Types of Timers in PLC | Retentative On Delay Timer (RTO)

 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 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)

Retentative On Delay Timer (RTO)

 The RTO instruction works a lot like the TON instruction with one main exception: When the rung goes false on the RTO instruction, it will retain the ACC value.  When the rung becomes true again, the ACC value will pick up from where it left off.  One good application for the RTO would be an hour meter to indicate total runtime for machinery.  

Since the RTO does not reset itself when the rung goes false, the RES instruction must be used to reset a timer.  Here is a practical application:

In this example, once the machine accumulates 1 hour of run time, a light might come on indicating that lubrication needs to be engaged.  Once the operator lubricates the machine, he can reset the hour meter

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