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How To: Set Up MDG Form For Data Acquisition

1. Start with a Modbus Data Group Form. You can create a new form or open an existing one.
  • To Create a new Modbus Data Group Form choosing File->New menu item, click the New Data Group toobar button or type the CTRL+N shortcut key combination.
  • To open an existing Modbus Data Group Form choosing File->Open menu item, click the Open Data Group toobar button or type the CTRL+O shortcut key combination and browse to/select the existing form (file name extension *.mdg).

  • 2.Configure the Modbus Data Group Form to reflect the type of data to be acquired and displayed.
    1. Set Device ID to the unit/slave ID of the device from which this form should acquire data. However note that most Modbus TCP devices ignore this field and will respond to requests
    with any device ID except zero.

    2.Choose an appropriate Modbus Data Type reflecting the nature of data to be acquired from the device. This information is generally published in the users manual as the address map
    of your device. The Tester decides on the Modbus Function Code to be used for the requests based on this parameter. Selecting an incorrect data type will result in an Exception
    Response from the device.

    3.Choose the Start Address and Length fields to define the range of Modbus addresses for which data is to be acquired. Select this range to reflect a logical group of addresses in your
    device. While the form permits configuration of a large number of addresses, it is suggested that all the addresses of interest be grouped and each group be displayed in different MDG
    forms. This makes it easier to view and analyse the acquired data. If any of the addresses in the configured range does not exist in the device, an exception response is received from
    the device causing data for existing Modbus addresses also to be unavailable.
    Important:
    The Start Address entered should be one more than the address as specified in the address map of the device. This is because the Tester decrements the start address by one (1) before using it in encoding Modbus packets as required by the Modbus specifications.
    4.Enter a duration (in milliseconds) for Poll Interval to set a periodic polling cycle. Setting too low a value can cause the device to be overloaded with Modbus requests and setting too
    high a value can cause data to be sluggishly updated on the display.
    3. Start data acquisition by checking the Start Polling checkbox or by clicking the right mouse button on the Data Display Panel and choosing the Poll Device
    menu item in the context menu.
    Tips:
    If you have multiple Data Group Forms open and would like to start or stop polling in all of them, you can use the Start Poll All Data Groups and Stop Poll All Data Groups toolbar buttons to do this.
    Verify that data acquisition is working fine.
  • If the address range and data type configured in the Data Group Form exist on the device, you should see data being displayed on the form. If the form is unable to acquire data, it displays ???? in place of data.
  • The upper section of the Data Group Form shows a set of Poll Counters. If data acquisition is working correctly the Polls counter must be equal to Resp (response) counter and the other remaining counters should not increment.
  • Notes:
  • You can create/open multiple Data Group Forms to view different sets of data. Each can acquire data at its own polling frequency.
  • Stability of a device's communication interface under heavy load conditions can be tested by setting a very low value (<10ms) for Poll Interval so as to bombard the device with Modbus requests.
  • Incorrect setting of Modbus Data Type, Start Add and/or Length can cause the device to respond with an Exception Response.
  • Next Steps

    Topic link View data in different display formats (hex numbers, integers, floats and so on)
    Topic link Change value of individual Modbus tags via the Data Group Form
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