Introduction

 

SLC 500 Training: In this section we will cover timers used in ladder logic programming. Timers are very important in ladder logic programming. Timers gives the precision in time. Timer on delay starts timing when instruction is true. Timers are used to track time when instruction are on or off. They could also keep track on a retentive base.

 

Definition

The following is a list of timer instructions in SLC 500:

• TON - Timer On Delay

• TOF - Timer Off Dealy

• RTO - Retentive Timer

TON Timer On Delay

Symbol

Definition

• Count time base intervals when the instruction is true.

• The Timer On Delay instruction begins to count time base intervals when rung conditions become true. As long as rung conditions remain true, the timer adjust its accumulated value (ACC) each evaluation until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go false, regardless of whether the timer has timed out.

Each Timer on Delay is made of a 3-word element.

Word 1 is the control word

• Bit 0-12: Internal Use

• Bit 13: Done (DN) this bit is on when the Accumulation value >= Preset Value

• Bit 14: Timer Timing (TT) this bit is on when the timer is timing

• Bit 15: Enabled (EN), this bit is on when the timer is energized.

Word 2 stores the preset value. (PRE)

• The programmer specifies this value. When the accumulated time reaches the preset value the controller sets the done bit. When the accumulated value becomes equal to or greater than the preset value, the done bit is set. Usually preset value is from 0 - 32,767

• If a timer-preset value is negative an error will occur.

· Word 3 stores the accumulated value. (ACC)

• This is the time elapsed since the timer was last reset. When enabled the timer updates this continually.

• Time Base: is the timing update interval, this can vary from 0 - 1 second.

TOF Timer Off Delay

Symbol

Definition

• Counts time base intervals when the instruction is false.

• The Timer Off Delay instruction begins to count time base intervals when the rung makes a true to false transition. As long as rung conditions remain false, the timer increments its accumulated value (ACC each scans until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go true regardless of whether the timer has timed out.

Each timer address is made of a 3-word element.

Word 1 is the control word

• Bit 0-12: Internal Use

• Bit 13: DN- Done

• Bit 14: TT - Timer Timing

• Bit 15: EN - Timer is enabled

Word 2 stores the preset value. (PRE)

• Specifies the value, which the timer must reach before the controller sets the done bit. When the accumulated value becomes equal to or greater than the preset value, the done bit is se.

• Preset value is from 0 - 32,767

• If a timer-preset value is negative an error will occur.

Word 3 stores the accumulated value. (ACC)

• This is the time elapsed since the timer was last reset. When enabled the timer updates this continually.

• Time Base: is the timing update interval, this can vary from 0 - 1 second.

RTO Retentive Timer

Symbol

Definition

• Counts time base intervals when the instruction is true and retains the accumulated value when the instruction goes false or when power cycle occurs.

• The Retentive Timer instruction is a retentive instruction that begins to count time base intervals when rung conditions become true.

• The Retentive Timer instruction retains its accumulated value when any of the following occurs:

• Rung conditions become false.

• Changing Processor mode from REM run /Test / program mode.

• The processor loses power while battery back up is still maintained.and a fault occurs.

Note:To reset the accumulated value in RTO, you must use a reset instruction (RES) with the same address.

Each Retentive Timer is made of a 3-word element.

Word 1 is the control word

• Bit 0-12: Internal Use

• Bit 13: DN- Done

• Bit 14: TT - Timer Timing

• Bit 15: EN - Timer is enabled

Word 2 stores the preset value. (PRE)

• Specifies the value, which the timer must reach before the controller sets the done bit. When the accumulated value becomes equal to or greater than the preset value, the done bit is se.

• Preset value is from 0 - 32,767

• If a timer-preset value is negative an error will occur.

Word 3 stores the accumulated value. (ACC)

• This is the time elapsed since the timer was last reset. When enabled the timer updates this continually.

• Time Base: is the timing update interval, this can vary from 0 - 1 second.