Modbus Function Codes; Data Addresses In Modbus Messages; Supported Function Codes - ABB RVT Serie Instrucciones De Instalación Y De Funcionamiento

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3 Modbus function codes

3.1 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
OXXXX
1XXXX
3XXXX
4XXXX
Input register address space will be mainly used for measurements.
Holding register address space will contain settings.
All data addresses in Modbus messages are referenced to zero.
For example:
The coil known as 'coil 1' in a programmable controller is addressed as coil 0000 in the
data address field of a Modbus message.
Coil 127 decimal is addressed as coil 007E hex (126 decimal).
Holding register 40001 is addressed as register 0000 in the data address field of the
message.
The function code field already specifies a 'holding register' operation. Therefore the
'4XXXX' reference is implicit.
Holding register 40108 is addressed as register 006B hex (107 decimal).

3.2 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
1 Read coil status
2 Read input status
3 Read holding registers
4 Read input registers
5 Force single coil
6 Preset single register
14 Modbus function codes ç RVT communication
Data Readable/writable
Output bit Read & write
Input bit Read
Input word Read
Output word Read & write
Address range/remark
0XXXX reads the on/off status of discrete outputs
1XXXX reads the on/off status of discrete inputs
4XXXX reads contents of output registers
3XXXX reads contents of input registers
0XXXX sets the status of a discrete output
4XXXX sets the value of a holding register
Modbus name
Coil status
Input status
Input register
Holding register