wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Controller Area Network 31-3
TABLE 31.1 Practical CAN Bus
Length for ISO 11898 Compliant
Transceivers and Standard Bus
Line Cables
Bit Rate
Bus Length (m)
1.Mbps 30
800.kbps 50
500.kbps 100
250.kbps 250
125.kbps 500
62,5.kbps 1000
20.kbps 2500
10.kbps 5000
Source: CiA, CAN physical layer, CiA
(CAN in Automation), Nuremberg,
Germany, 2001.
• SAE J2411 Single Wire—An unshielded single wire is defined as
the bus medium and the communication takes place with a nominal
data rate of 33.3.kbps. The standard defines up to 32 nodes per
network. The main application area of this standard is in comfort
electronics networks in vehicles.
• ISO 11992 Point-to-Point—This standard defines a point-to-point
connection for use mainly in vehicles with trailers. The nominal
data rate is 125.kbps with a maximum bus line length of 40.m.
The most popular CAN physical layer protocol, available in most
of the CAN transceivers, is the one defined in the ISO 11898-2 standard.
The maximum achievable bus line length in a CAN network,
represented in Table 31.1, depends on
• The loop delays of the connected bus nodes and the delay of the
bus lines
• The differences of the relative oscillator tolerance between nodes
• The signal amplitude drop due to the series resistance of the bus
cable and the input resistance of bus nodes
The CAN physical layer provides a two-state medium, where the bus can be either dominant or
recessive. Whenever two nodes simultaneously transmit bits of opposite value, then all nodes should
read dominant. Usually the dominant state is associated with the binary value 0 and recessive with the
binary value 1.
The physical layer has a number of built-in fault-tolerant features. CAN provides resilience against a
variety of physical faults such as one open wire, the short-circuit of the two signal wires, or even one of
the signal wires shorted to ground or power. Notice that not all CAN controllers are able to implement
these features, by switching from differential signaling to single-line signaling, at higher bus speeds. The
CAN differential electrical signaling mode is very resistant to electromagnetic interference (EMI) since
interference will tend to affect each side of a differential signal almost equally. However, the differential
electrical signaling does not fully prevent EMI to affect the signal on the bus in such a way that one or
more nodes on the bus will simultaneously read a different bit value from that which was transmitted.
A node detecting the error (possibly the transmitter) will invalidate the message by transmitting an
error frame. The number and the nature of EMI-induced transmission faults and the ability of the physical
layer to prevent them depends on several factors such as the cable type and length, the number of
nodes, the transceiver type, and the EMI shielding.
31.2.2 Data Link Layer
The data link layer of CAN includes the services and protocols required to assure a correct transfer
of information from one node to another.
There are four types of message on CAN: data frames, error frames, remote frames, and overload
frames. The latter two types of messages are rarely used in real application and, thus, will not be
described further.
The CAN protocol supports two message frame formats, the only essential difference being in
the length of the identifier. The CAN base frame format supports a length of 11 bits for the identifier
(formerly known as CAN 2.0 A), and the CAN extended frame format supports a length of 29 bits for the
identifier (formerly known as CAN 2.0 B).
Data on the bus are sent in data frames which consist of up to 8 bytes of data plus a header and a
footer. The frame is structured as a number of fields, as depicted in Figure 31.1.
© 2011 by Taylor and Francis Group, LLC