Lite-On Technology Corp.
Industrial Automation
195
8.3.
MODBUS protocol
For the RTU (Remote Terminal Unit) mode, the user sets the required protocol in Parameter PD-02. As for
functions supported by the drive, 03H Multiple Word Reading, 06H Single Word Writing and 10H Multiple
Word Writing. Refer to the description below.
Data structure of communication
The following is the definition for the data frame in the RTU communication mode:
RTU mode:
Start
Standstill period over 10 ms
Slave address
(communication
address)
Slave address (communication address): 1-byte
Function code:
Function code: 1-byte
Data (n-1)
Data (n-1)
…….
Data content: n-word =2n-byte, n<=10
Data (0)
Data (0)
CRC error check:
CRC error check: 1-byte
End 1
Standstill period over 10 ms
The RTU (Remote Terminal Unit) communication mode begins from a static signal and ends with another
static signal. The communication position, function code, data content, check for Cyclical Redundancy
Check (CRC) error...etc. are between the beginning and end.
Example 1, Function Code 03H Multiple Word Reading:
For the example below, the master passes down the command to No. 127 (7FH) Slave for reading the data
from two words in a row starting from the home address 0200H. The data content replied by the slave is
Location 0200H = > Content 1122H and Location 0201H=> Content 3344H. Up to 10 entries allowed to be
read at once.
RTU mode:
Request command: Response command:
Request
command:
Slave Address
(1 Bytes)
7FH
Function
(1 Bytes)
03H
Initial data
location
(2 bytes)
02H (high bit set)
00H (low bit set)
