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Old January 12th, 2021, 06:10 AM   #1
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xzen is offline
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Edge trigging in ST or SCL

What is your best practice for edge trigging in structured text language?

#condition := (Value1 > 0 OR NOT Tag1) AND Tag2;
IF #condition_positive_edge THEN
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Old January 12th, 2021, 06:22 AM   #2

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Use the built in functions (IEC 61131-3):
- R_Trig: Rising edge
- F_Trig: Falling edge
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Old January 12th, 2021, 07:19 AM   #3

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+1 R_Trig
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Old January 12th, 2021, 07:55 AM   #4
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If you wish to code it yourself then:

IF Condition AND NOT Condition_Memory
Some Code
End IF

Condition_Memory = Condition
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Old January 12th, 2021, 02:18 PM   #5

Lare is offline
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Jesper's post 9 years ago, same as Manglemenders If condition on ST.

Originally Posted by JesperMP View Post
Dont think there is a one-shot block for SCL.
But it is trivial to code.

signal_oneshot_ON := signal_input AND NOT signal_mem ; // generate the ON oneshot
signal_oneshot_OFF := NOT signal_input AND signal_mem ; // generate the OFF oneshot
signal_mem := signal_input ; store the status of the input
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Old January 13th, 2021, 01:35 PM   #6
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Not sure if it's in all ST packages but in CoDeSys (at least in v3.5) you can set a bit for a positive or negative edge trigger. The 'P' indicates positive edge, for Negative edge it would be an 'N'

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