Chapter 9 Communications
CRC (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: Exclusive OR the first 8-bit byte of the command message with the low order byte of
the 16-bit CRC register, putting the result in the CRC register.
Step 3: Extract and examine the LSB. If the LSB of CRC register is 0, shift the CRC register one
bit to the right. If the LSB of CRC register is 1, shift the CRC register one bit to the
right, then Exclusive OR the CRC register with the polynomial value A001H.
Step 4: Repeat step 3 until eight shifts have been performed. When this is done, a complete 8-
bit byte will have been processed, then perform step 5.
Step 5: Repeat step 2 to step 4 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
are the CRC value.
For example, reading 2 words from address 0101H of the AC servo drive with address 01H.
The final content of the CRC register from ADR to last data character is 3794H, then the
command message is shown as follows. What should be noticed is that 94H have to be
transmitted before 37H.
Command Message
ADR 01H
CMD 03H
01H (High)
Starting data address
01H (Low)
00H (High)
Number of data
(In Word)
02H (Low)
CRC Check Low 94H (Low)
CRC Check High 37H (High)
9-14 Revision January 2011
Call 1(800)985-6929 for Sales
Call 1(800)985-6929 for Sales
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