Device ID Enter the Modbus device ID of the destination Modbus device.
Server IP Address Specify the IP address of the destination Modbus TCP server for the specified
transaction.
Sever Port Specify the server port number used for Modbus TCP. The default/standard port
number is 502.
Response Timeout Enter the timeout (in milliseconds) to wait for a response to the specified transaction.
Response time should be configured in conjunction with the response time for the
serial ports if using TCP to RTU communications.
Comm Fail Register Enter the on-board I/O register number to store the communication status of the
specified transaction. The specified register will be set to 0 if communications is
successful, 0xFFFF if there is no connection to the specified server, or 0xFFxx where
“xx” is the Modbus exception code (see Appendix E).
Save Changes Save changes to non-volatile memory (reset is required to activate).
Save and Activate
Changes
Save to non-volatile memory and restart to activate changes.
NOTE When entering the local or destination registers, you do not need to enter in the full Modbus
address (for example, 30001 or 10001). Only the I/O address is needed because the function code
determines what type of command is being used.
For example, if you want to read from Destination register 30001 you need to select function code 04: Read
Inputs and then enter the destination register of 1. Or, if you want to read register 10501 you need to select
function code 02: Read Discretes and then enter the destination register of 501.
TCP Mapping Example
The system in Figure 61 shows that Unit B is a Modbus TCP client that will poll the TCP server at Unit C via the
Wireless Ethernet interface to gather the status of the on board DIO (digital input), which will then be reflected on its
own DIO (digital output).
Figure 61 Modbus Example
Enabling the Modbus TCP server within Unit B provides a register location for the polled values from Unit C to
be stored. It will also allow an external Modbus TCP client (DCS or SCADA) to monitor the stored I/O values from
units A and C. First, the Modbus TCP client must be enabled and a suitable scan rate be selected (Figure 62). The
default times will be 1000 msec, meaning that there will be a 1-second delay between the clients mappings
directed at any Modbus server.
