Ladder Logic Retentive Timer Instruction

RTO - Retentive Timer

• 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.

Example 1

Let's say we to have a timer on a motor that times the time the motor was on. What we want to do is to run the lubrication pump every 10 minutes the main motor has run, for 5 seconds.

Input / Output

• Selector I:1.0/0
• Motor O:2.0/0
• Lube Pump O:2:0/1

Ladder Logic Solution

When the selector is on the motor will go on.

When the motor goes on the RTO will start timing. We needed 10 minutes so 60 seconds * 10 minutes = 600 seconds.

When RTO is done and the 5-second timer on the lubrication is not done start timing on the lube system.

When the RTO is done starts the lubrication pump. Note that Rung 2 and 3 could have been combined. This is only to keep things simple.

Once the TON of the lube is done then we have to reset the RTO. So the whole thing will start over again.

Note

• When TON is energized it will start timing. If the RTO is de-energized the accumulated value will not reset since it is retentive. Therefore next time we energize the RTO the timer will accumulate to whatever value it was.
• As opposed to TON and TOF, RTO does not reset by just de-energizing the rung. We need to have the reset instruction with the timers address to reset it, as shown in Rung 4.