Packet / Frame Structure
An RF packet consists of fields arranged in a fixed order so that the receiver can capture and make sense of the signal: preamble, sync word, address, payload and checksum. In a remote control the payload is usually a few bytes; button state and mode information are carried. The packet is not accepted unless all of these fields are valid.
The fields of a typical frame and their duties are as follows:
- Preamble: an alternating bit sequence; needed for the receiver to set its gain and lock its bit clock.
- Sync word: marks where the packet actually starts; prevents random noise from being taken for a packet.
- Address / device identity: makes the receiver respond only to its own transmitter.
- Payload: the actual command data; a few bytes are enough in a remote control.
- Checksum (CRC): catches corruption during transmission.
This layered structure produces an important behaviour from a safety standpoint: if one of the fields does not hold, the packet is silently discarded. That is, no wrong command appears at the receiver output; the command is simply counted as not having arrived. If the signal loss continues, the timeout comes into play and the outputs are taken to a safe state; on the HCT402 set this period is stated as 250 ms.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety