Kongsberg Seapath 320, Installation Manual, Page: 123 / 152

Installation Manual
M300-62/rev.2 107
Element Scaling Format Bytes Value
Status word Bit-fields 1
Checksum Unsigned 2
Checksum is calculated as a 16-bit Block Cyclic Redundancy Check of all bytes
between, but not including the Header and Checksum fields. The CRC algorithm is
described in a separate section. Time is divided in an integer seconds part and a
fractional second part. The integer seconds part of time is counted from 1970-01-01
UTC time, ignoring leap seconds. Heave is positive down. The status field is zero if
heave is valid, non-zero if heave is invalid.
A.15 Cyclic redundancy check algorithm
The 16-bit Block Cyclic Redundancy Check algorithm used to calculate the checksum
in some formats is described in C and Fortran source code below.
C code:
#define POLY 0x8408
unsigned short blkcrc(
unsigned char *bufptr, /* message buffer */
unsigned long len /* number of bytes */
)
{
unsigned char i;
unsigned short data;
unsigned short crc = 0xffff;
if (len == 0L) {
return ~crc;
}
do {
for (i=0, data = (unsigned short) (0xff & *+);
i < 8;
i++, data >>= 1) {
if ((crc & 0x0001) ^ (data & 0x0001)) {
crc = (crc >> 1) ^ POLY;
} else {
crc >>= 1;
}
}
} while (--len);