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Modbus Function Codes

Each command starts with an address followed by a function code. These are used to read data from discrete inputs and input registers. They can also read/write data on coils and holding registers of each Modbus device you address.

The following is a table of object types provided by a Modbus slave device to a Modbus master device:

Object type Access Size
Coil Read-write 1 bit
Discrete input Read-only 1 bit
Input register Read-only 16 bits
Holding register Read-write 16 bits

Modbus Frame

The frame format is composed of an Application Data Unit (ADU), which encloses a Protocol Data Unit (PDU):

  • ADU = Address + PDU + Error check
  • PDU = Function code + Data

Most used Function Codes

All Modbus Function Codes

The various reading, writing and other operations are categorised as follows. The most primitive reads and writes are shown in bold. A number of sources use alternative terminology, for example Force Single Coil where the standard uses Write Single Coil. Prominent entities within a Modbus slave are:

  • Coils (1 bit): readable and writable, states: on/off
  • Discrete Inputs (1 bit): readable, states: on/off
  • Input Registers (16 bit): readable, essentially measurements and statuses
  • Holding Registers (16 bit): readable and writable, essentially configuration values

Function type Function name Function code
Data Access Bit access Physical Discrete Inputs Read Discrete Inputs 2
Internal Bits or
Physical Coils
Read Coils 1
Write Single Coil 5
Write Multiple Coils 15
16-bit access Physical Input Registers Read Input Registers 4
Internal Registers or
Physical Output Registers
Read Multiple Holding Registers 3
Write Single Holding Register 6
Write Multiple Holding Registers 16
Read/Write Multiple Registers 23
Mask Write Register 22
Read FIFO Queue 24
File Record Access Read File Record 20
Write File Record 21
Diagnostics Read Exception Status 7
Diagnostic 8
Get Com Event Counter 11
Get Com Event Log 12
Report Slave ID 17
Read Device Identification 43
Other Encapsulated Interface Transport 43

Contents retrieved from

  • https://en.wikipedia.org/wiki/Modbus

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