805U-E Wireless Ethernet
User Manual
Man_805U-E Rev 1.5
Page 50
“Reset Registers on Comms Fail” option is enabled with a timeout of 60 seconds, indicating
that any of the registers at unit B will be reset if a successful Modbus transaction involving
that register has not been executed in the last 60 seconds. The Modbus TCP to RTU Gateway
at B must also be enabled (see section “3.13.3 Modbus TCP to RTU Gateway”) to allow
Modbus communications with the serial device A.
Three “Modbus TCP Client Mappings” are also configured at B in order to perform the
required I/O transfer. The first mapping transfers the status of the onboard digital input at C
to the onboard digital output at B. Local Register 4320 specifies the register for the onboard
digital output at B (since B is the local unit at which the mapping is configured). I/O Count 1
specifies that only one I/O point is being transferred (i.e. the single digital I/O). Function
Code 02: Read Discretes specifies the standard Modbus function code to read discrete (i.e.
digital) inputs. Destination Register 4300 specifies the register for the onboard digital input
at unit C (since C is the destination unit for this mapping). Device ID 1is the ID of the
onboard Modbus TCP Server at C. Server IP Address 192.168.0.200 is the IP address of unit
C – which is the Modbus TCP Server we are reading from. Response Timeout 1000 ms
specifies that unit C must respond to this message within 1000ms. Comm Fail Register 0
specifies the local register where the communications status for this mapping will be stored.
The second mapping reads 8 registers from serial unit D into onboard registers in unit B.
Note that in this case the specified Device ID 6 is the Modbus Address of the serial device D,
while the Server IP Address 192.168.0.200 is the IP Address of unit C since the Modbus TCP
to RTU Gateway at unit C converts the Modbus TCP message to Modbus RTU and routes it
out the serial port to unit D.
The third mapping takes the 8 registers read by the second mapping and writes them to the
serial unit A. The specified Device ID 5 is the Modbus Address of the serial device A, and
the Server IP Address 192.168.0.196 is the IP Address of the local unit B since the local
Modbus TCP to RTU Gateway is to route the message out the serial port to unit A.
Since the 805U-E supports Modbus TCP Client and Server simultaneously, the Modbus TCP
Server for unit B above could also be enabled. This would allow one (or more) external
Modbus TCP Clients anywhere on the extended wired or wireless network to connect to unit
B and monitor the status of the I/O registers – including the I/O at units A, C, and D. This is a
very powerful and flexible feature which could, for example, be exploited by a central
monitoring facility or SCADA.
