Back to FBD referenceTypical FBD symbol for F_TRIG
Edges
F_TRIG
Falling-edge detector
The F_TRIG block compares the current IN value with the previous scan and emits a one-scan pulse on a falling edge.
Typical FBD symbol
IN
F_TRIG
Q
Inputs
| Name | Type | Required | Description |
|---|---|---|---|
| IN | BOOL | Yes | Signal whose falling edge is detected. |
Outputs
| Name | Type | Required | Description |
|---|---|---|---|
| Q | BOOL | Yes | TRUE only on the TRUE -> FALSE scan. |
Stateful
Yes, the block keeps internal state between scans.
Behavior rule
Q = TRUE for one scan when IN changes from TRUE to FALSE.
Scan table
Falling-edge example.
| Scan | IN | previous IN | Q |
|---|---|---|---|
| 1 | TRUE | FALSE | FALSE |
| 2 | TRUE | TRUE | FALSE |
| 3 | FALSE | TRUE | TRUE |
| 4 | FALSE | FALSE | FALSE |
Practical use
- reacting to a released button;
- part leaving a sensor;
- signal end detection.
What to watch
- Confusing it with R_TRIG. F_TRIG reacts to the falling edge.