Can anyone tell how PLC takes previous cycle data?


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As PLC starts its scan cycle by loading the input values into an image register; these values will remain fixed until the start of the next cycle (say 20 ms on average); logic processing and output image writing then follow. Then begin next cycle. But some algorithms in control system require previous cycle data. How they get this data. If there is any special storage for this? Also if there is any chance to lose previous cycle data during executing present time. If lose then how can rectify this problem?