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Before staring Installation Operation Specifications
3.1.3 MODBUS-RTU Protocol
Frame
heading
character
Instrument
no.
Function
code
Data
Frame
confirmation CRC
Frame
terminating character
none 8 bit 8 bit n x 8 bit 16 bit None
Function code Code detail
03 Register multiple reading (n EA)
06 Register single writing (1 EA)
08 Diagnosis function (LOOP-BACK TEST)
16 Register multiple writing (n EA)
Serial number Details Size Ex.
0 Frame head character None -
1 Device number 8 bit 01 h
2 Function code (03) 8 bit 03 h
3 Reading start register (high) 8 bit 75 h
4 Reading start register (low) 8 bit 36 h
5 Number of data to be read (high) 8 bit 00 h
6 Number of data to be read (low) 8 bit 05 h
7 Frame confirmation CRC (low) 8 bit XX h
8 Frame confirmation CRC (high) 8 bit XX h
9 Frame terminating character None -
Serial number Details Size Ex.
0 Frame head character None -
1 Device number 8 bit 01 h
2 Function code (03) 8 bit 06 h
3 Number of data read and transmitted 8 bit dd h
4 Read data 1 (high) 8 bit dd h
5 Read data 1 (low) 8 bit dd h
... ... ... ...
n - 4 Read data n (high) 8 bit dd h
n – 3 Read data n (low) 8 bit dd h
n – 2 Frame confirmation CRC (low) 8 bit XX h
n – 1 Frame confirmation CRC (high) 8 bit XX h
n Frame terminating character None ...
● Frame structure
● Function code
● Function code 03 (READ MULTIPLE REGISTERS)
CRC : Cyclic Redundancy Check
<Transmission frame>
With function code 03, consecutive register details can be read at once. Number of data to be read at
once varies depending on line condition and transmission speed within the frame. One transmission frame
should be less than 255 bytes.
<Reception frame>
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