14
Refer to Section 5.1.7 of this manual and/or refer to the document “Long Messages - Alternate
Header Formats.”
5.1.2 Transmit message format
To transmit a message onto the network, the message must be built by the operator and then sent
to the AVT-718.
The CRC or checksum byte will be automatically calculated and appended onto the message
when transmitted. In some operational modes this function can be enabled or disabled by
software command.
Any message destined for transmission must be preceded by a byte whose upper nibble is ‘0’
(zero) and lower nibble is the byte count of the message (unless one of the alternate header
forms is used). The message bytes then follow immediately.
It is up to the user to determine and know the proper messaging strategy that is used on the
external network to which the AVT-718 is attached.
5.1.2.1 Transmit Acknowledgment
Each time a message transmission is attempted the interface will issue some kind of transmission
acknowledgment. The type and meaning of this transmission acknowledgment is dependent on
the mode of operation.
For VPW and KWP modes, the transmit status byte definition is the same as the receive message
status byte.
CAN mode does not have a transmit status byte.
PWM does not have a transmit status byte, but the AVT-718 will forward any received
acknowledgments to the host computer.
In UBP mode of operation, when a message is transmitted onto the network, the AVT-718 will
issue a receive message report to the host, along with a status byte. The AVT-718 will also issue
a separate transmit status report.
Some operational modes permit the user to enable or disable the function “Forward messages
from this device.” When enabled, the host receives a complete copy of the message just
transmitted, with the status bits set accordingly. When disabled, the host receives just the status
byte (for example,
01 60
).
Refer to the “Master Command and Responses” document for detailed information on each
specific mode of operation.
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