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J_CMD
The J1850 command (J_CMD) register determines the messaging type, specifies the number of bytes
to be transmitted in the next message frame, and updates the status of the message transmission in
progress. This byte register can be directly addressed through windowing . You must write to this
register prior to transmitting every message.
7
AUTO
IFR
Bit
Bit
Number
Mnemonic
7
AUTO
6
IFR
5
IGNORE
4
ABORT
3:0
MSG3:0
Figure 8-16. J1850 Command (J_CMD) Register
IGNORE
ABORT
Automatic Transmit Retry
This bit, when arbitration is lost on the first byte of your message, prompts
the transmitter to automatically retry until the byte is successfully
transmitted. Automatic retry applies only to the first byte.
0 = normal operation
1 = enable automatic retry
In-frame Response Indicator
This bit signals that a normalization bit (NB) is to be sent after an end-of-
data symbol is detected on the bus and that the subsequent byte written to
the J1850 transmitter (J_TX) register is an in-frame response (IFR).
0 = standard messaging
1 = next byte written to J_TX is an IFR
Ignore Incoming Message
This bit instructs the bus to ignore the incoming message until an EOF
symbol is detected. The bit is cleared after an EOF symbol is detected.
0 = normal operation
1 = ignore incoming message
Abort Transmission
This bit aborts any transmission in progress and flushes the transmit buffer
(JTX_BUF). To prevent another node from mistakenly assuming that the
last byte was a CRC byte, two extra '1's are appended.
0 = normal operation
1 = abort transmission in progress
Message
These four bits specify the number of bytes to be transmitted in the next
message frame. This number includes the header, but not the CRC byte. In
normal messaging, the maximum number of bytes you can transmit in a
message frame is eleven.
MSG3:0
Operation
FH
Termination byte
EH
Block transmission Transmit unspecified number of bytes
DH
Reserved
CH
Reserved
B:0H
Normal messaging Transmit specified number of bytes
J1850 COMMUNICATIONS CONTROLLER
Address:
Reset State:
MSG3
MSG2
Function
Purpose
Terminate block transmission
—
—
1F51H
00H
0
MSG1
MSG0
8-17
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