Savch S2800 Series, User Manual

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J. CRC Checking-CRC(cyclical Redundancy Check)
In RTU frame format, the frame includes frame incorrect test field based on CRC
computing. The CRC field detects the content of whole frames. The CRC field is 2
bytes and includes 16-binary. It is added to frame after being computed by
transmission equipment. The receiving equipment recalculates the CRC which has
received frames and compare it with values in the received CRC field. If the two CRC
values are different, it indicates that there is transmission error received.
In CRC checking mode, it saves in 0xFFFF first, then uses a procedure and
processes the continuous 6 more than bytes in frames together with current register.
Only the 8-bit data in every character is available to CRC, the start bit, end bit and
parity checking are all unavailable to it.
In CRC production course, every 8-bit character is different from the register content
separately or(XOR). The result moves to the direction of lowest effective bit. And the
highest bit is filled with 0. LSB is picked up to test. If LSB is 1, the register is different
from the preset value alone. If LSB is 0, no actions. The whole process must be
repeated 8 times. As the last bit (8th bit) is finished, the next 8-bit character is
different from the present value of register again. The final value in register is the
CRC value after all bytes in frame have been executed.
This calculation method of CRC adopts international standard CRC checking rule.
The user can refer to the CRC algorithm of relative standard when edit the CRC
algorithm to edit the real CRC calculation program which complies with the
requirements.
Here is a simple function of CRC algorithm for users' reference (using C language
programming).
Unsigned int crc_ cal_ value(unsigned char *data_ value, unsigned char data_ length)
{
int I;
unsigned int crc_ value=0xffff; while(data_ length--)
{
Crc_ value^=*data_ value++;
For(i=0;i<8;i++)
{
If(crc_value&0x0001)
Crc_ value=(crc_ value>>1)^0xa001;
Else
Crc_ value=crc_ value>>1;
}
}
Return(crc_ value);s
}
In ladder logic, CKSM calculates CRC value according to frame content with table
look-up method. This program is simple and fast calculation, but occupies larger ROM
space. So please use it carefully for occasions with requirement to program space.