Modbus RTU via Terminal Server

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This article describes the configuration of a communication system where ClearSCADA is required to talk Modbus RTU (Modbus serial protocol) to a Modbus device that is connected serially to a terminal server i.e. a remote serial port, rather than one the local ClearSCADA computer. The terminal server is connected to ClearSCADA via an Ethernet link.

To configure the ClearSCADA objects, follow the steps below:

• Create a Modbus channel. Configure it as "In Service" and the "Primary Port" connection type is to be set to "TCP/IP".

• Create a Modbus Scanner. Configure the "Scanner" tab as follows:

• "In Service" ticked

• "Channel" should refer to the above Modbus Channel (in Step 1)

• Set the "Device Address" to the Modbus Slave address no

• On the "Scanner" tab you also need to configure the Data Address, Data Length and Location settings have to be configured to suit your requirements.  For example, to Scan Modbus addresses 30001 to 30010 (input registers) from the slave device you would set the following:

• Location: "Input Registers" meaning Address in the 30000 range)

• DataAddress: 1 (meaning start of the scan block is to be 30000+1)

• Length: 9 (meaning that the end of the scan block will be 30000+1+9=30010).

• On the "Primary port" make the configuration as follows:

• Connection type to be "Network"

• Host address to be the IP address of the terminal server.

• Port to be the identifier of the TCP port (at the terminal server) that is used for the Modbus serial communications port. (This number does not refer to the serial com port no)

• The communication type is to be "Modbus RTU." This is where you tell the driver that you will be talking serial Modbus protocol but it is initially going out on a Ethernet comms link. If you wanted to use direct ethernet communications to the PLC, you would choose Modbus TCP protocol from this pulldown selection.

• Create the Modbus points as required.

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