Hach ANATEL PAT700 User Manual - Section 11
Page 145
Section 11
Modbus Protocol
11.1 Introduction
11.1.1 Modbus
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
http://www.modbus.org
for information
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.
11.1.2 Modbus/TCP
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.