Delta Electronics AC Servo Drive ASDA-B User Manual

Page 248

Chapter 8 MODBUS Communications|ASDA-B Series

Revision February 2008, Doc. Name: 2006PDD23000009

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

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.

RTU Mode:

In RTU mode, a silent interval of more than 10ms indicates communication end.