Understanding Modbus Gateways; Protocol Overview - Advantech ADAM-4572 User Manual

2.1

Understanding Modbus Gateways

Networks have become increasingly vital for industrial automation applications. Many
control devices today do not have a network port and can only communicate with a
dedicated local PC or control panel. Advantech's revolutionary network-enabling
technology is now allowing control devices with serial ports to connect to the Ethernet
and share networks quickly and cost-effectively. The ADAM-4572 and EKI-122X
series are network-based, Modbus gateways for integrating new and existing
Modbus/RTU and Modbus/ASCII serial devices to newer TCP/IP networked-based
devices. Manufacturers, system integrators, and end users can now take advantage
of Modbus gateways to create networked applications for remote managing and
accessing data for their control devices that wasn't possible before.
2.1.1

Protocol Overview

Originally developed for PLCs in industrial automation and manufacturing control
application, Modbus has become one of the most popular open standard protocols in
use today. When it comes to planning data communication for open, multi-vendor
industrial control systems, Modbus is the first choice of end-users and integrators.
Although it's not the most powerful protocol available, its rare simplicity allows not
only rapid implementation, but also remains flexible enough to be applied in virtually
all industrial situations. The communication mode of Modbus can be ASCII, RTU, or
TCP/IP. Modbus gateways are used to support applications such as protocol conver-
sion between serial (Modbus/ASCII or Modbus/RTU) and networked (Modbus/TCP)
Modbus devices or it can be used to bridge Modbus serial devices over TCP/IP net-
work.
The Modbus/RTU and Modbus/ASCII protocols define how a "master" device polls
one or more "slave" devices to read and write real-time data over RS-232, RS-422, or
RS-485 serial data communication. The simplicity of Modbus/RTU not only allows
rapid implementation, but can also remain flexible enough to be applied in virtually all
industrial situations.
During Modbus network communication, the protocol determines how each controller
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 controller will construct the reply message and
send it back using Modbus protocol.
The way controllers communicate with each other is by using a master-slave tech-
nique, in which only one device (the master) can initiate queries. The other devices
(the slaves) respond by supplying the requested data to the master, or by taking the
action requested in the query. Typical master devices include host processors and
programming panels. Typical slaves include programmable controllers.
It is the master that can address individual slaves and initiate a broadcast message
to all slaves. On the other hand, slaves return a response to queries that are
addressed to them individually. Responses are not returned to broadcast queries
from the master.
The Modbus protocol has a definite format for the master's query, which incorporates
the device (or broadcast) address, a function code defining the requested action, any
ADAM-4572 & EKI-122X Series User Manual 6