CONTREX CX-1200 - Manual (Page 255)

7 - 16
Parameter Control Byte Definitions:
Bit 7 = (1) Negative Numbers are Possible (0) Positive Numbers Only
Bit 6 = (1) Leading Zero’s OK (0) No Leading Zero’s
Bit 5 = (1) Restricted (0) Not Restricted
Bit 4 = (1) Parameter Defined (0) Parameter is NOT Defined
Bit 3 = Not Used (Reserved) always 0
Bit 2 = (1) Floating Point Number (0) Fixed Decimal Point Number
Bit 1 = (1) Binary Number (0) Decimal Number
Bit 0 = (1) Integer (0) Non-Integer
Example of CRC-16 Calculation (in C):
#define CRC16 0x8005 /* CRC-16 Generating Poly */
/* function returns the accumulated CRC value calculated for the Buffer */
/* this value can be transmitted or compared to a CRC value received */
/* “*data” is a pointer to the Buffer of data bytes to calculate the CRC for */
/* “len” is the number of data bytes to use for the calculation */
unsigned int do_crc(unsigned char *data, int len)
{
int i, j; /* byte & bit counters */
unsigned int accum = 0xFFFF; /* CRC value accumulator */
unsigned int dat; /* holds data byte */
for(i = 0; i < len; ++i){ /* for each byte of data */
dat = *data++; /* get data byte & goto next */
accum ^= (dat << 8); /* put data into high byte */
j = 0; /* clear bit counter */
while(j++ < 8){ /* for each bit */
if(accum & 0x8000) /* if MSB set */
accum ^= CRC16; /* Modulus-2 math w/CRC 16 */
accum <<= 1; /* shift left 1 bit */
} /* end for each bit */
} /* end for each byte */
return(accum); /* return the CRC value */
} /* End do_crc function */
Note: This “CRC” must be converted to 4 ASCII characters before transmission. (Chars 0 to 9 and A to F
should be used). For all “ASCII HEX” values the A through F characters must be in Upper Case
when Transmitted in order to keep the conversions consistent.