Chapter 6 Applied Instructions CRC check code: 16#02 is stored in the low 8-bit in D106, and 16#07 is stored in the low 8-bit in D107. Additional remarks 1. The following table shows the format of the communication data in RTU mode. 2. CRC check code: The check code starts from the address to the data. The operation rule is shown in the following table. Step 1: Suppose the data in the 16-bit register (the register where the CRC check code is stored) is 16#FFFF. 6_ Step 2: The logical operator XOR takes the first 8-bit message and the low 8-bit data in the 16-bit register, and performs the logical exclusive OR operation on each pair of corresponding bits.
The operation result is stored in the 16-bit register. Step 3: The values of the bits in the 16-bit registers are shifted by one bit to the right. The value of the highest bit becomes 0. Step 4: If the value of the right-most bit that is shifted to the right is 0, the data from step 3 is stored in the 16-bit register. Otherwise, the logical operator XOR takes 16#A001 and the data in the 16-bit register, and performs the logical exclusive OR operation on each pair of corresponding bits. The operation result is stored in the 16-bit register. Step 5: Repeat step 3 and step 4, and perform the operation on the 8-bit message. Step 6: Repeat step 2–5, and then get the next 8-bit message.
Perform the operations on all messages. The final result in the 16-bit register is the CRC check code. Notice that the low 8-bit data in the 16-bit register is interchanged with the high 8-bit data in the 16-bit register before the CRC check code is put into the check code of the message. 6-531 START Time interval Address Communication address: 8-bit binary address Function Function code: 8-bit binary code DATA（n-1） Data: n×8-bit data ……. DATA 0 CRC check code: The 16-bit check code is composed of two 8-bit binary codes. CRC CHK Low CRC CHK High END Time interval