Grass Valley 2200 - Manual (Page 101)

5-11
Message Format
Message Format
A binary protocol with variable-length messages is used. Multiple messages may
be delivered in a single packet. To accommodate a packet that may contain
multiple variable-length messages, each message is preceded by a message
header. As shown in Figure 5-1, the message header is comprised of Þve Þelds.
The Version Þeld is used to identify the protocol version and allows changes in the
protocol while providing compatibility with older implementations. Currently,
only version 1 has been speciÞed.
The Token Þeld identiÞes the message.
The Return ID is a unique command identiÞer, that is used in conjunction with the
Return Code message. If a control point desires to know the result of issuing a
command, it places a non-zero value in the Return ID Þeld. The Return ID and the
command return code are sent back to the control point, in the form of a Return
Code status message. The sender should manage the return IDÕs value to avoid
receiving multiple return code messages with the same return ID.
NOTE: The switcher violates this utilization of the return ID.
The switcher does not respond with Return Code messages even if the return Id is
nonzero.
The switcher always responds to a Send Event On-line messages with a Return
Code message even if the return ID is zero. It inserts a zero return ID into that
Return Code message and sets the data value to EID_ONLINE (= 15) instead of
OK or FAILURE.
The switcher does specify a nonzero return ID in its Send Event On-line message
and expects the DPM to echo that return ID in a Return Code message, however
the switcher ignores the data value.
Only commands may request a Return Code message via the Return ID Þeld.
Status messages must insert a null return ID.
Figure 5-1. Message Format
Token
Return ID
Data Size
Header Checksum
Message
Message
Header
Data
2 bytes
Field Size
2 bytes
2 bytes
4 bytes
Variable-length
Version 2 bytes