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5.1.7 Alternate Header Formats
There are occasions where packets between the AVT-718 and the host computer are more than
15 bytes in length. To support these longer packets, two alternate forms have been developed.
The user may also refer to the document “Long Messages - Alternate Header Formats.”
To accommodate long messages, the format for communications between the AVT-718 unit and
the host computer has been modified. The host has three methods available for passing a packet
to the AVT-718 for transmission onto the vehicle network. All three methods are described here,
along with examples.
If the message is 15 bytes or less in length (total) then the 'normal' format may be used.
Normal Format:
0x aa bb cc ...
x - count of bytes to follow
aa bb cc ... message bytes.
Example:
05 81 F1 C1 48 9B
If the message is more than 15 bytes but less than 255 bytes in length, alternate format #1 is
available using a header byte of $11.
Alternate format #1: 11 xx aa bb cc ...
11 - header byte
xx - count of bytes to follow
aa bb cc ... message bytes.
If the message is more than 255 bytes in length, alternate format #2 is available using a header
byte of $12.
Alternate format #2: 12 xx yy aa bb cc ...
12 - header byte
xx - count of bytes to follow, most significant byte
yy - count of bytes to follow, least significant byte
aa bb cc ... message bytes.
These formats are backward compatible and may be used as desired.
For example, the host wants to transmit the following message onto the bus:
A1 B2 C3 D4 E5 F6 A7 B8 C9 DA EB FC AD BE
The following messages from the host to the AVT-718 are all equivalent. The header byte(s) are
bold and underlined for clarity.
0E A1 B2 C3 D4 E5 F6 A7 B8 C9 DA EB FC AD BE
11 0E A1 B2 C3 D4 E5 F6 A7 B8 C9 DA EB FC AD BE
12 00 0E A1 B2 C3 D4 E5 F6 A7 B8 C9 DA EB FC AD BE
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