Section 11 Modbus Protocol; Introduction; Modbus; Modbus/Tcp - Hach Anatel PAT700 User Manual

Section 11

11.1 Introduction

11.1.1 Modbus

11.1.2 Modbus/TCP

Modbus Protocol
This section provides a detailed explanation of the Modbus/TCP registers used by the
Anatel PAT700 TOC Analyzer.
Please visit the Modbus Organization website at
regarding the latest Modbus and Modbus/TCP specifications. The Modbus Protocol is a
messaging structure and it is used for master-slave/client-server communication between
intelligent devices.
Modbus devices communicate using a master-slave (client-server) technique in which
only one device (the master/client) can initiate transactions (called queries). The other
devices (slaves/servers) respond by supplying the requested data to the master, or by
taking the action requested in the query. A slave is any peripheral device (I/O transducer,
valve, network drive, or other measuring device) which processes information and sends
its output to the master using Modbus.
The Modbus messaging structure is the application protocol that defines the transmission
medium. TCP/IP refers to the Transmission Control Protocol and Internet Protocol, which
provides the transmission medium for Modbus TCP/IP messaging.
Modbus/TCP uses TCP/IP and Ethernet to carry the data of the Modbus message
structure between compatible devices. That is, Modbus/TCP combines a physical
network (Ethernet), with a networking standard (TCP/IP), and a standard method of
representing data (Modbus as the application protocol). Essentially, the Modbus/TCP
message is simply a Modbus communication encapsulated in an Ethernet TCP/IP
wrapper.
The Modbus/TCP protocol supports multiple types of data transactions, from reading
single bits per transaction, to advanced object-oriented operations. However, to ensure
the most compatible system available, the simplest function set is to be made available.
The Modbus/TCP has each transaction type classified in to conformation classes, to
ensure consistency and interoperability. Class 0 is the simplest, and allows for reading
and writing of multiple 16-bit registers. The Modbus/TCP feature of the Anatel PAT700
TOC Analyzers will support reading and writing of these 16-bit registers, which allows the
Anatel PAT700 TOC Analyzers to establish a block of data which contains all the process
variables, set points, alarms and input/output statuses that are to be made public to a
Modbus/TCP client. This block of data is packaged so that it can be read in 16-bit chinks
(or registers) at a time, regardless of the type of data within it. In the following sections,
the formatting, storing and reading of this data are described.
http://www.modbus.org for information
Page 145