Civil Engineering Reference
In-Depth Information
Fig. 2.3
Typical signals for a fault-tolerant low-speed CAN network
work? In principle, CAN_H and CAN_L work separately and in ant phase. CAN_H
has a termination resistor to ground (via switch RTH) and CAN_L has a termination
resistor via switch RTL to VCC (see Fig.
2.2
). The signal will be transmitted via
the CAN_H path and CAN_L low in ant phase. All receivers obtain the differential
signal if there is no short or open wire on the bus. In this mode, the common-mode
range and robustness is very high. If one short is detected, the transmitter deactivates
this path (for example, if CAN_H is shorted to ground, CAN_H will be disabled and
the communication will be transmitted over CAN_L only). For the termination, all
nodes are complete. The value for the termination resistor can be between 500 Ω
and 5 kΩ. In total, the value for all resistors (in a parallel connection) should be in
a range of 100 Ω. Values less than 100 Ω are not allowed.
A high level on pin TxD corresponds with a recessive level on the bus. A reces-
sive level on the bus means 0 V on CAN_H and 5 V on CAN_L. A low level on
pin TxD corresponds with a dominant level on the bus. A dominant level on the bus
means CAN_H and CAN_L are switched on and the levels are 4 V for CAN_H and
1 V for CAN_L. The absolute level depends on the busload. This ends in a differ-
ential level for recessive state of typical −5 V (V
CAN_H
− V
CAN_L
) and −3 V for the
dominant state. In Fig.
2.3
, the typical behaviour is demonstrated.
A fault-tolerant receiver consists of five different receivers which work in paral-
lel. These receivers are:
• A differential receiver (for standard communication), with threshold voltage at −3 V
• A single-ended receiver for CAN_H (used in case of CAN_H short), with thresh-
old voltage at 1.8 V
• Asingle-ended receiver for CAN_L (used in case of CAN_H short), with thresh-
old voltage at 3.2 V
