Civil Engineering Reference
In-Depth Information
• Transceiver
• Controller area network (CAN) choke (optional)
• Electro magnetic compatibility (EMC) and electrostatic discharge (ESD) protec-
tion devices (optional)
• Connector
• Network or data bus, e.g. a two-wire cable
There are two different concepts for the physical layer, which is, on the one hand,
the so-called
Fault-Tolerant Low-Speed CAN Physical Layer
and the
High-Speed
CAN Physical Layer
on the other. They are different in the maximum data rate
(1 Mbit/s for high-speed CAN and 125 kbit/s for low-speed CAN) and the concepts
for bus termination which enable the fault-tolerant low-speed CAN transceiver to
continue the communication in all error conditions. This is not feasible for high-
speed CAN communication if there is, for instance, a short circuit condition be-
tween the bus lines. Nevertheless, high-speed CAN physical layer meanwhile is
applied for the communication between all modules in cars. The area of application
of low-speed CAN is typically limited to the vehicle interior applications—the so
called “body”. The fault-tolerant low-speed CAN physical layer is preferably ap-
plied by European car manufacturers. However, American and Japanese automo-
tive manufacturers solely apply the more simple high-speed CAN physical layer.
Nevertheless, European car manufacturers have dropped low-speed CAN in the
meantime. In newer vehicle generations, they almost only apply the newer version
of the high-speed CAN transceivers. The basic characteristics of the transceivers
and the related physical layers are specified in the ISO 11898 standard.
2.1
Basic Elements
2.1.1
Transceiver
The transceiver transmits and receives the physical data to and from the bus. The ba-
sic concept of the transceiver is identical for high-speed CAN and low-speed CAN
transceiver. For this basic concept, a transmitter and a receiver are needed. The trans-
mitter is a buffer, which transforms the logical signal on pin TxD (transmit data) into
a slew rate-controlled analog signal on the pins CAN_H and CAN_L. Both are open
drain outputs. CAN_H is a high-side driver and CAN_L is a low-side driver. Both
have reverse polarity diodes to protect these outputs against reverse operation. The
minimum voltage range for both outputs is required to be between − 27 and + 40 V.
The maximum output current is controlled to protect this output stage and the CAN
coils (if implemented) against shorts to ground and supply…. The current will nor-
mally be limited between 40 and 200 mA, but in newer products the maximum value
is reduced to 100 mA to protect the 100-µH CAN coils too. The receiver is a dif-
ferential comparator and converts the differential signal into a logical signal on pin
RxD (receive data). A logical high on RxD corresponds with a recessive differential
level on the bus, and a logical low on pin RxD corresponds with a dominant signal
