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Overview: Modbus Data Group Form

The Modbus Data Group form is the Window thorugh which you view the data acquired from the Modbus device as well as change values of (i.e. Write to) tags in the device. Using this form you can:

  • Periodically acquire data from your Modbus device at a configurable poll interval
  • Group data tags (but of the same data type) and view their latest acquired values
  • Format the display to show the data in various formats (float, hex etc.)
  • Change values of (i.e. write to) data tags in your device
  • View raw Modbus packets being transmitted to and received from the device
  • Track communication status by way of a set of Event Counters

  • The Data Group Forms can be saved (along with all the settings made in them including the size and position of the Window) and reopened at a later time.

    A: Title Bar

    The title bar displayes the name of the Data Group Form. If the form is newly created (i.e. not yet saved) a default file name of the form GroupN where 'N' is a running serial number (e.g. Group1, Group4 and so on). If a form has not been saved or if some changes have occured in the form after it was last saved, a star (*) is suffixed to this name.

    B: Modbus Communication Parameters

    This set of parameters defines the Modbus data that will be acquired and displayed by this form.


  • Device ID: The Modbus Slave (Unit) ID of the device that this form will communicate with. This field accepts any value between 1 to 247. Note that most Modbus TCP devices will ignore the Slave
    ID field in a Modbus TCP packet.
  • Start Add: One Data Group Form can acquire and display data of any one type from a device. This field specifies the address of the first Modbus data item whose value will be acquired &
    displayed by this form.
  • Length: is the number of Modbus data items starting at address Start Add whose values will be acquired & displayed by this form.
  • Modbus Data Type: Users can choose the data type from four available choices - Coils, Discrete Inputs, Holding Registers Input Registers. Notice that one form can display data of one type
    only.
  • C: Poll Interval

    The periodic interval at which this form sends a Modbus request to acquire data for the configured Modbus addresses.

    Caution:
    Data acquisition is not guaranteed to happen precisely at this interval as the device being communicated to may have a much larger response time than the value of this parameter. A few other factors also affect the rate at which data can actually be acquired from the device - the network load, the number of Data Group Forms competing to communicate with the device and so on.
    Note:
    The controls for Modbus Communication Parameters and Polling Interval get disabled once polling is started. To change their values disable/stop polling first.
    D: Start Polling

    Checking this checkbox will cause the Data Group Form to start periodic data acquisition from the device. Unchecking it will stop the data acquisition.

    Note:
  • The Start Polling checkbox is enabled only if a connection has been made to the device.
  • If an existing connection breaks (dropped by the device or breaks due to network disturbances) the Data Group forms stop the data acquisition even though Start Polling is checked. Data acquisition resumes automatically when a new connection is made to the device.
  • E: Display Traffic

    Check this checkbox if you intend to see the raw Modbus packets being transmitted by the form to the device and the response packets received from the the device.

    F: Traffic Display Panel

    This panel displays the raw Modbus packets being transmitted by the form (prefixed with TX:)to the device and the response packets received from the the device (prefixed with RX:). Each byte of the packet is displayed as a hexadecimal number within two square brackets [ ].

    G: Data Display Panel

    This panel displays the current value of all the configured Modbus addresses in the currently selected Data Format. More on the Data Display Panel.

    H: Event Counters Panel

    This panel displays a set of event counters to indicate the health and status of communication of this Data Group with the Modbus device. A brief description of these counters follows:>

  • Polls: The total number of Modbus requests sent out by this data group. When polling is enabled this counter will increment roughly at the rate of the set polling frequency.
  • Resp: Short for "Responses". The total number of valid Modbus responses received by this data group (excluding Exception Responses). If the communication is error free, this counter should
    follow the Polls counter.
  • Timeout: The total number of Modbus requests for which the Data Group received no responses from the device. Ideally this counter should be zero. Possible reasons for the occurance of
    timeouts are:
  • Use of wrong device ID (one which does not match any devices on the network.
  • Too small a timeout setting in Communication Setup form. This will cause the Tester to see no response even if the device did respond (but after the timeout period).
  • Excp Resp: Short for "Exception Responses". The total number of Modbus requests for which the device responded with an Exception response. Exception responses indicate that the device
    does not support the requested function code or that atleast one address in the requested address range does not exist on the device. While a non-zero value of this counter does not indicate a
    problem in the communication per se, it does indicate an error in the configuration.
  • CRC Err: Short for "CRC Errors". This counter is unused in Modbus TCP communication and so is reserved for future use.
  • Other Err: Short for "Other Errors". The total number of Modbus requests for which an unknown error occured when trying to read a response.
  • I: Reset Counters Button

    This button is used to reset counters of this form to zero. You will use this when you start a test case afresh probably after fixing some error in the configuration or the setup.

    J: Splitter Bars

    The splitter bars are used to reorganise the panels on this form to get an optimal view of all the information shown on the form. How to use the splitter bars to customise the Data Group Form.

    Related Information

    Topic link How To: Setup Modbus Data Group Forms for Data Acquisition
    Topic link How To: Change values of Modbus variables from Data Group Form
    Topic link How To: Customise the look and feel (UI) of Data Group Forms
    Topic link Learn more about Data Display Panels
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