Freescale Semiconductor MPC5604B, Quick Start Manual

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When a device requests data from another by sending a frame with no data where RTR is
recessive, that frame is called a remote frame. The device which is identified with this frame
starts transmitting the requested data frame(with same ID).
1.3. Physical Layer
The physical layer can be of any medium but it has to support recessive/dominant signals for
using logical AND between different devices. In this sections we will talks about the typical
twisted-pair electrical medium with differential voltages.
Figure 113/ A typical CAN transceiver and CANH/CANL signals
In high-speed CAN standard, CANH and CANL are derived from TX to from a differential voltage
with two states, ‘0’ the dominant state (CANH=3.5V, CANL=1.5V), and ‘1’ the recessive state
(CANH=CANL=2.5V). The RX signal is determined from CANH-CANL as, if the differential
voltage is above 0.9V then RX=’1’ and if it is less than 0.5V then RX=’0’.
CAN Transceivers of the devices at the edges of the can bus (the first and the last station) has to
use resistors as bus termination; either a 120
Ω between CANH and CANL or a splitted
termination with two 60 Ω with a reference voltage in the middle.