3.1 Adding an Analog Input Tag to Represent the Booster Tank Level

You will begin to develop the operator interface by adding the first analog input tag to the system. Choosing the first basic I/O tag to add is somewhat arbitrary: in this example, you will start with the booster tank level tag, simply because in your system plan it is a relatively simple tag, having only 2 tags required to represent its data

 

Referring back to the Tag Table created earlier in the system plan (see: Create a Tag Table), the booster tank level tag will be named "AI20_1".

 

The Tag Browser should still be open from the conclusion of the last lesson, but if not, please re-open it now. If you are unsure how to do this, please glance back to the first steps of Lesson 2.

 

1.  With the Tag Browser open, choose Analog Input from the Types drop-down list.

Because no analog input tags have been created in the system yet, no tags will be displayed in the tag list.

2.  Click the New button of the Tag Browser.

Because "Analog Input" was chosen in the Types drop-down list, a new analog input tag properties folder will open as shown below.

Figure 22

Observe the new analog input tag properties folder. Again, its title bar indicates the type of tag to which this tag properties folder corresponds. Note that the ID tab and its identification properties is always the first to display. Once more, by default, "System" is selected in the Area drop-down list, as this was the last known entry for the Area property.

 

You will now configure the analog input tag using the properties decided upon in the system planning section, and as recorded in your Tag Table (see Create a Tag Table).

 

3.  Enter "AI20_1" in the Name field and press the Enter key.

4.  Enter "Tank" in the Area field and press the Enter key.

5.  Enter "Booster Tank Level" in the Description field and press the Enter key.

This description will help us to clearly identify the tag, since the tag names we are using are somewhat cryptic to anyone who doesn't know the convention we decided upon.. The description will make it easier to identify and locate this tag once we've established many tags in this application. It will appear in tooltips for the tag's drawn representations. The properties for the ID tab are now complete and the config folder should appear similar to the example shown in Figure 23.

Figure 23

6.  Click on the I/O tab.

Figure 24

The properties of the I/O tab are used to associate an analog input tag with a driver tag representing an I/O device (in this case, the virtual Modicon PLC).  In the address field, you will enter the address on the I/O device at which the data for this tag is to be found.

 

To see this, you will now connect this analog input tag to the virtual Modicon driver tag.

7.  Click the tag button to the right of the I/O Device field.  

A fresh instance of the Tag Browser opens and displays the names and descriptions of the available I/O device driver tags. In this example only one I/O device driver tag is available, the Modbus1.

Figure 25

8.  Ensure that "Modbus1" is selected in the Tag Browser's list of tags.

9.  Click the Select button. The Tag Browser closes, and the selected driver tag is displayed in the I/O Device field.

You must now supply the address at which this analog input tag can find the analog data for the booster tank level. Referring to your Tag Table (see: Create a Tag Table), the appropriate address is "40001".

 

10. Enter "40001" in the Address field and press the Enter key.

 

You will not change the Scan Interval value, but you should make note of it.  This is the interval at which the tag should check for data at the indicated address. By default, the Scan Interval is set to "1" (i.e. 1 second).

All data that is within the same memory area in the I/O device and that has the same scan interval will be read in the same poll. It is therefore recommended that you limit the number of different scan intervals for a particular I/O device. This will reduce the number of separate reads that must be done and will improve overall update performance. Throughout this tutorial, all scan intervals will be set to "1".

11. Enter "in" in the Engineering Units field and press the Enter key.

This represents the engineering units for the incoming value for the tag; the booster tank level is measured in inches, thus the "in" abbreviation. The remaining field (Manual Data) is not required at this time. Manual data is used to provide a static, user-defined value for the tag in place of live data.

The I/O tab is complete and should appear similar to Figure 26.

Figure 26

 

12. Click the Scaling tab of the new analog input tag properties folder.

The Scaling tab will look similar to Figure 27 (shown completed).

 

There are two "Unscaled" fields and two "Scaled" fields. The Unscaled Process Data Min and Unscaled Process Data Max fields represent the raw, unscaled minimum and maximum values incoming from the I/O device. The Scaled Process Data Min and Scaled Process Data Max fields indicate the scaled value according to the engineering units assigned on the I/O tab.

You need to know about your I/O device and your process in order to correctly specify values for these fields. For our analog tags in this application, the raw data is read from a 12-bit register, giving a range from 0 to 4095 (thus the default values displayed above). The diameter (height) of the booster tank is 120 inches, so the scaled range is from 0 to 120.

 

13. Enter "120" in the Scaled Process Data Max field and press the Enter key.

The remaining analog input tag properties folder tabs (Owner, Merit, Alarm, Script, and Logger) are used for more advanced operations, and are not necessary to the completion of this analog input tag.

Figure 27

14. Click the OK button on the analog input tag properties folder. The new analog input tag properties folder closes, and the name and description for the new analog input tag is now displayed in the tag list of the Tag Browser.

 

To review, you have just configured the properties required to establish a new analog input tag. In selecting the "Modbus1" I/O device driver tag in the analog input tag's I/O device property field, and configuring an Address property for the tag, you have connected the analog input tag to the virtual Modicon PLC.  The new tag is now reading simulated data from the virtual Modicon PLC.

 

You did not configure the remaining analog input tag properties on the Owner, Merit, Alarm, Script and Logger tabs of the analog input tag properties folder.  For your interest, refer to the table below for an understanding the use of these tabs. Information on configuring tags using these tabs and their related properties can be found in the VTS Developer's Guide.