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ADVANCED USER GUIDE
Commander SX
IP66/Nema 4X variable speed drive
OPERATION WITH MODBUS RTU
CONTROL TECHNIQUES
3854 en - 03.2008 / b
21.4.3 - Parameter mapping
Commander SX drives are configured using a
menu.parameter notation.
The "menu" and "param" indices can take the values 0 to 99.
The menu.param is mapped into a MODBUS RTU register
menu x 100 + parameter.
In order to map the parameters correctly, the slave
increments (+1) the address of the received register.
Example: X = menu; Y = parameter
21.4.4 - Data encoding
MODBUS RTU uses a "big-endian" representation for the
addresses and data items (except for the CRC which is "little-
endian"). This means that, when a numerical quantity "bigger"
than one byte is transmitted, the most significant byte is sent
first.
For example:
21.4.5 - Function codes
The function code determines the context and format of the
message data.
• Function code 3: Read multiple
Reads a contiguous array of registers. The slave imposes an
upper limit on the number of registers which can be read. If
the limit is exceeded, the slave does not respond.
Note: 99 parameters maximum can be read.
Frame sent by the Master:
Frame returned by the slave:
Where n = 3 + number of bytes to be read.
• Function code 6: Write single register
Writing of a value to a single 16-bit register. The normal
response is an echo of the request, returned after the register
contents have been written.
Frame sent by the master:
Frame sent by the slave:
Drive parameter
Register address
(protocol level)
X.Y (X x 100) + (Y – 1)
Examples:
1.02 101
1.00 99
0.01 0
12.33 1232
1
st
2
nd
…
16 – bits 0x1234 would be: 0x12 0x34
Function code
Description
Decimal Hexadecimal
3 03
Read multiple 16-bit registers or
words
6 06 Write single 16-bit register or word
16 10
Write multiple 16-bit registers or
words
23 17
Read and write multiple 16-bit
registers or words
Bytes Description
0 Slave address (1 to 247)
1 Function code 0x03
2 Start word address MS byte
3 Start word address LS byte
4 Number of words to be read MS byte
5 Number of words to be read LS byte
6 CRC LS byte
7 CRC MS byte
Slave no.
Command (03)
Address of 1st word
Number of words
CRC
1 hexadecimal
byte
2 hexadecimal
bytes
Bytes Description
0 Slave address
1 Function code 0x03
2 Number of bytes to be read
3 Word 0 MS byte
4 Word 0 LS byte
5 Word 1 MS byte
6 Word 1 LS byte
... …
n CRC LS byte
n + 1 CRC MS byte
Bytes Description
0 Slave address (0 to 247)
1 Function code 0x06
2 Register address MS byte
3 Register address LS byte
4 Register value MS byte
5 Register value LS byte
6 CRC LS byte
7 CRC MS byte
Bytes Description
0 Slave address
1 Function code 0x06
2 Register address MS byte
3 Register address LS byte
4 Register value MS byte
5 Register value LS byte
6 CRC LS byte
7 CRC MS byte
CRC
Slave no.
Command (03)
No. of bytes
Word number 0
Word number 1
…
1 hexadecimal
byte
2 hexadecimal
bytes
CRC
Slave no.
Command (06)
Register address
Register value
1 hexadecimal
byte
2 hexadecimal
bytes
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