Alcorn Mcbride Digital Video Machine 4, User Manual

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30 Ethernet Control
serial protocol command. For example, A DVM4 Control Message containing
the Command bytes 7@?V<CR> would cause the DVM4 on the network with
Unit ID 7 to send back a UDP packet containing its version information. Please
see the Pioneer / AMI Serial Protocol section for detailed information about the
rest of the command set.
The basic format of a DVM4 Control Message is as follows:
0xF1 0x01 0x04 <Pioneer / AMI Serial Command> <Checksum> 0xF2
The only bytes that change from message to message are the Command bytes
and the Checksum. The command corresponds exactly to an Addressed Pioneer
/ AMI serial command.
Control Message Checksum
The Checksum is the summation of the hexadecimal value of all of the bytes in
the Control Message except the first byte (0xF1), the last byte (0xF2) and of
course the Checksum itself. The value of the checksum may easily increase to a
value that cannot be represented by one byte. This situation calls for what we
call number expansion.
If the value of the Checksum is between 0x00 and 0xF9 (0-249), the value is
represented in only 8 bits (1 byte). If the value is between 0xFA and 0xFFFF
(250-65535), the value is represented in 16 bits (2 bytes) and is preceded by
0xFF to signify the number has been expanded to 16 bits. If the value is
between 0x10000 and 0xFFFFFF (65536-16777215), the value is represented in
24 bits (3 bytes) and is preceded by 0xFE to signify the number has been
expanded to 24 bits. The following example shows a Control Message with an
expanded Checksum.
0xF1 0x01 0x04 5 5 @ R J <CR> 0xFF 0x01 0x58 0xF2
The value of the checksum is calculated by adding the hexadecimal values of
each byte preceding it except the first byte (0xF1) as follows:
0x01 + 0x04 + 0x35 + 0x35 + 0x40 + 0x52 + 0x4A + 0x0D = 0x158.
The Checksum in this example is 0x158, but because it is greater 0xF9, the two
byte Checksum is preceded by 0xFF.