Modbus Tutorial

In contrast to other field busses, Modbus poses a simple alternative to transfer data. The Modbus protocol is open. It is based on Master / Slave communication. As a principle, every participant may transmit data in the network. In practice however, communication is always initiated by the master.

Types of transfer (selection)

  • Modbus RTU
    • Serial communication
    • Binary data transfer
    • RTU stands for Remote Terminal Unit
  • Modbus TCP
    • Extended Modbus RTU for TCP/IP networks

Modbus Master

  • Requests data from Modbus slaves
  • Acts as the client, as it doesn’t make data available

Modbus Slave

  • Makes data available
  • Acts as the server as it makes data available

Modbus Register

  • Caution: depending on the device, the registration addresses with a 1 or 0
  • A maximum of 216 registers are addressable, therefore 65536
  • Enables access to data
  • Access is via a register address and Modbus function and service respectively
  • Are specific to each device

Functions or services

  • Implements access to data
  • Distinguishes
    • Reading / writing / both
    • Data width
      • Data word, therefore 2 bytes
      • Bit
  • The implemented functions are dependent on the device

Request data

Configure the Modbus Master and Modbus Slave data connection

  • Modbus TCP – network configuration (static IP address, net template)
  • Modbus RTU – serial interface (baud rate, data bits, parity, stop bits)

The corresponding function has to be selected on Modbus Master to the one that Slave makes available

  • Modbus RTU Slave ID: between 1 and 247 (0 stands for broadcast)
  • Modbus TCP Unit ID: it has the same functionality as Slave ID. However there’s still the 255. In practice, it depends on the connected device which value has to be set. Then either a ModbusTCP Slave will be clearly identified in the network by means of its IP address. But when this Slave is a gateway of ModbusTCP on ModbusRTU, it can address several ModbusRTU Slaves. The Unit ID 255 is recommended to address the gateway itself.
  • Modbus register
    • The kind of register is specific to the device
    • Counted are either from 0 or from 1
  • Modbus function, e.g. READ_HOLDING_REGISTER

Example of necessary information for Modbus RTU

  • Serial interface parameter, e.g. 9600 Baud, 8 data bits, 1 stop bit, no parity
  • Slave address, e.g. 1
  • Modbus register, e.g. 1000
  • Modbus function e.g. READ_INPUT_REGISTER

Example of necessary information for Modbus TCP

  • Slave IP address, e.g.
  • Unit ID, e.g. 255
  • Slave register, e.g. 1000
  • Modbus function e.g. READ_HOLDING_REGISTER

Modbus in RevPi

The following virtual devices are available in RevPi Connect and can be configured via PiCtory:

  • ModbusRTU Master
    • Application: RevPi Connect requests data from another Modbus device
  • ModbusRTU Slave
    • Application: RevPi Connect makes data available that can be requested by a master
  • ModbusTCP Master
    • Application: RevPi Connect accesses data from another Modbus device
  • ModbusTCP Slave
    • Application: RevPi Connect makes data available that can be accessed by a master

A maximum of 32 registers per virtual device can be used for input and output data. Should you need more, then you can simply configure several virtual Modbus devices. A total of 30 virtual devices can be configured.