Cascaded SFC Program Behaviour


Thread Starter

Mark Meng

Dear List members,

This is a question related to the SFC run-time behaviour:

We use SFC (assume there are steps STP-B-1,2,3...) to write an Action (assume the associated step is called STP-A-1) of the main SFC. When STP-A-1 is active and the sub-level SFC program goes to STP-B-2, the STP-A-1 becomes inactive. The question is: next time when STP-A-1 is active again, should the sub-level program inside the Action start from beginning or continue from the break point STP-B-2?

Best Regards,
Mark Meng

Prof. J-M Faure

Next time when STP-A-1 is active again, the sub-level program inside the Action should start from the break point STP-B-2.

This answer is based upon our interpretation of the 61131 standard for there is no such example given in this document.

In the paragraph 2.6.4.(Actions) it is never mentioned that the variables elaborated by an action associated to a given step come back automatically to their initial state when this step becomes inactive. On the contrary several examples of the standard (for instance n° 2l and 2f of table 42) indicate that the developer shall introduce explicitly in the action program the step flag (S8.X) to obtain such a behaviour (variables coming back to their initial state when the step becomes inactive). It is the reason why we have chosen the behaviour hereabove expressed.

Nevertheless we suggest to anybody willing to use such a structure (SFC within an action block associated to as step, or action blocks described in ladder diagram) to test the actual behaviour of the machine he intends to use. Our experience trends to be careful with the interpretation of this part of the standard. In France an users association, named EXERA (, has developed behavioural tests in order to check PLC programs written in LD and SFC. As far as we know, these tests will be soon available in English

Professeur Jean-Marc Faure
Responsable de l'équipe CSAP
Laboratoire Universitaire de Recherche en Production Automatisee Ecole Normale Superieure de Cachan 61, avenue du President Wilson - 94235 Cachan Cedex - France tel : 33 (0)147402216 - fax : 33 (0)147402220
[email protected] -

Jean-Marc ROUSSEL Telephone : (33) 01 47 40 29 97
LURPA Fax : (33) 01 47 40 22 20
61, AV. Pt Wilson Email : [email protected]