RS-232 Communication

Note

The RS-232 interface requires a common ground connection between the USBoard-USS4 and the computer.

The RS-232-interface runs at 19200 Baud. Communication via RS-232 uses the same format as the CAN Communication except for the following addition:

Every message from the USBoard-USS4 begins with a start byte which must have the value 0xFF. After each message, made up of eight data bytes, a 16 bit checksum is sent. Because of this a complete message is 11 bytes long.

Byte 1	Bytes 2-9	Byte 10	Byte 11
0xFF	data bytes	checksum high byte	checksum low byte

The checksum is calculated with an CRC-CCITT algorithm using 8 databytes.

Messages to the USBoard-USS4 only contain the data bytes.

Example implementation of the checksum calulation (C code):

unsigned int getCheckSum(unsigned char *c, size_t iNumBytes){
	unsigned int uCrc16;
	unsigned char ucData[2];
	size_t i;

	uCrc16 = 0;
	ucData[0] = 0;
	for(i=0; i<iNumBytes; i++){
		ucData[1] = ucData[0];
		ucData[0] = c[i];

		if(uCrc16 & 0x8000){
			uCrc16 = (uCrc16 & 0x7fff) << 1;
			uCrc16^= 0x1021;
		}else{
			uCrc16 <<= 1;
		}
		uCrc16^= (unsigned int)(ucData[0]) | ((unsigned int)(ucData[1]) << 8);
	}

	return uCrc16;
}