Connect devices via Modbus

About Modbus

The Modbus-protocol is a widely used communication protocol that was developed in 1979 by Gould-Modicon It was developed. It is based on a client-server architecture, in which a master-device (usually a computer) is connected to one or more slave-devices. This protocol makes it possible to monitor and control various measuring devices from a central system. Data transmission can take place using one of three variants: ASCII, RTU or TCP.

In Eliona are mainly Modbus TCP as well as RTU over TCP used, but the platform also supports other schemes such as UDP and RTU over UDP, to enable flexible integration of older or special devices.

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Modbus TCP

With Modbus TCP data transmission takes place over Ethernet in binary form using TCP/IP packets. The standard port 502 is reserved for this purpose. For older devices with serial interfaces, a protocol converter or router can be used to integrate them into the TCP system.

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Roles in the Modbus protocol

Modbus Client (Master)

• Communicates with a remote device.

• Sends Modbus requests that are controlled by the application.

• Transfers these requests to the Modbus client interface.

• Usually, the client is the device that retrieves the data (e.g. a PC or Modbus Poll).

Modbus Server (Slave)

• Waits for Modbus requests (via TCP port 502).

• Reads and processes these requests.

• Returns Modbus response messages.

• Usually, the server is the device that provides the data (e.g. a PLC or a sensor).

Each slave has a unique address in the range of 1 to 247.

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Procedure for connecting devices via Modbus

To connect devices via Modbus with Eliona follow these steps:

1. Connect to a Modbus server (slave)

   To establish a connection to a Modbus server (slave), you need the IP address and the Port of the server. The standard port for Modbus TCP is 502.

2. Read or write registers

   After the connection has been successfully established, the Modbus client (master) can send requests to the Modbus server server. These requests are used for read or write reading registers on the server.

   • read: The client reads certain values from the server's register addresses.

   • write: The client sends data and writes it to the server's registers. ⚠ Note: All requests are initiated by the Client client.

3. Specify a unique device address (ID)

   Each device in the Modbus network has a unique address (ID) in the range of 1 to 247. This ID must be specified when configuring the client to ensure that requests are sent to the correct device.

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Internal Modbus versions in Eliona

In Eliona several Modbus protocols are supported to integrate different device types and networks:

• Modbus TCP: Enables communication over Ethernet with the standard port 502.

• RTU over TCP: Enables the integration of devices with the older RTU protocol via a TCP connection.

• UDP: Provides a connectionless transmission option for Modbus messages.

• RTU over UDP: Uses the RTU protocol over UDP to integrate devices with low connection overhead into the network.