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Introduction
The Modbus protocol was originally developed by Modicon (nowadays Schneider Electric) for the data transfer with their controllers. Data transfer was organized in terms of 16-Bit registers (integer format) or as status information in terms of data bytes. Over the years the protocol was extended and has been adopted by other manufacturers as well. New data types were added,especially to achieve a higher resolution for values to transmit. The protocol was adopted for new transfer media, dialects like Modbus Plus or Modbus/TCP arised. But for compatibility reasons the basic structure of the data area or the addressing mechanism of the protocol retained. The Modbus protocol is in fact a single master protocol. The master controls the complete transmission and monitors ifpossible timeouts (no answer from the addressed device) occur. The connected devices are slaves and are allowed to send telegrams only on master request. The following basics are limited to the protocols Modbus/RTU and Modbus/TCP. Also only functions supported by Modbus devices of the company Camille Bauer are described.

Contents
1. Modbus/RTUprotocol....................................................................................................................................................................2 1.1 Transmission mode.................................................................................................................................................................2 1.2 General message form...........................................................................................................................................................2 1.3 Data types ...............................................................................................................................................................................3 1.4 Dataaddressing......................................................................................................................................................................3 1.5 Cyclic redundancy check calculation (crc16) (Example in ‘C)’ ............................................................................................3 1.6 Error handling.........................................................................................................................................................................4 1.7 Telegram examples ................................................................................................................................................................4 2. Modbus/TCP protocol ....................................................................................................................................................................7 2.1 Generictelegram types...........................................................................................................................................................7 2.2 Communication management.................................................................................................................................................7 2.3 Error handling.........................................................................................................................................................................8 2.4 Telegram examples ................................................................................................................................................................8

MODBUS® - Modbus is a registered trade mark of Schneider Electric. Detailedprotocol specifications are available via the Website http://www.modbus.org

Änderung

Datum Vis.:

Typ: Bezeichnung:

Basics MODBUS

Nr.: 1

/9

gez.:

03.08.06 RR

Zeichnr.:

W2417e

1. Modbus/RTU protocol

1.1 Transmission mode
Character format: Normally configurable 1 start, 8 data, even parity, 1 stop bit 1 start, 8 data, odd parity, 1 stop bit 1 start, 8 data, no...