Which method is generally used to communicate between programs in Structured Text

Question

I am maintaining a project for a PLC written in ST. To implement a new feature I need to let cyclic program A know when an event happened in cyclic program B.

How is this generally done in ST? Do I simply use global variables or is there a different method? If I use global variables, how are these then protected from concurrent modification?

I use the X20 PLCs from B&R Automation.

Answer 1

Yes, you need to declare a variable that has a shared scope to both cyclic programs.

You can do this by using the existing global.var file or you can create a new variable file, and limit what programs can read or write to it by placing it within a "package" (folder in your project).

To create a new var declaration file...

-right click within the logical view

-add object

-select "file" category, and choose new file

-name, and change to "save as *.var" in the drop down

By default, the new variable declaration visibility will be limited to the package it is contained within. To verify this, right click the file and go to properties. Select the details tab.

There is no way to protect from concurrent modification, but you can use the cross reference tool to see where a selected variable is being written and read within your project. First build a cross reference, and then use the tab at the bottom.

Good luck!