Lessons · Industrial control · the off-delay timer
The off-delay
An off-delay (TOF) keeps its output on for a set time after the input turns off: a fan that runs two minutes after the machine stops.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
Cool-down, purge, hold: many outputs must outlive their trigger. TOF is the mirror of the on-delay TON, which waits before turning on.
How to think about it
Say which edge starts the clock. Delay before ON: TON on the input. Hold after OFF: an off-delay, or a TON timing while the input is FALSE with the output held by a branch until it is done.
Worked example
[/Machine] [TON Cool 2s]Time while the machine is off: the clock starts at the moment it stops.
{ [Machine] | [/Cool.DN] } (Fan)Fan runs while the machine runs, or until the off-time has elapsed: an off-delay built from a TON.Your turn
Hold the lamp 5 seconds after the door opens.
[/DoorOpen] [TON Hold ]
{ [DoorOpen] | [/Hold.DN] } (Lamp)Run a rung and watch the scan
The trap
A TOF's timing starts on the falling edge of its input. Triggering it with a pulse that never went true does nothing, because there was no fall.