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Use Modbus RTU to Communicate via a Terminal Server

In this example, ClearSCADA is configured to use Modbus RTU (the Modbus serial protocol) to communicate with a Modbus Slave device that is connected serially to a terminal server.

The Modbus Slave device is connected to ClearSCADA via a remote serial port, rather than directly to the local ClearSCADA computer. The terminal server is connected to ClearSCADA via an Ethernet link.

Example:

A Modbus Scanner is to scan addresses 30001 to 30010 inclusive on a Slave device’s Input registers.

Within ClearSCADA, a Modbus Channel is configured as follows:

Primary Port Connection Type: TCP/IP

Within ClearSCADA, a Modbus Scanner is configured as follows:

On the Scanner tab of the Modbus Scanner Form:

Channel: Configured to reference the Modbus Channel indicated above

Device Address: Configured with the Modbus Slave’s address number

Data Address: 1 (the start of the scan block is 30000 + 1 = 30001)

Data Length: 9 (the end of the scan block is 30000 + 1 + 9 = 30010)

Location: Input Registers (the scanner scans registers in the 30000 address range)

On the Primary Port tab of the Modbus Scanner Form:

Connection Type: Network

Host Address: Configured with the IP address of the terminal server

Port: Configured with the identifier of the TCP port at the terminal server that is used for the Modbus serial communications port. (The Port number is not the serial communications port number.)

Communications Type: Modbus RTU (as the Simple Modbus driver uses the Modbus serial protocol, but communicates initially via an Ethernet communications link). (If the driver was using direct Ethernet communications to talk to a Modbus PLC, the Communications Type would be configured as Modbus TCP.)

Modbus points are configured as required (see earlier in this section for Configuration Examples).


ClearSCADA 2015 R2