Grass Valley 2200 - Manual (Page 102)

5-12
Section 5 Ñ Digital Effects Interface
The Data Size Þeld speciÞes the size of the Message Data in bytes. It is this Þeld
which allows variable-length messages to be passed. The Header Checksum
provides the mechanism by which message integrity and alignment can be
veriÞed. This is extremely important when parsing packets with multiple
messages. The algorithm used with version 1 of the protocol is a simple
summation of the message header information. This algorithm is shown below.
Header Checksum = Version + Token + Return ID + Data Size
All messages are network-byte ordered (i.e. most signiÞcant byte transmitted Þrst)
with a maximum packet size of 256 bytes. All messages will Þt within this packet
size, although care must be taken to insure subscription response issues are
packed based on this maximum.
When constructing packets (HDLC frames) which contain multiple messages,
each message must be padded to guarantee 32-bit alignment.
Message Tokens
The values assigned to each message token are enumerated in Table 5-1. Other
message values are treated as an error by the switcher.
The following sections identify the messages used for the switcher/DPM
interface. It provides a brief overview of each message, as well as details the
contents of each message.
Table 5-1. Message Tokens
Token Value
Send Event 5
Set 6
Subscribe 8
Unsubscribe 10
Issue 16
Return Code 17