Chziri ZVF300H, User Manual

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3.5 CHK (check sum)
ASCII mode:
ASCII mode adopt LRC (Longitudinal Redundancy Check) check sum. The LRC
check sum is ADR1 to the last data content and the results is 256 as one unit . the
excess parts removes , (For example , the receiving results is hexadecimal 128H
only take 28H), Then calculate the two times complement and the getting result is
LRC check sum.
For example, the first example of inquiry information check sum: 01H +03 H +21
H+02 H+00 H+02 H = 29H, and then take 2's complement = D7H.
RTU mode
CRC (Cyclical Redundancy Check) is calculated by the following steps:
Step 1: Load a 16-bit register (called CRC register) with FFFFH.
Step 2: The first byte of the command message and 16-bit CRC make low byte
XOR arithmetic .
Step 3: Shift the CRC register one bit to the right with MSB zero filling. Extract and
examine the LSB.
Step 4: If the LSB of CRC register is 0, repeat step 3, else XOR or the CRC register
with the polynomial value A001H.
Step 5: Repeat step 3 and 4 until eight shifts have been performed. When this is
done, a complete 8-bit byte will have been processed.
Step 6: Repeat steps 2 to 5 for the next 8-bit byte of the command message.
Continue doing this until all bytes have been processed. The final contents of
the CRC register is the CRC value. When transmitting the CRC value in the
message, the upper and lower bytes of the CRC value must be swapped. i.e. the
lower order byte will be transmitted first.
The following is an example of CRC generation using C language. The function
takes two arguments:
Unsigned char* data
← a pointer to the message
Unsigned char length ← the quantity of bytes in the message. The function returns
the CRC value as a type of unsigned int.