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How To: Set Up Communications for Modbus RTU


  1. Open the Communication Setup form by choosing Communication->Setup menu item. This form can be opened by clicking the Communication Setup Toolbar button too.
  2. Click the Modbus RTU radio button under Modbus Mode. This will enable the panel containing fields for serial communication parameters and will disable the panel containing fields for setting IP address and port number.
  3. Select from the Port name list or enter manually the name of the COM port of your computer to which your Modbus device is connected.
  4. Select appropriate values for baud rate, parity, data bits and stop bits to match the settings that have been made in your device.
  5. Set a communication timeout period in the Timeout text box. Exercise caution in setting this parameter as a value too small will cause timeouts even though the device is responding correctly to Modbus requests. Device response times greatly vary from anywhere between 10ms to tens of seconds depending on the speed of the device's microcontroller and the medium of communication. For direct serial (RS232/RS485) and Ethernet connections a typical value of timeout is between 1 second to 5 seconds. However for slower media like GPRS you may have to set this to anywhere between 10 seconds to even a minute.
  6. If you want the Tester to automatically reconnect to the last configured device on startup, check the Reconnect on startup checkbox. This will save you the hassle of manually reconnecting every time the test harness is started.
  7. Note that the Autoreconnect on disconnection feature is not supported in Modbus RTU mode and hence this checkbox will be disabled.
  8. Click OK to save the configuration.
The Setup Communication form does not check for the existence of the entered port name. This check is done only when you try to connect to the device. So ensure that the port name is entered correctly and that the port is not in use.

Next Steps

Topic link Make a connection to your Modbus device

See Also

Topic link Setup Communication Parameters
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