Message Format¶

PPRZ format¶

The Pprzlink v2.0 standard message is defined as such:

PPRZ-message: ABCxxxxxxxDE
    A PPRZ_STX (0x99)
    B LENGTH (A->E)
    C PPRZ_DATA
      0 SOURCE (~sender_ID)
      1 DESTINATION (can be a broadcast ID)
      2 CLASS/COMPONENT
        bits 0-3: 16 class ID
        bits 4-7: 16 component ID
      3 MSG_ID
      4 MSG_PAYLOAD
      . DATA (messages.xml)
    D PPRZ_CHECKSUM_A (sum[B->C])
    E PPRZ_CHECKSUM_B (sum[ck_a])

PPRZ_STX (1 bytes) start byte, defined as 0x99.
LENGTH (1 bytes) full length of the message, from PPRZ_STX to PPRZ_CHECKSUM_B.
SOURCE (1 bytes) ID of the sender.
DESTINATION: (1 bytes) ID of the receiver.
CLASS: (half a byte) The message class id, coded on the 4 least significant bits. See messages.xml.
COMPONENT: (half a byte) The component id, coded on the 4 most significant bits. This is not yet broadly supported in paparazzi. Default value is 0.
MSG_ID: (1 bytes) The message id. See messages.xml.
MSG_PAYLOAD: (LENGTH-8 bytes) message payload. See messages.xml.
PPRZ_CHECKSUM_*: (2 bytes) message checksum.

aircraft ID¶

ID 0x00 is reserved for the ground.
ID 0xFF is the broadcast ID.

Checksum¶

PPRZ_CHECKSUM_A is computed as the 1 byte wrapping sum of all bytes from LENGTH to MSG_PAYLOAD.

PPRZ_CHECKSUM_B is computed as the 1 byte wrapping sum of all values of PPRZ_CHECKSUM_A.

In this example code, data are the bytes from LENGTH to MSG_PAYLOAD.

def ck(data):
    cka=0
    ckb=0
    for b in data:
        cka += b
        ckb += cka
    return cka,ckb

Xbee API format¶

Pprz data are the same as in the PPRZ format, just the encapsulation differs.

XBee-message: ABCDxxxxxxxE
    A XBEE_START (0x7E)
    B LENGTH_MSB (D->D)
    C LENGTH_LSB
    D XBEE_PAYLOAD
      0 XBEE_TX16 (0x01) / XBEE_RX16 (0x81)
      1 FRAME_ID (0)     / SRC_ID_MSB
      2 DEST_ID_MSB      / SRC_ID_LSB
      3 DEST_ID_LSB      / XBEE_RSSI
      4 TX16_OPTIONS (0) / RX16_OPTIONS
      5 PPRZ_DATA
        0 SOURCE (~sender_ID)
        1 DESTINATION (can be a broadcast ID)
        2 CLASS/COMPONENT
            bits 0-3: 16 class ID
            bits 4-7: 16 component ID
        3 MSG_ID
        4 MSG_PAYLOAD
        . DATA (messages.xml)
    E XBEE_CHECKSUM (sum[D->D])

   ID is AC_ID for aircraft, 0x100 for ground station

XBee destination ID is 2 bytes long. Use the paparazzi AC_ID for the LSB and 0x00 for the MSB, with 2 exceptions:

The ground address is 0x0100
The broadcast address is 0xFFFF.

Example¶

Lets take this “PPRZ” encoded message:

99 0C 07 00 01 02 03 00 01 02 1C C4

This message can be decomposed as :

STX	LENGHT=12	SOURCE=7	DESTINATION=0	CLASS=1, COMPONENT=0
99	0C	07	00	01

MSG_ID=2	MSG_PAYLOAD	CKA	CKB
02	03 00 01 02	1C	C4