Scenario

A building has a single elevator at the atrium, serving 4 floors.

Someone on the first (ground) floor who wants to ascend pushes a single button ("Up") to call the elevator. Similarly, someone on the 4th floor who wants to descend pushes a single button ("Down"). These two floors each have no other option.

Someone on the 2nd or 3rd floor could push either an Up or Down button to indicate his or her desired direction of travel.

Once on the elevator, the passenger can push a button for the desired arrival floor (1, 2, 3, 4). A passenger can also press/hold a "Door Open" button to signal the doors not to close.

The elevator has a safety interlock system. Any floor's door will not open unless the elevator is stopped at that floor. Similarly, the elevator's inside door will not open unless the elevator is stopped at a floor.If the elevator is going Up and there are any requests for floors above its current location, the elevator keeps moving in an upward direction until either all upward requests are satisfied or it has arrived at the 4th floor.

Similarly,if the elevator is going Down and there are any requests for floors below its current location, the elevator keeps moving in an downward direction until either all downward requests are satisfied or it has arrived at the 1st floor.The elevator will not reverse direction unless all requests related to the (untraveled portion of the) direction it was traveling have been satisfied.

Once a request has been handled, it may need to be cleared so later requests like it can be recognized.

Task

To analyze the description above and come up with a reasonable, first-cut design for controlling the elevator. Develop the necessary logic as one or more Truth Tables. Then derive the needed Logic Circuit(s), documented either as Sum-of-Products equation(s) or as Logic Circuit diagram(s).