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
C: Poll Interval
The periodic interval at which this form sends a Modbus request to acquire data for the configured
|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.
|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.
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
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
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.