8-9
J1850 COMMUNICATIONS CONTROLLER
Figure 8-6. J1850 Message Frames
A standard message frame is initiated by the responder and contains no more than 11 data bytes
to be transmitted. An IFR message is a request initiating the recipient(s) to respond by transmit-
ting data within the same frame. The following subsections describe each of the messaging forms.
8.4.1 Standard Messaging
A standard message frame can best be described as a “send mode only” format that is initiated by
the responder either to request information or to reply to a received message from a remote node.
In addition to the actual data that is being transmitted, the standard message is composed of a
header (1–3 bytes), a CRC byte, and a series of start and end symbols.
8.4.1.1 Header
The header provides general information on the physical network and the necessary interface re-
quirements. For a complete description of the header, refer to the Society of Automotive Engineer-
ing (SAE) J1850 specifications (revised May 1994).
8.4.1.2 CRC Byte
The CRC byte, calculated through the cyclic redundancy check generator, is a checksum value
that verifies the accuracy of the data message transmitted onto the bus. The CRC byte is appended
to all data messages and optionally appended to IFR response messages. Upon reception, the
CRC byte is compared with the value C4H. If the values match, the transmitted message is valid;
otherwise, it is invalid, and an error flag in the J_STAT register is set.
8.4.1.3 Normalization Bit
The normalization bit (NB), found only in IFR messaging, defines the start of the IFR message
response data. The NB is triggered by bit J_CMD.6 and is transmitted after an end-of-data (EOD)
symbol is detected on the bus. The timing format of the NB is assigned by the J_CFG register
A5225-01
S
O
F
I
F
S
1 Byte
CRC
E
O
F
E
O
D
1-3 Bytes
Header 1-11 Bytes
Data
†
Standard Frame
0-1 Byte
CRC
E
O
D
S
O
F
E
O
F
I
F
S
The number of data bytes to be transferred is unspecified if 0EH is written to J_CMD3:0.
1-32 Bytes
IFR Data
1 Byte
CRC
E
O
D
N
B
1-3 Bytes
Header 1-11 Bytes
Data
†
In-frame Response (IFR) Frame
†