Modbus is a de facto standard for device communication in automation technology, introduced in 1979. It is an open communication protocol based on a client/server architecture. In the OSI reference model, the Modbus protocol is located on the application layer and is defined independently of the physical transmission layer. In practice, the protocol is transmitted via different transmission media/fieldbuses, such as Ethernet, RS232 and RS485.

For data transmission, a distinction is made between three transmission types:

Modbus RTU

Here the data is transmitted in a compact binary coded data format. This format is well suited for machine-to-machine communication, but cannot be read easily by humans.

Modbus ASCII

In Modbus ASCII format, data is encoded in ASCII code, making it directly readable by humans. In contrast to binary coding, more data must be transmitted, which reduces data throughput.

Modbus TCP/IP

The Modbus TCP/IP variant has been standardized in the IEC 61158 fieldbus standard since 2007. The data is coded in TCP/IP packets similar to Modbus RTU data. By default, TCP port 502 is reserved for this protocol. Since 2018, there is also a secured variant called Modbus TCP Security, which is based on the TLS protocol (Transport Layer Security). By default, TCP port 802 is reserved for this.

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