TOFR — Off-Delay Timer

Structured Text off-delay timer function block. Uses FBD_TIMER; the ST equivalent of ladder TOF.

Operands

Name	Type
timer	FBD_TIMER

FBD_TIMER Tag Members

Member	Type	Meaning
.EnableIn	BOOL	Forced to 1 each scan by the instruction call — setting it 0 has no effect
.TimerEnable	BOOL	1 primes the timer (ACC held at 0); 0 runs the off-delay, counting ACC up to PRE
.PRE	DINT	Preset target time in milliseconds
.Reset	BOOL	1 clears EN, TT, DN and sets ACC to PRE
.EnableOut	BOOL	1 while the instruction is executing normally
.ACC	DINT	Accumulated time in milliseconds
.EN	BOOL	1 while TimerEnable is 1 and Reset is 0
.TT	BOOL	1 while the timer is actively counting toward PRE
.DN	BOOL	Done status — each timer (TONR, TOFR, RTOR) decides when this turns on
.Status	DINT	Packed status word — combines InstructFault and PresetInv bits
.InstructFault	BOOL	1 when the instruction hit an execution error
.PresetInv	BOOL	1 when PRE is outside its valid range

How It Works

TOFR is the Structured Text function-block equivalent of the ladder TOF instruction. It operates on an FBD_TIMER tag.

While .TimerEnable is 1, the timer is primed — .DN is 1 and .ACC stays at 0. When .TimerEnable drops to 0, .ACC counts up; .DN stays 1 until .ACC reaches .PRE, then goes 0. So .DN holds 1 for the preset duration after TimerEnable drops to 0.

Example

ExhaustTimer.TimerEnable := MachineRunning;
ExhaustTimer.PRE := 30000;
TOFR(ExhaustTimer);

ExhaustFan := ExhaustTimer.DN;

Practice

Try off-delay timing in the Off Delay exercise — keep an output on for two seconds after its input drops.

Common Mistakes

• Trying to pause with .EnableIn := 0. The instruction call forces .EnableIn back to 1 every scan, so the assignment has no effect. .TimerEnable is the control: 1 holds the output on, 0 begins the off-delay before the output turns off.
• Assigning .ACC from logic to change it (MyTimer.ACC := 0). Structured Text recomputes .ACC every scan, so a runtime write is ignored (the initial .ACC default is still honored). Reset with .Reset := 1. The same write does take effect on the ladder TOF — so don't rely on it either way; it isn't portable.

Related

• TOF — the ladder-logic equivalent
• TONR, RTOR — other ST timer function blocks