Modbus connector

Modbus general principles

Modbus is a communication used for PLC networks. Historically, it was designed for communications on a serial interface (RS232, RS422, RS485) and supports of TCP mode has bee added later on. OIBus integrates the most recent version of the protocol in TCP mode.

In this TCP mode, Modbus sets up a client-server connection: the server makes data available to addresses but remains passive. The Modbus client will look for the data values. OIBus Modbus South Connectot is a Modbus client. You must therefore indicate to the connector the host ( host , ie the IP address of the Modbus server machine) and the port ( 502 by default).

  • The address of a point corresponds to the reference of a data item on the Modbus server machine.
  • The point id corresponds to the data reference for the target application (destination endpoint).
  • The scan mode indicates the time interval between two requests to the server .

The Mode of Addressing

Modbus data addresses follow the Modicon convention 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]

When adding a point in OIBus, it is necessary to specify the complete MCN (in hexadecimal or in decimal). Here are some examples:

  • For the data Holding Register 16001 (0x3E81), you must enter 403E81 (address 0x403E81 in extended MCN) or 43E81 (address 0x43E81 in standard MCN).
  • For the data Coil 156 (0x009C), you must enter 00009C (address 0x00009C in extended MCN) or 0009C (address in 0x0009C standard MCN).

To go further

You can consult the website of the Modbus Organization and more particularly the document Modicon Modbus Protocol Reference Guide .