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Cooper Bussmann 945U-E Wireless Ethernet Modem & Device Server User Manual
47
Rev Version 2.14
Figure 41 Modbus
An example of the Modbus functionality of the 945U-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 single-bit registers from
Modbus serial device D will be transferred to A.
Figure 42 Modbus Mappings
Unit B is configured with Modbus TCP server enabled and device ID = 1. Unit C is configured as shown above.
• The first will write the register 4300 (local digital input) to server IP address 192.168.0.200 (Unit B), device
ID #1, register 4320 (local digital output).
• The second mapping shows a Modbus read command of 8 discretes, starting at register 1 (destination reg) on
device ID #6 connected to IP address 192.168.0.123 (itself), and store the values at register #1 locally.
• The third mapping shows the Modbus write command (write coils), which is writing the local 8 I/Os starting at
register 1 across to server IP address 192.168.0.200, device ID #5, destination reg #1.
The configuration of unit B is shown below (accessible via the I/O Transfer configuration page). It can be seen that
Modbus TCP client has been enabled with a 500-msec scan rate, meaning that there will be a 500-msec delay
between each of the mappings directed at any server. The “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 “RS-232 / RS485 Modbus TCP/RTU Converter” 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 (the single digital I/O).
Function Code 02: Read Discretes specifies the standard Modbus function code to read discrete (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 1 is 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 1000 ms. Comm Fail Register 0 specifies the local
register where the communications status for this mapping will be stored.