Multithread Considerations with VBA

Cicode is pre-empted and executed on an instruction-by-instruction basis. This means that execution of a simple unnested Cicode thread can only switch to another thread after the current Cicode instruction has completed execution.

VBA code is pre-empted and executed on a line-by-line basis (as opposed to an instruction-by-instruction basis), and pre-empting can only occur after the current line has completed execution.

Each line of VBA script is handled as a separate thread in Plant SCADA. Therefore multiple procedures placed on one line may not complete before another subsequent thread is processed in a multithreading environment. This could cause unpredictable results and consequences, including data invalidation and corruption.

If, for example, you were reading or setting some variable or point in a multi-statement thread, and further processing that data in a later thread,that data might become invalid or incorrect. For this reason, you should separate every statement onto separate lines in VBA.

For example, it is better to write:

A = Motor1.speed() + Motor4.speed() + Motor5.speed()

as

A = Motor1.speed()
A = A + Motor4.speed()
A = A + Motor5.speed()

in situations where the method speed()may take a long time to execute.

In the first example above, the VBA thread executes for three times longer before it can be pre-empted than in the latter example.

Note: This does not apply to Cicode because the Cicode engine can pre-empt aggregated code.

See Also

Using VBA in Command or Expression Fields