LD UDFB Guided Learning Q09: – Identify the implicit one shot in a state machine

Automation and Control
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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 9

Identify the implicit one shot in a state machine.

Answer

An implicit one shot occurs on every state transition. Consider the second branch of Figure 3 which activates on a fault when the motor starter fails to close within timClose. The timer output will be high for one and only one program cycle scan. The fact that we move to a new state ensures that the timer will reset on the next scan.

With regard to a state machine, this is an important consideration when we want things to occur once, and only once, on the state transitions. As example is the S_MOVE which is only activated for a single program scan.

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