Message tokens – Grass Valley Switcher Products User Manual
Page 114
114
Switcher Products — Protocols Manual
Section 4 — DPM CPL Protocol
Figure 9. Message Format
The Data Size field specifies the size of the Message Data in bytes. It is this
field which allows variable-length messages to be passed. The Header
Checksum provides the mechanism by which message integrity and align-
ment can be verified. 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 significant byte trans-
mitted first) with a maximum packet size of 256 bytes. All messages will fit
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 mes-
sages, each message must be padded to guarantee 32-bit alignment.
Message Tokens
The values assigned to each message token are enumerated in
.
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 105. Message Tokens
Token
Value
Send Event
5
Set
6
Subscribe
8
Unsubscribe
10
Issue
16
Return Code
17
Token
Return ID
Data Size
Header Checksum
Message
Message
Header
Data
2 bytes
Field Size
8063_07_09_
r0
2 bytes
2 bytes
4 bytes
Variable-length
Version
2 bytes