User manual

PAT700 Total Organic Carbon Analyzer – Modbus Protocol Page 143
Anatel Operator Manual
Chapter 11 Modbus Protocol
11.1 Introduction
Modbus
This chapter 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.
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.