Modbus message structure


Modbus message structure

Modbus is one of the most common communication protocols. It is based on the Master/Slave architecture. This standard is widely used to establish communication between various devices and transfer information.

The Modbus message structure consists of four main components. It looks like this.

• Slave address. This refers to the device that serves as a recipient. The response begins with its address. Each slave can have an address field between 1 and 247. Besides, the slave devices may be assigned 0, as well as addresses in the range of 285…255, but they are reserved. For example, 0 is provided for broadcast messages.
• Function code. The function code field is one-byte. Due to this component, the Slave device understands what operations are required by the Master equipment.
• Data. The field contains the necessary information that the Slave device needs to perform the current function. Also, it may contain information that the Slave should send to the Master. The function code determines the length and format of this field.
• Error-checking field. The CRC provides the checksum. It is calculated taking into account the previous bytes. Two algorithms are used for this. The first one is a circular shift, and the second one is a bitwise exclusive OR operation.

The Modbus protocol has four standard functions. They imply reading several discrete inputs and outputs, temporary registers, analog inputs and outputs. Each of them is given a unique code.

Modbus OPC Server

Modbus OPC Server allows you to improve the functionality and convenience of the automation system. It is designed to collect and exchange data between devices and applications that use this communication protocol. One of the main advantages of this software product is an embedded SQL server. Also, the software provides a text editor.

You can buy a licensed Modbus OPC Server on our website. To do this, add the item to the cart and place an order. For details, please contact the specified phone number.

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