Working in Steady State

Most of the expressions that you create in tags and graphics work in steady state. For example, the code behind any page in VTScada. If none of the tag values change, nothing on the page will change. Deep in the VTScada engine, code is watching for changes to occur, but at the level of your page, all the code is simply sitting there. It does nothing, and takes no CPU time until a change occurs, such as an input tag reporting a new value. This triggers a linked widget to update, showing that value. Only the code for one tag and then one widget is processed. This is an important part of what makes VTScada so efficient. If you write code for a new display feature, it is extremely unlikely that you would need to write a loop to repeatedly check for new values.

Note that there are ways to get into trouble with steady state. For example, perhaps you might create two I/O and Calculation tags, both as analog calculations. You name them Calc1 and Calc2.

In Calc1, you suppose that you have created an expression that uses some other analog tag's value and also the value of Calc2. In Calc2, you have an expression that uses the value from Calc1. If you were working in an ordinary programming language, this would not be a problem. Your code would update Calc1, then Calc2 and then stop. But in steady state, expressions are triggered whenever any of the components change. What will happen in VTScada is:

  1. The analog tag changes value.
  2. This triggers the expression in Calc1.
  3. The change in Calc1's value triggers the expression in Calc2 and so Calc2's value changes.
  4. This triggers the expression in Calc1.
  5. And, so on...

The process will never stop on its own, therefore you must make sure that you never create circular references like the one in this example.

Pegging a CPU

(Do not do this experiment within a production application.)

The following demonstrates how poorly-designed expressions can cause trouble:

  1. Create two numeric calculation tags. Name the first, "Calc1" and the second, "Calc2".
  2. Enter the following expression for Calc1:

[Calc2] + 1

  1. Enter the following expression for Calc2:

PickValid([Calc1] + 1, 0)

  1. Let the system run for a few moments while you check the CPU statistics using the Windows Task Manager.
  2. Delete the tags.
    In an extreme case, you might need to use the Windows Task Manager to stop VTScada.

Break the Steady-State Rule for Expressions

The rule is that you must use Steady State functions in your expressions, excepting optimized Tag Parameter expressions. What if you need a script function, like CurrentTime? Then you can take advantage of a useful hack.

The Workstation Status driver tag provides a way to process script functions and display them in a steady-state page. It was given this feature originally because the memory information from the operating system is not an accurate reflection of memory in use. To obtain an accurate value, it was necessary to use the VTScada Memory() function, which runs only in script mode. A side benefit of this enhancement to the Workstation Status driver is that you can now use nearly any script-mode function with the Workstation Status tag and retrieve a result. Additionally, that result will update on a set schedule, almost as if it were running in Steady State.

For further information, refer to: Workstation Status Driver I/O Addressing