Grass Valley 2200 - Manual (Page 107)

5-17
Message Format
Parameter Article
The parameter subscription service data consists of one or more parameter
articles. Each article contains the value of one parameter. Multiple articles may be
included in the same issue as long as the request address and request ID for all
articles are the same. Remember that the request address and ID are provided by
the requesting control point and must be returned with the issue.
The data associated with each parameter article is shown below.
The parameter ID Þeld identiÞes which parameter is contained in the article. The
pad Þeld 32-bit aligns the device Þeld. The device Þeld itself identiÞes the device
associated with the speciÞed parameter. The last Þeld contains the parameter
value. In order to accommodate parameters of varying size, the value Þeld is
variable length. When parameter service issue contains more than one parameter
article, each article is 32-bit aligned within the packet.
1
To maximize link bandwidth, articles should be combined into a single issue
whenever possible. However, this practice should not delay delivery of an article.
That is, if no more articles require delivery, send a partially Þlled issue.
When a large number of subscriptions are active and many parameters are
simultaneously changing value, it is easy to exceed the maximum packet size.
When adding an article to an existing issue would exceed the maximum packet
size, the article must be added to a new issue message.
Table 5-7. Parameter Article Data
Field Size Value
Parameter ID 2 bytes refer to Appendix B
alignment pad 2 bytes any
Device 4 bytes refer to Appendix B
Parameter value variable refer to Appendix B
1. Refer to “Parameters” on page 5-19 for more information concerning parameters.