Elevator Control System（1）

电梯问题作业，第一篇
The general requirement is to design and implement a program to schedule and control four elevators in a building with 40 floors. The elevators will be used to carry people from one floor to another in the conventional way.

Efficiency: The program should schedule the elevators efficiently and reasonably. For example, if someone summons an elevator by pushing the down button on the fourth floor, the next elevator that reaches the fourth floor travelling down should stop at the fourth floor to accept the passenger(s). On the other hand, if an elevator has no passengers (no outstanding destination requests), it should park at the last floor it visited until it is needed again. An elevator should not reverse its direction of travel until its passengers who want to travel in its current direction have reached their destinations. (As we will see below, the program cannot really have information about an elevator's actual passengers; it only knows about destination button presses for a given elevator. For example, if some mischievous or sociopathic passenger boards the elevator at the first floor and then presses the destination buttons for the fourth, fifth, and twentieth floor, the program will cause the elevator to travel to and stop at the fourth, fifth, and twentieth floors. The computer and its program have no information about actual passenger boardings and exits.) An elevator that is filled to capacity should not respond to a new summon request. (There is an overweight sensor for each elevator. The computer and its program can interrogate these sensors.)
Destination button: The interior of each elevator is furnished with a panel containing array of 40 buttons, one button for each floor, marked with the floor numbers (1 to 4 These destination buttons can be illuminated by signals sent from the computer to t panel. When a passenger presses a destination button not already lit, the circuitry behind the panel sends an interrupt to the computer (there is a separate interrupt for each elevator). When the computer receives one of these (vectored) interrupts, its program can re the appropriate memory mapped eight-bit input registers (there is one for each interrupt hence one for each elevator) that contains the floor number corresponding to the desti tion button that caused the interrupt. Of course, the circuitry behind the panel writes t floor number into the appropriate memory-mapped input register when it causes the v tored interrupt. (Since there are 40 floors in this application, only the first six bits of ea input register will be used by the implementation; but the hardware would support a bui ing with up to 256 floors.)
Destination button lights: As mentioned earlier, the destination buttons can be illu nated (by bulbs behind the panels). When the interrupt service routine in the progr receives a destination button interrupt, it should send a signal to the appropriate panel illuminate the appropriate button. This signal is sent by the program's loading the number the button into the appropriate memory-mapped output register (there is one such regis for each elevator), The illumination of a button notifies the passenger(s) that the syst has taken note of his or her request and also prevents further interrupts caused by ad tional (impatient?) pressing of the button. When the controller stops an elevator at a floor should send a signal to its destination button panel to turn off the destination button for t floor.

Floor sensors: There is a floor sensor switch for each floor for each elevator sh When an elevator is within eight inches of a floor, a wheel on the elevator closes the swi for that floor and sends an interrupt to the computer (there is a separate interrupt for the of switches in each elevator shaft). When the computer receives one of these (vectore interrupts, its program can read the appropriate memory mapped eight-bit input regis (there is one for each interrupt, hence one for each elevator) that contains the floor numb corresponding to the floor sensor switch that caused the interrupt.
Arrival lights: The interior of each elevator is furnished with a panel containing one ill minable indicator for each floor number. This panel is located just above the doors. T purpose of this panel -is to tell the passengers in the elevator the number of the floor which the elevator is arriving (and at which it may be stopping). The program should illu nate the indicator for a floor when it arrives at the floor and extinguish the indicator for floor when it leaves a floor or arrives at a different floor. This signal is sent by the progra loading the number of the floor indicator into the appropriate memory-mapped output regi ter (there is one register for each elevator).
Summons buttons: Each floor of the building is furnished with a panel containing su mon button(s). Each floor except the ground floor (floor 1) and the top floor (floor 40) is f nished with a panel containing two summon buttons, one marked UP and one mark DOWN. The ground floor summon panel has only an UP button. The top floor summ panel has only a DOWN button. Thus, there are 78 summon buttons altogether, 39 UP b tons and 39 DOWN buttons. Would-be passengers press these buttons in order to summ an elevator. (Of course, the would-be passenger cannot summon a particular elevator. T scheduler decides which elevator should respond to a summon request.) These summ buttons can be illuminated by signals sent from the computer to the panel. When a passenger presses a summon button not already lit, the circuitry behind the panel sends a vectored interrupt to the computer (there is one interrupt for UP buttons and another for DO buttons). When the computer receives one of these two (vectored) interrupts, its program can read the appropriate memory mapped eight-bit register that contains the floor number corresponding to the summon button that caused the interrupt. Of course, the cir cuitry behind the panel writes the floor number into the appropriate memory-mapped inpu register when it causes the vectored interrupt.
Summon button lights: The summon buttons can be illuminated (by bulbs behind the panels). When the summon button interrupt service routine in the program receives an UP or DOWN button vectored interrupt, it should send a signal to the appropriate panel to illuminate the appropriate button. This signal is sent by the program's loading the number of the button in the appropriate memory-mapped output register, one for the UP buttons and one for the DOWN buttons. The illumination of a button notifies the passenger(s) that the system has taken note of his or her request and also prevents further interrupts caused by additional pressing of the button. When the controller stops an elevator at a floor, it should send a signal to the floor's summon button panel to turn off the appropriate (UP or DOWN) button for that floor.

Elevator motor controls (Up, Down, Stop): There is a memory-mapped control word for each elevator motor. Bit 0 of this word commands the elevator to go up, bit 1 commands the elevator to do down, and bit 2 commands the elevator to stop at the floor whose sensor switch is closed. The elevator mechanism will not obey any inappropriate or unsafe command. If no floor sensor switch is closed when the computer issues a stop signal, the elevator mechanism ignores the stop signal until a floor sensor switch is closed. The computer program does not have to worry about controlling an elevator's doors or stopping an elevator exactly at a level (home) position at a floor. The elevator manufacturer uses conventional switches, relays, circuits, and safety interlocks for these purposes so that the manufacturer can certify the safety of the elevators without regard for the computer controller. For example, if the computer issues a stop command for an elevator when it is within eight inches of a floor (so that its floor sensor switch is closed), the conventional, approved mechanism stops and levels the elevator at that floor, opens and holds its doors open appropriately, and then closes its door. If the computer issues an up or down command during this period (while the door is open, for example), the manufacturer's mechanism ignores the command until its conditions for movement are met. (Therefore, it is safe for the computer to issue and up or down command while an elevator's door is still open.) One condition for an elevator's movement is that its stop button not be depressed. Each elevator's destination button panel contains a stop button. This button does not go to the computer. Its sole purpose is to hold an elevator at a floor with its door open when the elevator is currently stopped at a floor. A red emergency stop switch stops and holds the elevator at the very next floor it reaches irrespective of computer scheduling. The red switch may also turn on an audible alarm. The red switch is not connected to the computer.
Target machine: The elevator scheduler and controller may be implemented for any contemporary microcomputer capable of handling this a lication.