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Modbus

Modbus is a communication protocol used for PLC networks. Historically, it was designed for communication on a serial interface (RS232, RS422,RS485) and was extended to support the TCP mode.

OIBus uses the jsmodbus library in TCP mode only.

Connection settings​

In this TCP mode, Modbus sets up a client-server connection: the server provides data referenced by addresses but remains passive. It is the Modbus client that fetches the data values. The Modbus connector is a Modbus client. It is therefore necessary to indicate to the connector:

  • The host (IP address or hostname of the Modbus server machine)
  • The port (502 by default)
  • The slave ID to identify the Modbus source machine if necessary (1 by default).

PLC settings​

Depending on the PLC, several settings are possible on how to access the data. These settings are common to the whole PLC. Here are the following parameters:

  • Address offset: For most of the PLCs, there is no offset (Modbus option). Some PLCs start the address range at 1 instead of 0 (JBus option).
  • Endianness: Indicates the type of bit encoding (Big Endian or LittleEndian)
  • Swap Bytes: Indicates whether the bytes within a group of 16 bits (a word) should be inverted or not
  • Swap Words: Indicates whether the words (16-bit group) should be inverted or not within a 32-bit group.

Points and Modbus addresses​

The Modbus connector retrieves values from specific addresses. These can be added in the Points section (in the upper right corner).

In this list, points can be added with:

  • Point ID: the name of the data in the result (example: My point variable)
  • Address: the address of the data in the machine
  • Modbus type: coil, discrete input, input register or holding register (default)
  • Data type: Used in case of holding registers or input registers (ignored otherwise). This parameter indicates the type of data retrieved from the register: UInt16 (default), Int16, UInt32, Int32, UInt64, Int64, Float or Double.
  • Multiplier Coefficient (default 1)
  • Scan mode: the request frequency. To define more scan modes, see Engine settings.

The address corresponds to the address of the variable in the PLC, in hexadecimal without the data type digit. For example:

  • For the holding register data 0x40001, enter the address 0x0001 (omit the digit 4) and specify the Modbus type holdingRegister.
  • For the coil data 0x009C, enter 0x009C and specify the Modbus type coil.

Modbus data addresses follow the Modicon Convention Notation (MCN):

  • Coil: [0x00001 - 0x09999] ; from 1 to 39,321
  • Discrete Input: [0x10001 - 0x19999] ; from 65,537 to 104,857
  • Input Register: [0x30001 - 0x39999] ; from 196,609 to 235,929
  • Holding Register: [0x40001 - 0x49999] ; from 262,145 to 301,465

An extended version of MCN allows the user to specify larger address spaces:

  • Coil: [0x000001 - 0x065535]
  • Discrete Input: [0x100001 - 0x165535]
  • Input Register: [0x300001 - 0x365535]
  • Holding Register: [0x400001 - 0x465535]