Cf. https://www.plctalk.net/qanda/showthread.php?t=124552

Process
=======
Boxes on a conveyor
Three independent discrete inputs:
- Two PhotoEye (PE) station discrete inputs:  upstream BOX_AT_PE1; downstream BOX_AT_PE2
  - 1 => box is present at station; 0 => no box is present at station
- A third discrete input REJECT_BOX_AT_PE1 determines whether a box at the upstream station either
  - is a reject (1), and to be later diverted at PE2,
  OR
  - is okay (0), and not to be diverted.
Rising edge at upstream station BOX_AT_PE1 station is where each box's {1:reject;0:okay} status is set
- Based on the status of REJECT_BOX_AT_PE1
When each box later generates a rising edge at downstream station BOX_AT_PE2
- divert if box status from BOX_AT_PE1 was [reject]
- do not divert if box status BOX_AT_PE1 was [okay]
There will be an arbitrary number of boxes that have triggered BOX_AT_PE1 but not yet triggered BOX_AT_PE2
- These are the only boxes this program can keep track of with the available inputs
- The number of boxes will range from 0 to no more than approximately 10

Implementation data structures
=========================
Integer index TRACKED_BOX_COUNT (N199:0) is the count of tracked boxes
- I.e boxes that have triggered BOX_AT_PE1 but not yet triggered BOX_AT_PE2
Bit array (FIFO) in file #N99
- Only up bits up to to N99:0/[TRACKED_BOX_COUNT-1] represent tracked boxes
- N99 bit count = 64, whcih is much greater than the maximum possible value of TRACKED_BOX_COUNT
- Bit value is 1 if a tracked box is a [reject] and is to be diverted at PE2
- Bit value is 0 if a tracked box is [okay] and is not to be diverted at PE2
- FIFO content and shifting are controlled with BSR only, not FFL/FFU
- BSR always pushes 0-valued bit (status [okay]) at front of FIFO
  - Which will aways be well upstream of N99:0/[TRACKED_BOX_COUNT]

Implementation events
==================
At BOX_AT_PE2 rising edge
- Pop reject/okay bit
  - Set (or leave) DIVERTED bit state to (as) that popped bit
- Decrement TRACKED_BOX_COUNT
At PE1 rising edge
- Ensure bit N99:0/[TRACKED_BOX_COUNT] has same current value as REJECT_BOX_AT_PE1
- Increment TRACKED_BOX_COUNT
 
Implementation assumptions
=======================
There is adequate physical space between boxes, so there is exactly 1 rising edge per box at photoeyes, both at PE1, and at PE2
No boxes are added or subtracted between PE1 and PE2, nor coffee cups blocking either PE