ProSoft Technology 3170-MBS - Manual (Page 11)

3170-MBS ♦ Flex Platform Functional Overview
Modbus Slave Interface Module User Manual
ProSoft Technology, Inc. Page 11 of 55
October 27, 2010
2 Functional Overview
2.1 Modbus Addressing Concepts
Modicon developed the Modbus addressing scheme around the data table and
I/O structure in Modicon PLCs. As a result, the Modbus protocol supports access
to the various data spaces in the Modicon PLC.
By far the most common data space used is the 4xxxx space using the Function
Codes 3, 6 and 16. This space is used to transfer 16 bit register values and can
be used to transfer bit mapped data. Using formal Modbus addressing
terminology, this data space actually starts at address 40001.
Access to the different data spaces is determined by the Function Code that is
used. The following chart shows the four different types of data spaces, the
numerical range of these spaces, and the Function Codes that are used to
execute read and write instructions within these data spaces. The following
illustration shows the relationship between the Modbus Function Codes and the
Modbus addressing scheme.