Westermo MDI-112-F4G - Manual (Page 160)

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supports Modbus TCP/IP that it can be polled through Ethernet. Thus the Modbus
TCP master can read or write the Modbus registers provided by the Industrial
Ethernet Switch.
The Managed DIN-Rail Ethernet Switch has implemented Modbus TCP/IP register in
the firmware. Those register mapping to some of Ethernet Switch’s operating
information, includes description, IP address, power status, interface status, interface
information and inbound/outbound packet statistics. With the register supports, user
can read the information through their own Modbus TCP/IP based progress/ display/
monitor applications and monitor the status of the switch easily.
The configuration of Modbus TCP/IP only present in CLI management mode and the
no extra user interface for Web configuration.
5.3.1 Modbus Function Code
The Modbus TCP/IP device uses a subset of the standard Modbus TCP/IP function
code to access device-dependent information. Modbus TCP/IP function code is
defined as below.
FC Name Usage
01 Read Coils Read the state of a digital output
02 Read Input Status Read the state of a digital input
03 Read Holding Register Read holding register in 16-bits register format
04 Read Input Registers Read data in 16-bits register format
05 Write Coil Write data to force a digital output ON/OFF
06 Write Single Register Write data in 16-bits register format
15 Force Multiple Coils Write data to force multiple consecutive coils
The Managed Switch supports the function code 04, which name is Read Input
Registers. With this support, the remove SCADA or other Modbus TCP/IP application
can poll the information of the device and monitor the major status of the switch.
5.3.2 Error Checking
The utilization of the error checking will help eliminate errors caused by noise in the
communication link. In Modbus TCP/IP mode, messages include an error-checking
field that is based on a Cyclical Redundancy Check (CRC) method. The CRC filed
checks the contents of the entire message. It applied regardless of any parity check
method used for the individual BYTE acters of the message. The CRC value is
calculated by the transmitting device, which appends the CRC to the message. The
receiving device recalculates a CRC during receipt of the message, and compares the