805U-E Wireless Ethernet
User Manual
Man_805U-E Rev 1.5
Page 48
port (see section “3.13.3 Modbus TCP to RTU Gateway”). Modbus TCP Client functionality
allows connections to a maximum of 25 different Modbus TCP Servers.
Modbus TCP Server (Client) enables the 805U-E to accept connections from one or more
Modbus TCP Clients (Masters). All Modbus transactions routed to the onboard Modbus TCP
Server are directed either to/from the onboard general purpose I/O registers. The Modbus
TCP Server is shared with the Modbus TCP to RTU Gateway, so that the Modbus “Device
ID” is used to determine if a Modbus transaction is to be routed to the onboard Modbus TCP
Server or to a Modbus RTU device connected to the serial port. Care should therefore be
taken that all serially connected Modbus devices use a different Modbus Device ID (i.e.
Modbus Slave Address) to the onboard Modbus TCP Server. Up to 32 separate connections
to the Modbus TCP Server are supported.
Modbus RTU (serial) Master functionality is achieved by combining the Modbus TCP Client
(Master) and Modbus TCP to RTU Gateway. Simply specify a Modbus TCP Client (Master)
connection to a Modbus TCP Server where the server is the address of any 805U-E with
Modbus TCP to RTU Gateway enabled. Care should be taken to ensure that the Device ID
(i.e. Modbus Address) of the serial device is different to the Device ID of the onboard
Modbus TCP Server of the 805U-E that the serial device is connected to.
The 805U-E provides a configurable option to automatically reset the value of the onboard
I/O registers to zero in the event of a communications failure. If a valid Modbus transaction
directed to/from a given register has not been completed for longer than a configurable
timeout, then the value of that register will be reset to zero.
An example of the Modbus functionality of the 805U-E is illustrated below. In this example
the status of the onboard digital input at C will be reflected at the onboard digital output at B.
Also, 8 I/O registers from Modbus serial device D will be transferred to A.
Modbus
RTU Slave
Device ID 6
A
C
192.168.0.196
Modbus TCP
Client
192.168.0.200
Modbus TCP
Server
Device ID 1
D
B
Modbus
RTU Slave
Device ID 5