Appendix I
RS485 Communication Protocol
114
low-order
‘8’
LRC CHK Hi
‘C’
LRC CHK Lo
‘2’
END Hi
CR
END Lo
LF
Parity mode - CRC mode: CRC (Cyclical Redundancy Check)
Use RTU frame format, the message includes error check field based on the CRC
method. The CRC field checks the whole content of message. The CRC field has two
bytes containing a 16-bit binary value. The CRC value calculated by the transmitting
device will be added into to the message. The receiving device recalculates the value of
the received CRC, and compares the calculated value to the actual value of the received
CRC field, if the two values are not equal,then there is an error in the transmission.
The CRC firstly stores 0xFFFF and then calls for a process to deal with the
successive eight-bit bytes in message and the value of the current register. Only the 8-bit
data in each character is valid to the CRC,the start bit and stop bit, and parity bit are
invalid. During generation of the CRC, each eight-bit character is exclusive OR (XOR)
with the register contents separately, the result moves to the direction of least significant
bit (LSB), and the most significant bit (MSB) is filled with 0. LSB will be picked up for
detection, if LSB is 1, the register will be XOR with the preset value separately, if LSB
is 0, then no XOR takes place. The whole process is repeated eight times. After the last
bit (eighth) is completed, the next
eight-bit byte will be XOR with the register's current value separately again. The final
value of the register is the CRC value that all the bytes of the message have been
applied.
When the CRC is appended to the message, the low byte is appended firstly,
followed by the high byte. CRC simple functions is as follows:
unsigned int crc_chk_value
（
unsigned char *data_value,unsigned char length
）
{
unsigned int crc_value=0xFFFF;
int i;
while
（
length--
）
{
crc_value^=*dat+;
for
（
i=0;i<8;i++
）
{
if
（
crc_value&0x0001
）
{
c r c _ v a l u e =
（
c r c _ v a l u e > > 1
）
^0xa001;
}
else
{
