Siemens SIMATIC S7 System Manual page 379

TOF timer operation
The TOF instruction is used to delay turning an output OFF for a fixed period of time after the
input turns OFF. When the enabling input turns ON, the timer bit turns ON immediately, and
the current value is set to 0. When the input turns OFF, timing begins and continues until the
current time equals the preset time.
● When the preset is reached, the timer bit turns OFF and the current value stops
incrementing; however, if the enabling input turns ON again before the TOF reaches the
preset value, the timer bit remains ON.
● The enabling input must make an ON-OFF transition for a TOF timer to begin timing the
OFF-delay time interval.
● If a TOF timer is inside an SCR region and the SCR region is inactive, then the current
value is set to 0, the timer bit is turned OFF, and the current value does not increment.
Type
TON
TONR
TOF
The retentive timer current value can be assigned for retention through a power cycle. See Config-
1
uring the retentive ranges for details (Page 141).
Note
Using the Reset instruction with timer instructions
The TONR timer can only be reset with the Reset (R) instruction.
The TON and TOF timers can be reset by the timer's enable input and also the Reset (R)
instruction.
The Reset instruction performs the following actions:
• Timer bit = OFF
• Timer current value = 0
• After a reset, TOF timers require the enable input to make the transition from ON-to-OFF
in order restart the OFF-delay timer.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Current >= Preset
Timer bit ON
Current value continues
timing to 32,767
Timer bit ON
1
Current value continues
timing to 32,767
Timer bit OFF
Current = Preset, stops
timing
State of IN, the enabling input
ON: Current value = timing value
OFF: Timer bit OFF, current value
= 0
ON: Current value = timing value
OFF: Timer bit and current value
maintain last state and value
ON: Timer bit ON, current value =
0
OFF: Timer begins timing after
ON-to-OFF transition
Program instructions
7.17 Timer
Power cycle / first scan
Timer bit = OFF
Current value = 0
Timer bit = OFF
Current value can be
maintained
1
Timer bit = OFF
Current value = 0
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