Back to FBD referenceTypical FBD symbol for TOF
Timers
TOF
Off-delay timer
The TOF block turns Q on immediately when IN is TRUE, then keeps Q TRUE for the configured scan or time delay after IN becomes FALSE.
Typical FBD symbol
IN
TOF
Q
Inputs
| Name | Type | Required | Description |
|---|---|---|---|
| IN | BOOL | Yes | Timer hold signal. |
Outputs
| Name | Type | Required | Description |
|---|---|---|---|
| Q | BOOL | Yes | Output with delayed turn-off. |
Parameters
| Name | Type | Required | Default | Description |
|---|---|---|---|---|
| PT_SCANS | number | No | 1 | Off-delay threshold in scans. |
| PT | number | No | - | Delay in milliseconds when a time preset is used. |
Stateful
Yes, the block keeps internal state between scans.
Behavior rule
When IN = TRUE, Q is immediately TRUE; after IN changes to FALSE, the timer keeps Q = TRUE until the configured delay expires.
Scan table
Example for PT_SCANS = 2.
| Scan | IN | elapsed after off | Q |
|---|---|---|---|
| 1 | TRUE | 0 | TRUE |
| 2 | FALSE | 1 | TRUE |
| 3 | FALSE | 2 | FALSE |
| 4 | FALSE | 0 | FALSE |
Practical use
- fan turn-off delay;
- holding a signal after a sensor disappears.
What to watch
- Confusing it with TON. TOF delays turn-off, not turn-on.