Grass Valley Switcher Products - Manual (Page 113)

Switcher Products — Protocols Manual 113
Message Format
The switcher subscribes to information using several request IDs. It is
important for the DPM to include the proper request ID in the status
update issue messages. Different parameter articles may be sent in the
same issue as long as the subscriptions for all the data had the same request
ID. Articles destined for different request IDs must be placed in separate
issue messages.
It is also important to limit issue messages to less than 256 bytes or the
message will be discarded by the switcher.
Message Format
A binary protocol with variable-length messages is used. Multiple mes-
sages 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 9 , the message header is com-
prised of five fields.
The Version field is used to identify the protocol version and allows
changes in the protocol while providing compatibility with older imple-
mentations. Currently, only version 1 has been specified.
The Token field identifies the message.
The Return ID is a unique command identifier, 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 field. 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 ID’s 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 mes-
sage, however the switcher ignores the data value.
Only commands may request a Return Code message via the Return ID
field. Status messages must insert a null return ID.