Chapter 8 MODBUS Communications
Revision June 2010
8-13
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 01H
CMD 03H
01H (Upper byte)
Starting data address
01H (Lower bytes)
00H (Upper bytes)
Number of data
(Count by word)
02H (Lower bytes)
CRC Check Low
94H (Lower bytes)
CRC Check High
37H (Upper bytes)
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.
Summary of Contents for ASD-B2-0121-B
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Page 17: ...Chapter 1 Unpacking Check and Model Explanation 1 4 Revision June 2010 ECMA Series Servo Motor...
Page 87: ...Chapter 4 Display and Operation 4 12 Revision June 2010 This page intentionally left blank...
Page 131: ...Chapter 6 Control Modes of Operation 6 22 Revision June 2010 Time Domain...
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