wtw TresCon UNO A111, Operating Manual

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TresCon UNO Appendix
8 - 7 ba43134e08 05/2017
Message field This field contains the commands to the "Slave" (see table). The CRC
immediately follows subsequent to the message (2 bytes).
CRC16 field The CRC is the checksum of all the transmitted bytes. The CRC is
transmitted as 2 binary bytes. The higher value bytes are transmitted
first followed by the lower value bytes. The transmitted CRC 16 is con-
structed so that it always supplements the total CRC to 0000 (hex). A
string is only valid if the CRC calculated over the entire received string
is = 0000 (hex).
Otherwise, the message is ignored.
CRC (C program) Procedure to create a CRC (C program):
Unsigned short crc16 (unsigned char *bytes, int byte_anz)
{
unsigned short crc;
unsigned char byte;
unsigned short merker;
int i;
crc = 0;
while(byte_anz--)
{
byte = *bytes++;
for(i = 7; i >= 0; i--, byte <<= 1)
{
merker = crc & (1 << 15);
crc <<= 1;
if(byte & (1 << 7))
crc++;
if(merker)
crc ^= 0x1021;
}
}
return crc;
}
7 6 5 4 3 2 1 0
"0" Cont. Length of the message field incl. CRC 16
Bit 5,4,3,2,1,0 The length field indicates the length of the message
or the length of the m 2 Bytes (CRC bytes):
max. block length 65 bytes.
Bit 6 Cont.: length of the message field
≤
1 block --> bit
6 = 0 , for length of the message field > 1 block -->
bit 6 = 1.
The continuation bit is set if another block follows. The last data block
sets the data bit to "0".
Length field