While Omron has a Keep Relay with both a Set and with a Reset input controlling a single output coil shown as a single rectangle, in contrast Allen-Bradley has them split into two parts. Here the only distinction is wether to energise or de-energise the one coil. I am saying that the effect is progressive and instantaneous all the way to the hardware, progressive down the ladder. This has implications for the LinuxPLC scan and intrascan design, and for the LogicEngine timing and design. The existing PLCs don't care about multiple instances, or do not care about dupecoils, because they seem to just solve the Ladder rung by rung mindlessly.
Intrascan functions have immediate effect at the mid-position of the scan.
The theory of only passing the solved output bits on towards the output hardware at the end of the scan, is not applicable to all bits. So really persistence might need to be in active operation on a time scale much smaller than the time scale of a scan. The idea of a simple SRAM card with built in battery is looking very nice, plus it give a place to put a WatchDog Timer too. Even when PLCs were young, and SRAM was expensive and came in smaller chips, they did not selectively choose portions to be battery backed. The point is that the design choice taken was to simply give Everything persistence, and additionally at the time of PowerUpAgain there is a housekeeping routine that selectively zeros the bits that have no right to have Persistence.