RKC INSTRUMENT PG500, Инструкция по эксплуатации системы связи

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4. MODBUS COMMUNICATION PROTOCOL
IMR02F04-E1
33

Example of a CRC calculation in the ‘C’ language
This routine assumes that the data types ‘uint16’ and ‘uint8’ exists. Theses are unsigned 16-bit integer
(usually an ‘unsigned short int’ for most compiler types) and unsigned 8-bit integer (unsigned char).
‘z_p’ is a pointer to a Modbus message, and ‘z_messaage_length’ is its length, excluding the CRC. Note
that the Modbus message will probably contain NULL characters and so normal C string handling
techniques will not work.
uint16 calculate_crc
（
byte *z_p, unit16 z_message_length
）
/* CRC runs cyclic Redundancy Check Algorithm on input z_p */
/* Returns value of 16 bit CRC after completion and */
/* always adds 2 crc bytes to message */
/* returns 0 if incoming message has correct CRC */
{
uint16 CRC= 0xffff;
uint16 next;
uint16 carry;
uint16 n;
uint8 crch, crcl;
while (z_messaage_length--) {
next = (uint16) *z_p;
CRC ^= next;
for (n = 0; n < 8; n++) {
carry = CRC & 1;
CRC >>= 1;
if (carry) {
CRC ^= 0xA001;
}
}
z_p++;
}
crch = CRC / 256;
crcl = CRC % 256
z_p [z_messaage+] = crcl;
z_p [z_messaage_length] = crch;
return CRC;
}