NOV_B messages

Format:

This protocol uses the following frame format:

#	Offset	Field	Type	Unit	Example	Description
-	0	`sync1`	uint8_t	-	`0xaa`	Sync byte 1 (always `0xaa`)
-	1	`sync2`	uint8_t	-	`0x44`	Sync byte 2 (always `0x44`)
-	2	`sync3`	uint8_t	-	`0x12`	Sync byte 3 (always `0x12`)
-	3	`header_len`	uint8_t	bytes	`28`	Length of the header (always `28`)
-	4	`msg_id`	uint16_t	-	`1234`	Message ID (defines the structure of the payload)
-	6	`msg_type`	uint8_t	-	-	Bitfield:
0	-	`msg_source`	bits 4…0	-	`0`	Measurement source, where applicable: 0 = GNSS1, 1 = GNSS2
-	-	`reserved0`	bits 7…5	-	-	Reserved, ignore
-	7	`reserved1`	uint8_t	-	-	Reserved, ignore
-	8	`payload_len`	uint16_t	bytes	`20`	Total size in bytes of all payload fields
-	10	`reserved2`	uint16_t	-	-	Reserved, ignore
-	12	`reserved3`	uint8_t	-	-	Reserved, ignore
1	13	`time_status`	uint8_t	-	`0xb4`	Time (`gps_wno` and `gps_tow`) validity, `0xb4` = time known, otherwise unknown/invalid
2	14	`gps_wno`	uint16_t	-	`0`	GPS week number
3	16	`gps_tow`	int32_t	ms	`0`	GPS time of week
-	20	`reserved4`	uint32_t	-	-	Reserved, ignore
-	24	`reserved5`	uint16_t	-	-	Reserved, ignore
-	26	`reserved6`	uint16_t	-	-	Reserved, ignore
4…	28	…	any type	-	-	Any number of payload fields
-	x	`checksum`	uint32_t	-		CRC32 checksum (see below)

An alternative “short header” format is available for some cases:

#	Offset	Field	Type	Unit	Example	Description
-	0	`sync1`	uint8_t	-	`0xaa`	Sync byte 1 (always `0xaa`)
-	1	`sync2`	uint8_t	-	`0x44`	Sync byte 2 (always `0x44`)
-	2	`sync3`	uint8_t	-	`0x13`	Sync byte 3 (always `0x13`)
-	3	`payload_len`	uint8_t	bytes	`20`	Total size in bytes of all payload fields
-	4	`msg_id`	uint16_t	-	`2269`	Message ID
1	6	`gps_wno`	uint16_t	-	`0`	Week number
2	8	`gps_tow`	int32_t	ms	`0`	Time of week
3…	12	…	any type	-	-	Any number of payload fields
-	x	`checksum`	uint32_t	-		CRC32 checksum (same as for normal header)

Notes:

All fields are little endian
Signed fields are 2’s complement
Possible integer types are: int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t
Possible float types are: float (32 bits) and double (64 bits), as per IEEE 754

Cyclic Redundancy Check (CRC):

The user must calculate a checksum at the end of a data package for the Vision-RTK 2's wheelspeed interface to verify the integrity of the message. We use cyclic redundancy check (CRC) as our error-detecting code to achieve this. The CRC allows the detection of possible errors introduced during the transmission or storage of the message. The user can determine whether the data has been altered by comparing the computed checksum with the original checksum (previously computed and stored with the data).

For the NOV_B message, the checksum is a 32 bit cyclic redundancy check (CRC) with the polynomial 0x04c11db7. You can use the following script to generate the CRC:

CODE

// Generated using https://github.com/madler/crcany.git with the following parameters:
// width=32 poly=0x04c11db7 init=0x00000000 refin=true refout=false xorout=0x00000000 check=0x00000000 residue=0x000000 name="NOVB-32"
static const uint32_t sCrcNovb32[256] =
{
    0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
    0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
    0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
    0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
    0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
    0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
    0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
    0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
    0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
    0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
    0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
    0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
    0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
    0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
    0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
    0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
    0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
    0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
    0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
    0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
    0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
    0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
    0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
    0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
    0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
    0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
    0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
    0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
    0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
    0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
    0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
    0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
    0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
    0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
    0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
    0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
    0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
    0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
    0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
    0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
    0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
    0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
    0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};

uint32_t crcNovb32(const uint8_t *data, const int len)
{
    uint32_t crc = 0;
    for (int ix = 0; ix < len; ix++)
    {
        crc = (crc >> 8) ^ sCrcNovb32[(crc ^ data[ix]) & 0xff];
    }
    return crc;
}

The following CRC tables were generated using the following repository and catalog: 'https://github.com/madler/crcany.git ', 'https://reveng.sourceforge.io/crc-catalogue/all.htm '. The polynomials were extracted from 'http://users.ece.cmu.edu/~koopman/crc/crc32.html '.