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Types of Timers in PLC | OFF Delay Timer (TOF)

 PLC Timers are generally used for delaying an event from taking place, or to delay a device from shutting off either on an on transition or an off transition.  There are three types of PLC timers:  The Timer ON delay (TON), Timer Off delay (TOF), and the Retentative Timer On delay (RTO).

By default, timers are stored in the T4 Data file, however other time files can be created. A timer consists of the following components:  Preset word (PRE), Accumulate word (ACC), Done bit (DN), Timer Timing bit (TT), and Enable bit (EN).  For Timers, the Enable bit follows the rung condition.









The entire timer is addressed by it's element (example: T4:0) Pieces of the timer can be used in logic however such as the DN bit on an XIC (T4:0/DN), or the Accumulated value in a MOV statement (T4:0.ACC)

PLC Timer Off Delay (TOF)

 The Off Delay Timer is generally used to delay an event from shutting off.  Image a lube system on a large motor.  As long as the main motor is turning, the lube pump should be running.  When the main motor shuts off, you wouldn't want to shut off the lube pump immediately because the main motor needs time to coast down to zero RPM's.  The Main motor could run off the EN bit, and the Lube motor could run off the DN bit.

On the Off delay timer, as soon as the rung goes true, The EN bit goes true as it does for all timers.  Since the Off delay timer does not delay the DN bit from shutting off, the DN bit goes high immediately.  Remember, the TOF instruction delays the DN bit from shutting off, not turning on.  (Plus if we are delaying the DN bit from shutting off, it needs to be high to begin with).  While the rung is true, the timer is not timing, and the ACC value is at zero.

When the rung is shut off, the EN bit shuts off immediately.  The ACC value will start timing until it reaches PRE then the DN bit will shut off.

Here is what the TOF instruction might look like in logic:





When the motor switch is energized, both the main motor and the lube motor will energize immediately.  When the main motor switch is shut off, the main motor shuts off immediately, but since the TOF delays the DN bit from shutting off, the Lube motor will shut off 30 seconds later.   Warning:  Using the RES instruction on a TOF instruction could cause unpredictable operation.

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