Grass Valley 2200 - Manual (Page 104)

5-14
Section 5 Ñ Digital Effects Interface
Subscribe
The subscribe command is used to solicit information from a device. This
information is delivered in the form of a subscription issue
1 . A subscription
issue is sent after receiving a subscription request and thereafter when the
requested information changes. As shown below, the Subscribe command
data consists of a request address, a request ID, a serial number, a
service ID, subscribe flags, and optional service specific data.
The request address Þeld speciÞes where the requested information is to be
delivered. This Þeld is only used when subscribing to information via KrystalÕs
ethernet interface and is unused for HDLC communications.
The request ID is a user-deÞned subscription request identiÞer. It is used in
conjunction with the request address to uniquely identify a subscription. Because
the request ID is sent back as part of the subscription issue, it is commonly used
for subscription issue routing within a control point. The serial number is also sent
back to a control point as part of a subscription issue. A control point can use this
Þeld to qualify a subscription issue. For example, when a control point changes a
subscription, it should modify the serial number from its previous state. If a
subscription issue is received that contains the old serial number, then that issue
should be discarded.
The service ID identiÞes the desired subscription service. Only parameter service
is supported for the switcher/DPM interface. The parameter service is used to
obtain and/or monitor the present value of one or more parameters.
The ßags Þeld modiÞes the behavior of the Subscribe command. When the
subscribe immediate ßag is set, the requested information is returned
immediately. In essence, the information is returned once upon request without
further updates. The replace service ßag is used to replace a current service
subscription (identiÞed by request address, request ID, and service ID) with a new
request. For example, one could replace an existing parameter service subscription
with a new group of parameters.
1. Refer to “Issue” on page 5-16 for a complete description of a subscription issue.
Table 5-3. Subscribe Command Data
Field Size Value
Request Address 16 bytes (not used)
Request ID 2 bytes any (requestor’s choice)
echoed in Issue message
Serial Number 2 bytes any (requestor’s choice)
Service ID 2 bytes 3 = Parameter Service
Flags 2 bytes 0 = normal service
1 = Subscribe Immediate
2 = Replace Service
3 = combination of 1 & 2
Service Data variable refer to Parameter Service below