XC912BC32CFU8 Motorola Semiconductor Products, XC912BC32CFU8 Datasheet - Page 235

• Manufacturer Part Number: XC912BC32CFU8
• Description: M68HC12B Family Data Sheet
• Manufacturer: Motorola Semiconductor Products

15.7.2 J1850 Frame Format
15.7.2.1 SOF — Start-of-Frame Symbol
15.7.2.2 Data — In-Message Data Bytes
15.7.2.3 CRC — Cyclical Redundancy Check Byte
M68HC12B Family — Rev. 8.0
MOTOROLA
IDLE
SOF
PRIORITY
(DATA0)
All messages transmitted on the J1850 bus are structured using the format shown
in
J1850 states that each message has a maximum length of 101 PWM bit times or
12 VPW bytes, excluding SOF, EOD, NB, and EOF, with each byte transmitted
most significant bit (MSB) first.
All VPW symbol lengths in the following descriptions are typical values at a
10.4-Kbps bit rate.
All messages transmitted onto the J1850 bus must begin with a long-active 200 µs
period SOF symbol. This indicates the start of a new message transmission. The
SOF symbol is not used in the CRC calculation.
The data bytes contained in the message include the message priority/type,
message ID byte (typically, the physical address of the responder), and any actual
data being transmitted to the receiving node. The message format used by the
BDLC is similar to the 3-byte consolidated header message format outlined by the
SAE J1850 document. See SAE J1850 – Class B Data Communications Network
Interface specification for more information about 1- and 3-byte headers.
Messages transmitted by the BDLC onto the J1850 bus must contain at least one
data byte, and, therefore, can be as short as one data byte and one CRC byte.
Each data byte in the message is eight bits in length and is transmitted MSB (most
significant bit) to LSB (least significant bit).
This byte is used by the receiver(s) of each message to determine if any errors
have occurred during the transmission of the message. The BDLC calculates the
CRC byte and appends it onto any messages transmitted onto the J1850 bus. It
also performs CRC detection on any messages it receives from the J1850 bus.
Figure
DATA
Figure 15-4. J1850 Bus Message Format (VPW)
MESSAGE ID
(DATA1)
15-4.
Byte Data Link Communications (BDLC)
DATA
N
CRC
O
D
E
Byte Data Link Communications (BDLC)
N
B
OPTIONAL
IFR
BDLC MUX Interface
EOF
F
S
I
Data Sheet
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