specification in the form of a national or
international standard or guideline. Nevertheless, this is subject to the correspond-
ing legal or manufacturers internal requirements of the related devices or assembly
groups, for which these specifications would apply to.
But as a speciality for vehicle electronics applications, car manufacturers specify
specific requirements on the basis of which semiconductor devices are to be tested.
For this purpose, the technical specification IEC 62228 TS was developed and pub-
lished. This standard is the basis for EMC release of CAN transceivers for German
vehicle manufacturers.
2.4.2
Factors Effecting EMC of CAN Buses
The major factors influencing EMC of CAN buses are bus topology, termination
concept, bus lines, bus filter and transceiver. All these areas require an optimal
layout with regard to their EMC behaviour in order to meet the higher level require-
ments for safety-relevant applications as they are, e.g. for automotive electronics.
2.4.2.1
Bus Topologies and Termination Concepts
Passive bus topology is the mostly preferred topology in industrial automation
with both ends of the bus lines being terminated by line impedance resistors.
Any further bus node is connected to the bus by short stub lines (high-impedance
