TransCore MPRX Series - Page 52

The MPRX accepts four ASCII < ` > characters (60 hex) as a wild card CRC value in lieu of a valid four-
character CRC value to facilitate testing and diagnostic checkout.
The MPRX implements the algorithm with a 512-byte lookup table to reduce the processing overhead
requirements.
To simplify the implementation of the CRC algorithm by host software developers, several examples of the
calculation are provided in C source code on the following pages. The calculation may be performed with
or without a lookup table, depending on the trade-o between code memory and processing overhead.
Example 1 presents an example of a function (CALCCRC) that calculates the CRC value through a call to a
separate function (UPDCRC).
unsigned short calccrc(char *message)
{
unsigned short crc = 0;
for ( ; *message != (char)0;message++) crc =
updcrc(*message & 0xff, crc);
return (crc)
}
Example 2 shows an example of UPDCRC that does not require a lookup table.
#dene BITS_PER_CHAR 8
unsigned short updcrc (unsigned short ch, unsigned short crc)
{
register short counter = BITS_PER_CHAR;
register short temp = crc;
while (--counter >= 0) if
(temp & 0x8000) {
temp <<= 1;
temp += (((ch <<= 1) & 0x0100) != 0);
temp ^= 0x1021;
}
else { temp
<<= 1;
temp += (((ch <<= 1) & 0x0100) != 0);
}
return(temp);
}
MPRX User Guide
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