Omega DP41-B Series User Manual page 80

ModBus Communication Option Supplement
Note: To Enable the MODBUS PROTOCOL via Front LED Display Panel Push Button
Menu, enter MOdbUS submenu and select option "yES" in the "COMM" Communication
Configuration Menu.
1. Introduction
Modbus protocol defines a message structure that this Universal Input Meter will
recognize and use, regardless of the type of networks over which they communicate. It
describes the process a meter 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 this Universal Input Meter uses
for parsing messages. During communications on a Modbus network, the protocol
determines how each meter 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 meter 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.
This Universal Input Meter communicates on standard Modbus networks using RTU
(Remote terminal unit) transmission mode.
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 this Meter in RTU
mode:
2.1. Eight-bit binary, Hexadecimal (0 ... 9, A ... F)
2.2. Two hexadecimal characters contained in each eight-bit field of the message
2.3. 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
START 1
•
LSB – Least Significant Bit sent first
2 3 4
M - 1
5 6 7
MSB
8 STOP