Intel 87C196CA, Supplement To User’S Manual

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8-17
J1850 COMMUNICATIONS CONTROLLER
J_CMD
Address:
Reset State:
1F51H
00H
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 0
AUTO IFR IGNORE ABORT MSG3 MSG2 MSG1 MSG0
Bit
Number
Bit
Mnemonic
Function
7 AUTO 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
6 IFR 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
5 IGNORE 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
4 ABORT 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
3:0 MSG3:0 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 Purpose
FH Termination byte Terminate block transmission
EH Block transmission Transmit unspecified number of bytes
DH Reserved —
CH Reserved —
B:0H Normal messaging Transmit specified number of bytes
Figure 8-16. J1850 Command (J_CMD) Register