Part 6: Modbus Protocol; Introduction; Rtu Mode - Omega i-SERIES User Manual

PART 6
MODBUS PROTOCOL
To Enable the Modbus Protocol, set Modbus menu item to "Yes" in the Bus
Format Submenu of the Communication Menu.

6.1 Introduction

Modbus Protocol defines a message structure that i-Series devices will recognize and
use, regardless of the type of networks over which they communicate. It describes the
process a device uses to request access to another device, how it will respond to
requests from the other devices, and how errors will be detected and reported. It
establishes a common format for the layout and contents of message fields.
The Modbus Protocol provides the internal standard that the i-Series devices use for
parsing messages. During communications on a Modbus network, the protocol
determines how each instrument will know its device address, recognize a message
addressed to it, determine the kind of action to be taken, and extract any data or other
information contained in the message. If a reply is required, the i-Series will construct the
reply message and send it using Modbus protocol.
Modbus defines a digital communication network to have only one MASTER and one or
more SLAVE devices. Either a single (point-to-point) or multi-drop network (multipoint) is
possible.
i-Series devices communicate on standard Modbus networks using RTU (Remote
Terminal Unit) transmission mode.

6.2 RTU Mode

In 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.
The following format used for each byte sent and received by i-Series instrument
in RTU Mode:
1. Eight-bit binary, Hexadecimal (0 ... 9, A ... F)
2. Two hexadecimal characters contained in each eight-bit field of the message
3. 1 start bit, 8 data bits, 1 Stop Bit (No Parity Bit)
The figure below shows the bit sequences when byte transmitted in RTU Mode.
LSB – Least Significant bit sent first
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