The RTO, also know as Retentive Timer, an instruction is used to keep track of time just like the TON Instruction. However, there is a key difference between the two. The TON instruction will automatically reset the accumulated time upon a transition from HIGH to LOW. The RTO will simply “pause” the accumulated value during this transition. In other words, the timer will retain the value it currently has until it is energized again or forced to reset. Due to this capability, it’s an excellent tool for a process which may be interrupted but needs to keep track of a continuous event. For example, you can easily create a timer which would allow a certain valve to only open for a certain duration even though it is stopped in between.
This is a companion discussion topic for the original entry at https://www.solisplc.com/tutorials/plc-programming-intermediate-instructions-rto-retentive-timer