[Al Skierkiewicz]

Oliver,
The nature of the signaling on the CAN bus uses switching in the transceivers to cause a device to enter a 'dominant' state for transmit and a 'recessive' state for receive. It is my understanding this is accomplished by driving the bus in a way that causes current flow in the terminating resistors. The CAN terminating resistors also provide the transmission line termination to allow 1 Mb/s traffic on a buss 40 meters long. Microchip has a nice application note, AN228, describing one of their products and the CAN signaling.