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In-frame Response (IFR) Frame
S
1-3 Bytes
O
Header
F
†
The number of data bytes to be transferred is unspecified if 0EH is written to J_CMD3:0.
††
Each D
block in the IFR data field represents a byte of data from a different remote node.
X
8.4.2.3
IFR Messaging Type 3: Multiple Bytes, Single Responder
IFR messaging type 3 (Figure 8-11) is ideal for requesting large amounts of information from a
single source in your system. You can compile up to 12 bytes of data from a remote node on a
single request. In our example, for the same amount of CPU overhead as IFR type 1 messaging
exhausted (4.96 ms), you can gather up to twelve times as much information.
In-frame Response (IFR) Frame
S
1-3 Bytes
O
Header
F
†
The number of data bytes to be transferred is unspecified if 0EH is written to J_CMD3:0.
8.5
TRANSMITTING AND RECEIVING MESSAGES
The J1850 controller can transmit and receive messages in either standard or IFR form.
8.5.1
Transmitting Messages
To transmit a standard message, prepare the message in register RAM and then write it to the
J1850 transmit (J_TX) register (Figure 8-12) one byte at a time.
1 Byte
1-11 Bytes
†
CRC
Data
Figure 8-10. IFR Type 2 Message Frame
1 Byte
1-11 Bytes
†
CRC
Data
Figure 8-11. IFR Type 3 Message Frame
J1850 COMMUNICATIONS CONTROLLER
††
IFR Data Field
E
N
. . . . . . . . . .
D
D
D
O
0
1
31
B
D
E
1-12 Bytes
N
O
IFR Data
B
D
E
E
I
0-1 Byte
O
O
F
CRC
D
F
S
A5227-01
E
E
I
0-1 Byte
O
O
F
CRC
D
F
S
A5228-01
8-13
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