Lantronix XPort APS: Modbus Protocol User Guide - Page 7

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2: Modbus When it comes to planning data communication for open, multi-vendor industrial control systems, Modbus is the first choice of end users and integrators alike. The Modbus/RTU protocol defines how a "master" device polls one or more "slave" devices to read and write data in real time by means of RS232, RS422, or RS485 serial data communication. Although not the most powerful protocol available, its rare simplicity allows not only rapid implementation but also enough flexibility to apply in virtually all industrial situations. Modbus/TCP, an extension of Modbus/RTU, defines how Modbus/RTU and Modbus/ASCII messages are encoded within and transported over TCP/IP-based networks. Modbus/TCP is just as simple to implement and flexible to apply as the original Modbus/RTU. You can find the specification for both online at www.telemecanique.com. The IAP Device Server allows users to integrate new and existing Modbus/RTU and Modbus/ASCII serial devices with newer TCP/IP network-based devices. The next chapter describes a system that integrates four Modbus/RTU devices with four Modbus/TCP devices. Extended Modbus System Example Figure 2-1. Extended Modbus System Example Figure 2-1. Extended Modbus System Example Figure 2-1. Extended Modbus System Example shows four specific styles of Modbus operations. Modbus/RTU devices are traditionally split into two groups. (CoBox Modbus refers to an IAP Device Server.) Modbus slave devices generally are the workhorse devices. They perform their tasks 24 hours a day, 365 days a year, for example, tasks such as flow metering, temperature control, batch loading, or even running entire automated assembly lines. The slave devices are not called "slaves" because they work all the time; they are called slaves because as far as the data communications is concerned, they function as passive servers. Modbus slave devices passively sit and wait for a remote Modbus master device to ask them to report existing data values (Read) or accept new data values (Write). Modbus Protocol User Guide 7

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Modbus Protocol User Guide
7
2: Modbus
When it comes to planning data communication for open, multi-vendor industrial control systems,
Modbus is the first choice of end users and integrators alike. The Modbus/RTU protocol defines
how a “master” device polls one or more “slave” devices to read and write data in real time by
means of RS232, RS422, or RS485 serial data communication. Although not the most powerful
protocol available, its rare simplicity allows not only rapid implementation but also enough
flexibility to apply in virtually all industrial situations. Modbus/TCP, an extension of Modbus/RTU,
defines how Modbus/RTU and Modbus/ASCII messages are encoded within and transported over
TCP/IP-based networks. Modbus/TCP is just as simple to implement and flexible to apply as the
original Modbus/RTU. You can find the specification for both online at
www.telemecanique.com
.
The IAP Device Server allows users to integrate new and existing Modbus/RTU and
Modbus/ASCII serial devices with newer TCP/IP network-based devices. The next chapter
describes a system that integrates four Modbus/RTU devices with four Modbus/TCP devices.
Extended Modbus System Example
Figure 2-1. Extended Modbus System Example
Figure 2-1. Extended Modbus System Example
Figure 2-1. Extended Modbus System Example
shows four specific styles of Modbus operations.
Modbus/RTU devices are traditionally split into two groups. (CoBox Modbus refers to an IAP
Device Server.)
Modbus slave devices
generally are the workhorse devices. They perform their tasks 24 hours a
day, 365 days a year, for example, tasks such as flow metering, temperature control, batch
loading, or even running entire automated assembly lines. The slave devices are not called
“slaves” because they work all the time; they are called slaves because as far as the data
communications is concerned, they function as passive servers. Modbus slave devices passively
sit and wait for a remote Modbus master device to ask them to report existing data values (Read)
or accept new data values (Write).