CODEWITHSUNDEEP
plcstbr-automation-studio
martinhans
martinhans

Reputation: 1213

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

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.

Upvotes: 4

Views: 515

Answers (2)

Ira Baxter
Ira Baxter

Reputation: 95410

Asynchronous communication is tricky.

So imagine a global A_DONE initialized to false, with B inspecting it occasionally. A runs, and sets A_DONE. B can react to this event... but what does it do if it needs to handle another event?

If you believe that the event that tells A to signal A_DONE occurs only long after B sees A_DONE, B can simply reset A_DONE to false (assuming this always happens before the next A_DONE event) and the cycle can repeat.

If A_DONE can occur "again" while B is handling the results of seeing A_DONE, B cannot just reset A_DONE: you might get a timing splinter in which B reads A_DONE, A sets A_DONE again and B then clears A_DONE; now you've lost an event. If that event is controlling your reactor emergency rods, this could be pretty bad because poof, B missed it.

In this case you will likely need a handshake from A to B and back. We add a signal from B back to A, call it A_DONE_SEEN, to let B tell A that it has processed the event. Then A sets A_DONE, waits for A_DONE_SEEN; A clears A_DONE, waits for A_DONE_SEEN to go false, and continues its business. If A_DONE needs to be set while A_DONE is already set, or A_DONE_SEEN is set, we know we missed an event and some disaster recovery procedure can be run. B watches for A_DONE, handles the A_DONE action, sets A_DONE_SEEN, watches for A_DONE going false, and sets A_DONE_SEEN_FALSE.

I don't know about your specific PLCs, but in many systems there are atomic operations that increment counts, etc. You could use this instead of the handshake.

Upvotes: 2

tkezy
tkezy

Reputation: 162

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!

Upvotes: 2

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