Axpert-Eazy AC Variable Frequency Drive
11.15
10
11
Byte 3 and 4 has right justified packed value of parameter number.
5 00h No of parameters High Byte
6 0Ah No of parameters Low Byte. For 10 parameters to be write then 0Ah is
used.
7 14h Total 20 data bytes transmitted after byte 7.
8-27 20 Data Bytes.
Total 20 Data Bytes transmitted. First two for parameter 400084, next
two for 40085 and so on.
28 CRC Low CRC Low Byte
29 CRC High CRC High Byte. The High and Low bytes are reversed in CRC
transmission. The method of calculating CRC is described in the next
section.
If the query is received properly then the response is returned as below. The first six bytes are same
as query.
Number
1 01h First byte is the address of the modbus slave device referred as station
number.
2 10h Indication of multiple write operation
3 00h Starting address High byte
4 53h Starting Address Low byte.
5 00h No of parameters High Byte
6 0Ah No of parameters Low Byte.
7 CRC Low CRC Low Byte
8 CRC High CRC High Byte. The High and Low bytes are reversed in CRC
transmission. The method of calculating CRC is described in the next
section.
6. CRC Generation
The Cyclical Redundancy Check (CRC) field is two bytes, containing a 16-bit binary value. The CRC
value is calculated by the transmitting device, which appends the CRC to the message. The receiving
device recalculates a CRC during receipt of the message, and compares the calculated value to the
actual value it received in the CRC field. If the two values are not equal, an error results.
The CRC is started by first preloading a 16-bit register to all 1's. Then a process begins of applying
successive 8-bit bytes of the message to the current contents of the register. Only the eight bits of
data in each character are used for generating the CRC. Start and stop bits, and the parity bit, do not
apply to the CRC.
During generation of the CRC, each 8-bit character is exclusive ORed with the register contents. Then
the result is shifted in the direction of the least significant bit (LSB), with a zero filled into the most
significant bit (MSB) position. The LSB is extracted and examined. If the LSB was a 1, the register is
then exclusive Ored with a preset, fixed value. If the LSB was a 0, no exclusive OR takes place.
This process is repeated until eight shifts have been performed. After the last (eighth) shift, the next 8-
bit character is exclusive ORed with the register's current value, and the process repeats for eight
more shifts as described above. The final contents of the register, after all the characters of the
message have been applied, it the CRC value.
A procedure for generating a CRC is:
1, Load a 16-bit register with FFFFh hex (all 1's). Call this the CRC register.
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