To Test The Communication; Modbus Protocol Overview; Overview; Transactions On Modbus Networks - Hitachi Abb Rvt Installation Und Betriebsanleitung

4.7. To test the communication

Depending on the way the controller is connected to the Modbus network, different ways of testing may be chosen.
The user can find much application on the internet to scan Modbus devices and test communication.
In case of problem, please refer to the troubleshooting paragraph.

5. Modbus protocol overview

5.1. Overview

MODBUS RTU is a non-proprietary serial communications protocol that is widely used in the process control industry.
The protocol was developed by Modicon for PLC communications and later released for public use.
This protocol is available in all major Human Machine Interface (HMI) software packages and terminals. Many of the
major controller and PLC manufacturers also offer MODBUS protocol as a standard or optional protocol in their instru-
mentation.
The hardware over which MODBUS RTU communications are performed is not defined by the protocol. MODBUS
RTU is supported on RS-232, RS-422, RS-485, Ethernet and other electrical standards. It should be noted that
MODBUS RTU, MODBUS ASCII and MODBUS Plus are unique communication formats, and are not compatible with
each other. This document will discuss MODBUS RTU only.

5.1.1. Transactions on Modbus Networks

Modbus protocol uses a master–slave technique, in which only one device (the master) can initiate transactions
(called '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.
The master can address individual slaves, or can initiate a broadcast message to all slaves.
Slaves return a message (called a 'response') to queries that are addressed to them individually. Responses are not
returned to broadcast queries from the master.
The Modbus protocol establishes the format for the master's query by placing into it the device (or broadcast) ad-
dress, a function code defining the requested action, any data to be sent, and an error–checking field. The slave's
response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data
to be returned, and an error–checking field. If an error occurred in receipt of the message, or if the slave is unable to
perform the requested action, the slave will construct an error message and send it as its response.

5.1.2. Serial Transmission Mode

The transmission mode defines the bit contents of message fields transmitted serially on the networks. It determines
how information will be packed into the message fields and decoded.
Modbus defines two transmission modes: ASCII or RTU.
Only RTU mode will be used here. The mode and serial parameters must be the same for all devices on a Modbus
network.

5.2. Data Addresses in Modbus Messages

Modbus defines 4 address spaces: 2 address spaces for bit addressable data and 2 address spaces for 16 bits ad-
dressable data.
STATUS
Approved
SECURITY LEVEL
Public
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