Monday, November 02, 2009

CAN BUS MESSAGE FRAMES - Standard Data Frame

Standard Data Frame

The CAN standard data frame is shown in Figure 2-1.
As with all other frames, the frame begins with a Start-
Of-Frame (SOF) bit, which is of the dominant state and
allows hard synchronization of all nodes.
The SOF is followed by the arbitration field, consisting
of 12 bits: the 11-bit identifier and the Remote
Transmission Request (RTR) bit. The RTR bit is used
to distinguish a data frame (RTR bit dominant) from a
remote frame (RTR bit recessive).
Following the arbitration field is the control field,
consisting of six bits. The first bit of this field is the
Identifier Extension (IDE) bit, which must be dominant
to specify a standard frame. The following bit, Reserved
Bit Zero (RB0), is reserved and is defined as a dominant
bit by the CAN protocol. The remaining four bits of the
control field are the Data Length Code (DLC), which
specifies the number of bytes of data (0 – 8 bytes)
contained in the message.
After the control field is the data field, which contains
any data bytes that are being sent, and is of the length
defined by the DLC (0 – 8 bytes).
The Cyclic Redundancy Check (CRC) field follows the
data field and is used to detect transmission errors. The
CRC field consists of a 15-bit CRC sequence, followed
by the recessive CRC Delimiter bit.
The final field is the two-bit Acknowledge (ACK) field.
During the ACK Slot bit, the transmitting node sends
out a recessive bit. Any node that has received an
error-free frame acknowledges the correct reception of
the frame by sending back a dominant bit (regardless
of whether the node is configured to accept that
specific message or not). The recessive acknowledge
delimiter completes the acknowledge field and may not
be overwritten by a dominant bit.

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