Edge Trigger

Operator Menu Location: Logic Functions

Operator Bitmap:
 

Functional Description:
The Edge Trigger operator generates a pulse when it detects a change in the logic state of its input. It can be configured to generate the pulse on a rising edge (when the input state switches from LOW to HIGH) or on a falling edge (when the input state switches from HIGH to LOW) or on both the falling edge and rising edge.

The operator output is HIGH only for one schema compute cycle, immediately following the edge detection. Otherwise the output is LOW. (The width of the HIGH pulse is equal to the schema loop time.) For example, for the input signal shown in figure (a) below, the Edge Trigger operator output will be as shown in figure (b) when configured for rising edge detection only, as shown in figure (c) when configured for falling edge detection only, and as shown in figure (d) when configured for both rising edge and falling edge detection.

(a) Input signal

(b) Output signal when configured for Rising Edge Detection

(c) Output signal when configured for Falling Edge Detection

(d) Output signal with both Rising Edge and Falling Edge Detection

The Edge Trigger operator is useful for example, when multiple transition events in a state-transition diagram are triggered by the same signal source, but only one transition is desired whenever the signal source is active. For example, consider the schema diagram in the figure below containing a state-transition diagram with a start state and a stop state. The controller is to switch state whenever the same start/stop push button is pressed. Suppose a push-button operator were connected directly to the boolean variable that triggers the event transitions between the start and stop states (as shown in figure (b)). With this implementation, the controller will bounce back and forth between the start and stop states, as long as the push button is held down. This is generally not desirable behavior. A preferable outcome would be a single event transition each time the start/stop button is depressed, regardless of how long it is held down. This can be accomplished by inserting an edge-trigger operator (configured for rising-edge activation) between the push-button and boolean event trigger. In this way, the boolean variable is only set to true for one schema compute cycle, rather than as long as the button is depressed.
Design Pad will issue an error upon processing if the input pin is not connected.

User-Defined Properties:
Object Name. A string label that identifies the operator

Trigger On. Determines the edge condition on the input that results in an output pulse. If set to Rising Edge, output pulse occurs when input value switches from LOW to HIGH. If set to Falling Edge, output pulse occurs when input value switches from HIGH to LOW. If set to Both Edges, output pulse occurs whenever input value changes.

Comments: None.

See Also: Snapshot Recorder, Strip-Chart Recorder