INFICON Guardian EIES Controller User Manual

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N 07

Guardian Co-Deposition Controller Operating Manual

The CRC is computed using the following algorithm:
·

The CRC is initialized to 3FFF hex.

Each character in the message is examined, bit by bit, and added to the
CRC in the following manner:

The character is exclusive OR'd with the CRC.

The CRC is shifted right one bit position.

If a character's least significant bit is a 0 then the CRC is exclusive OR'd
with 2001h.

Steps b and c are repeated for each of the 8 bits in the character.

The CRC contains 14 significant bits. This is split into two characters of 7 bits
each, and then a decimal 34 is added to offset the character outside the range of
the Sync Character.

All messages are terminated with a LF character (0x0a).

The following is notional software (in java form) which describes the generation
and decoding of Guardian messages

public class SigmaAPI_Guardian
{
public static boolean useCheckSum = true;

private static short calcCRC (byte[] str) {
short crc = 0;
short tmpCRC;
int length = str[1] - 34;
if (length > 0) {

crc = (short) 0x3fff;

for (int jx = 1; jx <= length; jx++) {
crc = (short) (crc ^ (short) str[jx]);

for (int ix = 0; ix < 8; ix++) {
tmpCRC = crc;
crc = (short) (crc >> 1);
if ((tmpCRC & 0x1) == 1) {
crc = (short) (crc ^ 0x2001);
}
}
crc = (short) (crc & 0x3fff);
}
}
// 14 bit CRC of contents of str

return crc;
}