Annex; Modbus-Rtu Protocol Introduction; Modbus Storage Area; Modbus Function Code - odot DPM01 User Manual

7 Annex

7.1 Modbus-RTU Protocol Introduction

For user, it is important to understand that Modbus has 8 important function codes
corresponding to 4 areas: 4 read, 2 write a single bit or register, and 2 write multiple
bits or multiple registers (Address description adopts PLC address).

7.1.1 Modbus Storage Area

The storage area of the controller (or Modbus device) involved in Modbus is
identified by 0XXXX, 1XXXX, 3XXXX and 4XXXX.
Storage ID Name
0XXXX Output Coil
1XXXX Discrete Input
3XXXX Input Register
Output/Holding
4XXXX
Registers

7.1.2 Modbus Function Code

Modbus messages are relatively fixed, so it could know the structure after reading a
few messages, and users can inquire about it when necessary.
(1) Read output coil status
Function Code：01H
Master station inquiry message format:
Function Start Address
Address
Code High Byte
0x11 0x01
Function: Read slave station output coil 0XXXX status.
Note: Some device coil start address is 00000, which corresponds to address 00001 in
the device, and the sequence is postponed.
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Data Type
Bit
Bit
Word
Word
Start Address
Low Byte
0x00 0x13
69 / 76
Read/Write Storage Unit Address
00001~0XXXX，
Read/Write XXXX：related to the
10001~1XXXX，
Read Only XXXX：related to the
30001~3XXXX，
Read Only XXXX：related to the
40001~4XXXX，
Read/Write XXXX：related to the
Number of coils Number of coils
High Byte Low Byte
0x00 0x25
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device
device
device
device
CRC
xxxx