Delta ASD-B2-0121-B User Manual page 262

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.
NOTE
1) 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.
2) 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
CMD
Starting data address
Number of data
(Count by word)
CRC Check Low
CRC Check High
End1, End0 (Communication End)
ASCII Mode:
In ASCII mode, (0DH) stands for character '\r' (carriage return) and (0AH) stands for
character '\n' (new line), they indicate communication end.
Revision June 2010
01H
03H
01H (Upper byte)
01H (Lower bytes)
00H (Upper bytes)
02H (Lower bytes)
94H (Lower bytes)
37H (Upper bytes)
Chapter 8 MODBUS Communications
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