RS485 Modbus protocol overview - 15
5.2 Data Addresses in Modbus Messages
Modbus defines 4 address spaces: 2 address spaces for bit addressable data and 2
address spaces for 16 bits addressable data.
Address space Data Readable/writable Modbus name
0XXXX Output bit Read & write Coil Status
1XXXX Input bit Read Input Status
3XXXX Input word Read Input Register
4XXXX Output word Read & write Holding Register
5.3 Supported function codes
The following table gives the Modbus functions, which are implemented and supported.
The code is the one used in function field of the Modbus message.
The address space concerned and the purpose of the function are given below.
Code Function Address range/remark
1 Read Coil Status 0XXXX Reads the on/off status of discrete outputs
2 Read Input Status 1XXXX Reads the on/off status of discrete inputs
3 Read Holding Registers 4XXXX Reads contents of output registers
4 Read Input Registers 3XXXX Reads contents of input registers
5 Force Single Coil 0XXXX Sets the status of a discrete output
6 Preset Single Register 4XXXX Sets the value of a holding register
7 Read Exception Status Device specific (see Modbus data table of a device)
8 Diagnostics Checks the communication system between the master and the slave
11 Fetch Comm. Event Ctr. Returns amount of successful read/write operations on data points
12 Fetch Comm. Event Log Returns log registers of communication events
15 Force Multiple Coils 0XXXX Sets the status of multiple discrete outputs
16 Preset Multiple Registers 4XXXX Sets the value of multiple holding registers
17 Report Slave ID Device specific (see Modbus data table of the device)
22 Mask Write 4X registers 4XXXX And/Or write of a holding register
23 Read/Write 4X registers
4XXXX Reads a set of holding registers and writes a set of holding
registers in one query
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