The block diagram for an elevator controller for a

Chapter 16, Problem 16.29

(choose chapter or problem)

The block diagram for an elevator controller for a two-floor elevator follows. The inputs \(F B_1\) and \(F B_2\) are 1 when someone in the elevator presses the first or second floor buttons, respectively. The inputs \(C A L L_1\) and \(C A L L_2\) are 1 when someone on the first or second floor presses the elevator call button. The inputs \(F S_1\) and \(F S_2\) are 1 when the elevator is at the first or second floor landing. The output UP turns on the motor to raise the elevator car; DOWN turns on the motor to lower the elevator. If neither UP nor DOWN is 1 , then the elevator will not move. \(R_1\) and \(R_2\) reset the latches (described below); and when DO goes to 1 , the elevator door opens. After the door opens and remains open for a reasonable length of time (as determined by the door controller mechanism), the door controller mechanism closes the door and sets DC = 1. Assume that all input signals are properly synchronized with the system clock.

(a) If we were to realize a control circuit that responded to all of the inputs \(F B_1, F B_2, C A L L_1, C A L L_2, F S_1, F S_2\), and DC, we would need to implement logic equations with nine or more variables (seven inputs plus at least two state variables). However, if we combine the signals \(F B_i\) and \(C A L L_i\) into a signal \(N_i\) (i = 1 or 2 ) that indicates that the elevator is needed on the specified floor, we can reduce the number of inputs into the control circuit. In addition, if the

                                                         

signal \(N_i\) is stored so that a single pulse on \(F B_i\) or \(C A L L_i\) will set \(N_i\) to 1 until the control circuit clears it, then the control circuit will be simplified further. Using a D flip-flop and a minimum number of added gates, design a storage circuit that will have an output 1 when either input \(\left(F B_i\right.\) or \(\left.C A L L_i\right)\) becomes 1 and will stay 1 until reset with a signal \(R_i\).

(b) Using the signals \(N_1\) and \(N_2\) that indicate that the elevator is needed on the first or second floor (to deliver a passenger or pick one up or both), derive a state graph for the elevator controller. (Only four states are needed.)

(c) Realize the storage circuits for \(N_1\) and \(N_2\) and the state graph.