TC
cFE Telecommand packets carry commands from ground to spacecraft. They extend the SPP primary header with a secondary header containing command identification and integrity checking.
Packet Structure
| Field | Size |
|---|---|
| Primary Header (SPP) | 6 bytes |
| Secondary Header | 2 bytes |
| Payload | variable |
| Minimum Total | 8 bytes |
Secondary Header
| Field | Size |
|---|---|
| Function Code | 1 byte |
| Checksum | 1 byte |
| Total | 2 bytes |
| Field | Size | Description |
|---|---|---|
| Function Code | 1 byte | Command identifier within the APID |
| Checksum | 1 byte | 8-bit XOR checksum of entire packet |
Fixed Header Values
Telecommand packets always have:
- Packet Type = 1 (Telecommand)
- Secondary Header Flag = 1 (Present)
- Sequence Flags = 0b11 (Unsegmented)
Checksum Algorithm
The cFE uses an 8-bit XOR checksum:
- Set checksum field to 0
- XOR all bytes of the packet together
- Store result in checksum field
Validation: XOR of all bytes (including checksum) equals 0.
Constraints
- Maximum payload: 65534 bytes
- Function code is application-specific (0-255)