The object of this assignment is to write a program that will drive the Elevator from one floor to the other.

The list of inputs and outputs are as follows.

IP$0$ External Elevator call button

OP$0$ Elevator motor UP

IP$1$ Elevator on level $2$ sensor

OP$1$ Elevator motor DOWN

IP$2$ Elevator on level $1$ sensor

OP$2$ Elevator door motor OPEN

IP$3$ Internal Elevator button

OP$3$ Elevator door motor CLOSE

IP$4$ Door obstruction sensor

OP$4$ Alarm

IP$5$ Lift door closed sensor IP$6$ Lift stuck alarm button

The operation of the elevator is as follows: Assuming that the elevator is at level $1,($IP$2$ energised$),$ there are two options;

$1.$ The elevator could be called from level $2$ by the external call button IP$0$ being pressed. The elevator doors should then close via OP$3,$ once closed IP$5($doors closed sensor$)$ should energise. At this point the elevator should start to move up via OP$0($elevator motor UP$).$ Once the elevator reaches level $2,$ IP$1$ would energise and this should stop the elevator motor. After a short pause the elevator doors should open via OP$2.$

$2.$ The elevator could also be operated from level $1$ via the internal button IP$3.$ The elevator doors would then close $($OP$3)$ energising IP$5($doors closed sensor$).$ The elevator should then move up to level $2$ via OP$0.$ Once level $2$ sensor IP$1$ energises, the elevator doors can then open via OP$2.$

Note: When the elevator is on level $2$ it will be necessary to drive the elevator down to level $1$ using OP$1($elevator motor DOWN$).$

Other facilities:

$($a$)$ Alarm: Should the elevator get stuck between floors or the doors not open, then by pressing IP$6$ the alarm OP$4$ should operate.

$($b$)$ Door Obstruction: If the doors are obstructed the elevator motors OP$0$and OP$1$ should be disabled. You can test this by clicking on the box and obstructing the doors.

When writing the program use the inputs and outputs as listed, use any flags, timers, and counters as required. As usual use the comments facilities to document your program.