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What is Modbus protocol?

This page describes about Modbus protocol basics and mention Hart to Modbus converter.

The Modbus protocol typically follows a master and slave based architecture. Master transmits a request message to a slave and waits for a response messsage. This architecture provides complete control to the master for flow of information. This has great merits over the previous multidrop networks using RS422/RS485 etc.

This Modbus protocol is used in SCADA style of network communication between the devices. As mentioned this protocol is designed as request response protocol.


Modbus protocol

As shown in the figure-1 Modbus request message is a layered set of data. Controllers can be setup to communicate on standard Modbus networks using either of two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode, etc), during configuration of each controller. The mode and serial parameters must be the same for all devices on a Modbus network.

The selection of ASCII or RTU(remote terminal unit) mode pertains only to standard Modbus networks. It defines the bit contents of message fields transmitted serially on those networks. It determines how information will be packed into the message fields and decoded.

When controllers are setup to communicate on a Modbus network using ASCII mode, each eight-bit byte in a message is sent as two ASCII characters. The main advantage of this mode is that it allows time intervals of up to one second to occur between characters without causing an error.

When controllers are setup to communicate on a Modbus network using RTU mode, each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII for the same baud rate. Each message must be transmitted in a continuous stream.

In either of the two serial transmission modes (ASCII or RTU), a Modbus message is placed by the transmitting device into a frame that has a known beginning and ending point. This allows receiving devices to begin at the start of the message, read the address portion and determine which device is addressed , and to know when the message is completed. Partial messages can be detected and errors can be set as a result.
Refer Modbus message frame format➤➤.

Hart to Modbus converter

This converter converts Hart protocol to Modbus protocol. Following companies are the Hart to Modbus converter manufacturers.
• Emerson Process Management
• SCAN ELECTRONIC SYSTEMS
• ICP DAS USA
• Moore Industries-International, Inc.

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