CNAOB-Communications-Protocol-Notes

Comm Port Binary Mode

Is necessary to use binary mode. Serial port in ASCII mode chokes on null = chr(0) chars

• Serial port return handling of binary data into array
• extensive binary byte array in lieu of string functions handling page

Links for CRC CheckSum Algorithm

• CRC calculation source code in VB using array lookup - ModBus-CRC-Local-Version-routine-v1
• CRC calculation source code in VB using all calculation - ModBus-CRC-Local-Version-routine-2

16-bit CRC verification code Algorithm

Only 8 data bits are involved in CRC calculation, with the exclusion of start bit and end bit. Algorithm of CRC code:

• 1) Presetting a 16-bit register to hex FFFF. The register is called CRC register

• 2) XORing the first byte of the communication message frame with the low 8-bit of 16-bit CRC register.....then storing the result in CRC register

• 3) Right-shifting the register data by one bit and filling the highest bit with 0, then checking the shift-out bit

• 4) If the shift-out bit is 0, repeat step 3 (right-shifting one more bit);

If the shift-out bit is 1, XOR the CRC register data with polynomial A001 (1010 0000 0000 0001) = A001

• 5) Repeating step 3 and step 4 until all of the 8-bit data have been processed after 8 right-shift operations;

• 6) Repeating step 2 to step 5 to process the next byte of the communication message frame;

• 7) When calculation procedures of the first 5 bytes in the communication message frame are completed, the 16-bit CRC verification

code will be generated in the 16-bit CRC register.

CRC Example Calculations: 1, 06, 40, 03,E8, 18, 22 Hex

• 1111 1111 1111 1111 CRC
• 0000 0000 0000 0001 Data 1
• ----------------------------- XOR
• 1111 1111 1111 1110 CRC
• 0111 1111 1111 1111 CRC 32767 after right shift / shifted bit = 0
• 0011 1111 1111 1111 CRC after right shift / shifted bit = 1
• 1010 0000 0000 0001 polynomial = A001
• ----------------------------- XOR
• 1001111111111110 CRC

keep doing till 8 shifts accomplished