Technical Reference Manual - InduSoft

Communication with Other Devices
1. The new values on the client are buffered.
2. The original values on the server are read to the client — that is, the client is synchronized
with the server.
3. The new values are written from the buffer to the server.
4. The new values on the server are read to the client — that is, the client is again synchronized
with the server.
At the end of each scan, the values reflect what's happening in your project even though,
technically speaking, the project is merely staying in sync with the server.
Both options should be enabled in most projects. In some projects, however, this may cause
items to bounce between the original values and the new values. If this is a problem, try moving
those items to another OPC worksheet where the Read before writing and Read after writing options are
disabled.
• Accept Tag Name in the Item column checkbox: When this option is checked, the text configured between curly
brackets in the Item field is resolved as a Tag Name (string tag). In this case, the value of this tag is used
as the name of the item from the OPC Server, allowing the user to point to different item names during
runtime, by changing the value of the tag(s) configured in the OPC Client worksheet (Item column).
When the Accept Tag Name in the Item column option is unchecked, all characters configured in the Item column
are considered part of the Item name (including the curly brackets).
• Tag Name: Type the names of tags linked to the server items.
• Item: Enter the name of the server's items. After selecting an OPC Server, you can select items from the
Server using the OPC Browser. Right-click in the Item field and select the OPC Browser option.
Tip: You can configure a tag name between curly brackets (e.g., {TagName} ) in this field,
allowing the user to change the item names dynamically, during runtime.
• Scan field: Specify the condition under which the tag value is read from the remote device or server and
then updated in the project database, using one of the following options:
• Always means the tag is read and updated during every scan of the communication worksheet,
regardless of whether the tag is used in any other project screens, scripts, or worksheets.
This option is recommended for tags that must be continuously monitored in the background, such as
tags that trigger alarms, tags used in recipes, tags that are recorded in the historical database, and so
on.
• Screen means the tag is read and updated only if it is being used in at least one open project screen,
either locally or on another client station.
This option is recommended for tags that are used in screen objects, because the project may not
need to update tags that are not being visualized anywhere. Selecting this option can improve project
performance.
• Auto means the project will automatically choose either Always or Screen, depending on where the tag is
used in your project. If the tag is only used in a screen object on a project screen, then the scan will
default to Screen. But if the tag is configured in any other interface (e.g., Script, Math, Alarm, Trend,
Recipe, Report, Scheduler), then the scan will switch to Always and remain there until the project is
stopped.
If you are not sure of which option to select, select Always. This will guarantee the tag is read and updated.
• Div field: Specify the division constant when scale adjustment is required. This value is a division factor in
a read operation and a multiplication factor in a write operation.
• Add field: Specify the addition constant when scale adjustment is required. This value is a addition factor
in a read operation and a subtraction factor in a write operation.
To run the OPC Client runtime task, you can choose to run it automatically on start up, or run the task
manually by clicking Tasks (either local or remote) on the Home tab of the ribbon. After running this program,
a small icon displays in your system tray.
To close the OPC Client runtime task, right-click the icon in the system tray, and click Exit.
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