Blackmagic Design ATEM Manuel D'utilisation Et D'installation page 198

Blanking Encoding
A message group is encoded into a SMPTE 291M packet with DID/SDID x51/x53 in the active
region of VANC line 16.
Message Grouping
Up to 32 messages may be concatenated and transmitted in one blanking packet up to a
maximum of 255 bytes payload. Under most circumstances, this should allow all messages to
be sent with a maximum of one frame latency.
If the transmitting device queues more bytes of message packets than can be sent in a single
frame, it should use heuristics to determine which packets to prioritise and send immediately.
Lower priority messages can be delayed to later frames, or dropped entirely as appropriate.
Abstract Message Packet Format
Every message packet consists of a three byte header followed by an optional variable length
data block. The maximum packet size is 64 bytes.
Destination device (uint8)
Command length (uint8)
Command id (uint8)
Reserved (uint8)
Command data (uint8[])
Padding (uint8[])
Receiving devices should use the destination device address and/or the command identifier to
determine which messages to process. The receiver should use the command length to skip
irrelevant or unknown commands and should be careful to skip the implicit padding as well.
Device addresses are represented as an 8 bit unsigned integer.
Individual devices are numbered 0 through 254 with the value 255
reserved to indicate a broadcast message to all devices.
The command length is an 8 bit unsigned integer which specifies the
length of the included command data. The length does NOT include the
length of the header or any trailing padding bytes.
The command id is an 8 bit unsigned integer which indicates the
message type being sent. Receiving devices should ignore any
commands that they do not understand. Commands 0 through 127 are
reserved for commands that apply to multiple types of devices.
Commands 128 through 255 are device specific.
This byte is reserved for alignment and expansion purposes. It should
be set to zero.
The command data may contain between 0 and 60 bytes of data.
The format of the data section is defined by the command itself.
Messages must be padded up to a 32 bit boundary with 0x0 bytes.
Any padding bytes are NOT included in the command length.
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