Welch Allyn LC101 CO2 Module - User Manual - Manual (Page 60)

LC101 CO
2
Module OEM Implementation Manual
Page 60 Confidential Welch Allyn OEM Technologies
CRC
The CRC is performed on the byte stream, not on the corresponding values of the ASCII hex
characters. The CRC check should be performed at a very low level within the host's software.
The host calculates a CRC of the actual transmitted bytes excluding the check byte (which is sent
as two ASCII HEX characters).
For the byte string:
057h represents a single character, in this case the ASCII letter "W". 0x25 represents the
resulting value of the ASCII HEX representation of the parameter value. Thus, 0x25 is actually
transmitted as 32h (ASCII letter "2") followed by 35h (ASCII letter "5").”
CCITT/CRC Code Calculation
The 8-bit CCITT/CRC code is based on the CCITT/CCITT/CRC polynomial which uses the
conventional right shifting (high to low) method. The polynomial is:
G(x) = x^8 + x^7 + x^2 + 1
The feedback constant is 0xA1 (hex). CCITT/CRC values are initialized to 0xFF (hex).
The CCITT/CRC update value is implemented by a table look up scheme. The following is an
implementation example in ‘C’ language. Usage:
#define updateCCITT/CRC_8(value,CCITT/CRC) (CCITT/CRC_table_8[value^CCITT/CRC])
#define MAX_SIZE 23
/*choose any size*/
unsigned char CCITT/CRC_table_8[256];
unsigned char data[MAX_SIZE];
/*data to be CCITT/CRC checked*/
unsigned char calc_CCITT/CRC(char data,int n)
{
unsigned char CCITT/CRC;
/*make_table_8( );*/
/*do this at least once*/
CCITT/CRC=0xFF;
for(i=0;i
CCITT/CRC=updateCCITT/CRC_8(data[i],CCITT/CRC);
}
return(CCITT/CRC);
}
{STX} “W” xx yy CCITT/CRC {ETX} waveform byte string
0x25 0x80 0x79 {STX}W258079{ETX}
02h 57h 32h35h 38h30h 37h39h 03h