Physical Layer - CAN System Engineering - page 56

focus on the signal integrity. Analytic considerations and optimization measures
shall give possibilities of a specific judgement and enhancement of the signal qual-
ity of the CAN network.

2.2.1 CAN Network Architecture

Although CAN bus is not one of the high-speed data networks, some phenomena
show that the laws of physics apply here. Especially at speeds above 250 kbit/s and
at cable lengths above 50 m, it must be analysed more closely and will require some
calculation rules. The actual physical properties of the CAN bus lines as well as the
circuitry of the connected control devices have to be considered. If the electrical
specifications of these components are available, the quality of the received signals
can be derived from the transmit signals. To reduce the complexity of calculations,
some assumptions can be made, which are explained below.
Simulations may be used to avoid the need for such calculations. However, fun-
damental knowledge of all influencing parameters is important to predict the impact
of changes or modifications on existing CAN networks.

2.2.1.1 Transmission Line Theory

Theoretical considerations of the electrical lines are based on the electromagnetic
waves forming the signals, which spread along the line with a characteristic speed.
Therefore, voltage and current pulses of CAN messages also move from transmitter
to receivers with a characteristic propagation speed. For very slow operations, i.e.
switching off light, this can be neglected. For the fast CAN signals, the propagation
speed limits the allowable cable length. Signals are classified as fast, if the rise time
is shorter than the propagation time of the corresponding line.
The specific propagation delay results from the material properties of the cable
in use. The dielectric constant of the insulating material and geometric parameters
plays an essential role. Hence, a wave impedance can be determined, which de-
scribes the relationship between current and voltage at any point on the line, and the
propagation speed of pulses on the line. On closer inspection, these parameters are
frequency dependent, which can have an impact on very fast signals (above about
5 Mbit/s). For CAN signals, this effect can usually be neglected. The impedance
thus can be assumed to be a resistance.
The transmission of CAN signals is usually performed in a differential method.
Two wires are used, where the data signal is derived from the voltage between the
conductors. To simplify the calculation of the transmission signals, the signals of
the two conductors can be decomposed in a differential signal and a common-mode
signal .
The differential signal—also called odd mode signal—and the common-mode
signal can be calculated if the voltage of each single CAN signal line is known, as
shown in [2.1]: