SI-Ethernet User Guide 135
Issue: 3
Safety
information
Introduction
Mechanical
installation
Electrical
installation
Getting started Parameters
Key features
and Protocols
PC Tools
Applications
Security Diagnostics
Glossary of
terms
Index
7 Key features and Protocols
This section details the key features and protocols supported by the onboard Ethernet interface and
the SI-Ethernet option module.
7.1 PC/PLC considerations
If the subnet of the host PC/PLC is different to the subnet of the Ethernet interface, then both the
Ethernet interface and the PC/PLC, must be configured with the address of a gateway that allows
communication between the two devices.
7.2 Modbus TCP/IP
Modbus TCP/IP is one of the most widely supported industrial Ethernet based protocols offering the
functionality and simplicity of the Modbus protocol, with the flexibility of Ethernet. Table 7-1 shows
the supported Modbus function codes.
Modbus TCP/IP uses the standard Protocol Data Unit (PDU) but without the CRC bytes and
encapsulates it within a Modbus TCP/IP Application Data Unit (ADU) for transmission. This means
that the Modbus PDU is the same for both standard (RTU) and Ethernet based transmission.
Table 7-1 Supported Modbus function codes
7.2.1 Modbus TCP/IP port
The port number used for Modbus TCP/IP may be reconfigured to a different port number using
Pr S.15.005 see Modbus Listening Port (S.15.005) on page 92 for more information. A timer is
available to allow a loss of Modbus communication to be managed (see Modbus Timeout
(S.15.009) for more information).
7.2.2 Data structure
Communication between devices is based upon Modbus Application Data Units (ADUs), the ADU
consists of 2 parts, the Modbus Application Protocol (MBAP) header and the Modbus Protocol
Data Unit (PDU).
Figure 7-1 Modbus Data Structure
Code Description
3 Read multiple 16 bit registers.
6 Write single 16 bit register.
16 Write multiple 16 bit registers.
23 Read and write multiple 16 bit registers.
Modbus ADU
Function CodeMBAP Header Data
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