Two Modbus transmission modes


Today Modbus protocol is one of the most popular protocols in the world. The standard is widely accepted due to the ease of software implementation, the absence of special interface controllers and high availability. In Russia, Modbus has only one competitor – Profibus protocol.

The main feature of the Modbus protocol is the ability to work in two serial data transmission modes:

    • ASCII – American standard code for information interchange;
    • RTU – remote terminal unit.

It is provided that the RTU mode is included in the protocol by default, and ASCII is optional. The user has the right to choose any mode, but all modules connected to the network must work in the same transmission mode.

Comparison of ASCII and RTU modes

This mode is used for slow communication lines, in which one byte is encoded by two ASCII symbols. A special symbol indicates the start of a new service package. The pause between transmissions lasts a few seconds (if there are no errors during transmission). In Russia, the ASCII mode is almost never used.

The RTU mode speeds up serial data transmission by about two times. The package data is not changed and is transmitted as a binary code. The flow of information is transmitted without pauses, which allows you to transmit up to 1400 bytes per second.

Do not confuse ASCII with DCON protocol. This protocol works on the basis of ASCII encoding, but it does not comply with the Modbus standard.

OPC server for Modbus

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