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Version: v3

Modbus

Modbus is a communication protocol utilized in PLC networks. Originally, it was developed for serial interfaces like RS232, RS422, and RS485, and later expanded to include support for TCP mode.

OIBus uses the jsmodbus library in TCP mode only.

Specific settings​

Here are the Modbus connector settings:

  • Host: IP address or hostname of the Modbus server machine.
  • Port: The port to use for connection (502 by default).
  • Retry interval: Time to wait between reconnections after a connection failure.
  • Slave ID: Identifies the Modbus source machine, default is 1.
  • Address Offset: For most PLCs, there is no offset (Modbus option). Some PLCs may start the address range at 1 instead of 0 (JBus option).
  • Endianness: Specifies the type of bit encoding (Big Endian or Little Endian).
  • Swap Bytes: Determines whether the bytes within a group of 16 bits (a word) should be inverted or not.
  • Swap Words: Indicates whether the words (16-bit groups) should be inverted or not within a 32-bit group.

Item settings​

  • Address: The hexadecimal address of the data within the device.
  • Modbus type: Specifies whether it's a coil, discrete input, input register, or holding register (default).
  • Data type: Relevant for holding registers or input registers. It defines the type of data fetched from the register, with options such as Bit, UInt16 (default), Int16, UInt32, Int32, UInt64, Int64, Float, or Double.
  • Bit index (Bit data type only): The index of the bit to retrieve from the read value.
  • Multiplier Coefficient: Multiplies the retrieved value (default is 1).

About the Modbus address​

The address should match the variable's address in the PLC, represented in hexadecimal without the data type digit. For instance:

  • For holding register data at 0x40001, input the address as 0x0001 (excluding the digit 4) and specify the Modbus type as holdingRegister.
  • For coil data at 0x009C, use 0x009C as the address and specify the Modbus type as coil.

Modbus data addresses are structured according to the Modicon Convention Notation (MCN):

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

An extended version of MCN allows for larger address spaces:

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