LD UDFB Guided Learning Q11: – What is state fall through for a state machine?

Automation and Control
1

This article is part of a guided learning series on building and analyzing User-Defined Function Blocks (UDFBs).

:pushpin: Canonical Article: Building a User-Defined Function Block (UDFB) Using Siemens TIA Portal
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You are reading: Question 11

What is state fall through for a state machine

Clarification

Suppose we are in state A, The A network changes the state variable to B. Can we expect both network A and network B to execute? Is this an important consideration? What could be done to prevent this behavior if it was important?

Answer

This is directly related to the “last rung wins” gotcha that we commonly encounter in PLC programming.

If we change states, we risk falling through to the next state thereby vising both states in a single program scan. This could be a problem if we expected events to occur on the transition from A to B. However, if we design a true Moore state machine the hazard is eliminated.

If events must occur on the state transitions, it may be better to include a state between A and B. This transitionAtoB state could then handle the logic using easy to follow Moore logic.

Why Moore vs Mealy matters

  • Moore: A rudimentary state machine where the outputs are entirely dependent on the state.

  • Mealy: For our PLC state machine application, we define Mealy as a machine with outputs depending on both input and state. This includes the state variable itself. In PLC language could associate a Mealy machine with a one shot that occurs on a state change.

  • Output: All memory locations that may be impacted by the state machine. This includes the output for the featured UDFB. It also includes static variables and even uiState.

:writing_hand: Article by Aaron Dahlen, LCDR USCG (Ret.), Application Engineer at DigiKey