Transmit Data Input
2 GND Ground
3 VCC Supply Voltage
4 RXD Receive Data Output
5 Vref Reference Voltage Output
6 CANL Low-Level CAN Voltage In/Out
7 CANH High-Level CAN Voltage In/Out
8 RS Slope Resistor Input
The PCA82C251 is the interface between the CAN protocol controller and the physical bus.
It is primarily intended for applications up to 1 Mb in trucks and buses.
The device provides differential transmit capability to the bus and differential receive capability to the CAN
It is fully compatible with the “ISO 11898-24 V” standard.
A current limiting circuit protects the transmitter output stage against short-circuit to positive and negative
Although the power dissipation is increased during this fault condition, this feature will prevent destruction of
the transmitter output stage.
If the junction temperature exceeds a value of approximately 160 C, the limiting current of both transmitter
outputs is decreased.
Because the transmitter is responsible for the major part of the power dissipation, this will result in a reduced
power dissipation and hence a lower chip temperature.
All other parts of the IC will remain operating.
The thermal protection is particularly needed when a bus line is short-circuited.
The CANH and CANL lines are also protected against electrical transients which may occur in an automotive
Pin 8 (Rs) allows three different modes of operation to be selected: high-speed, slope control or standby.
For high-speed operation, the transmitter output transistors are simply switched on and off as fast as
In this mode, no measures are taken to limit the rise and fall slope. Use of a shielded cable is
recommended to avoid RFI problems.
The high-speed mode is selected by connecting pin 8 to ground.
The slope control mode allows the use of an unshielded twisted pair or a parallel pair of wires as bus lines.
To reduce RFI, the rise and fall slope should be limited.
The rise and fall slope can be programmed with a resistor connected from pin 8 to ground. The slope is
proportional to the current output at pin 8.
If a HIGH level is applied to pin 8, the circuit enters a low current standby mode. In this mode, the transmitter
is switched off and the receiver is switched to a low current.
If dominant bits are detected (differential bus voltage >0.9 V), RXD will be switched to a LOW level.
The microcontroller should react to this condition by switching the transceiver back to normal operation (via
pin 8). Because the receiver is slower in standby mode, the first message will be lost at higher bit rates.