2. Design an elevator controller for a simple two-floor elevator. On the 1st flo

Question

2. Design an elevator controller for a simple two-floor elevator. On the 1st floor, there is only an up button, and on the 2nd floor, there is only a down button. Within the elevator, there are two buttons: door open and door close. There are sensors that detect the status of door and location of the elevator. (a) Draw a block diagram with all the necessary input and output. State all your assumptions. (b) Draw the state transition diagram using either Moore or Mealy style, and explain the meaning of each state.

Explanation / Answer

Storing Mechanism: 1. Loading and unloading of the requests should be independent of clock hence a LATCH IS USED AS STORING MECHANISM 2. If we reach floor we should unload the request for that floor. 3. D_LATCH: Standard d-latch is used If control = 0, no change in output If control = 1, the value at input terminal of the latch is loaded The input of the latch is provided as (Floor loading request +the request in that floor)* & (unload request) 4. With two 3 * 8 decoders, load and unload decoders and the above D LATCH and also 1 latch per floor we implement Storing mechanism 5. Clear is uses to preset the value at the output terminal of latch to 0. so that an unknown value is not there at output terminal of the latch Control Circuit: 1. 1 hot assignment is used i.e. 1 flip flop per state is used at any time exactly one state was 1 and rest all were 0. 2. State transitions are synchronized with clock pulse 3. Time period of the clock pulse = time taken by lift to go from 1 floor to another. Inputs of the control circuit 1 Supply: If supply is 0 then no state transition takes place it denotes the fact that if Power goes of then lift remains at its present location and whenever we switch on the supply control resumes from the state where the control stopped originally. 2 Clear: It is used to initially load the T [0] state to the flip flop outputs. It is also used to initialize direction of lift. 3. Pfloor: value of the present floor. 4. queue_op: stores the request of the unvisited floors. Note: States are a direct consequence of state diagram, since we are using One hot assignment Output module T: The value of the states depending on the state as evaluated by control logic, the present floor is either incremented or decremented or remains same. Unload reachfloor: Depending on whether we have reached our destination or not, we unload that floor's entry from the queue. Other important variables: 1. xl: It is 1 denotes that there is at least one unattended request for a floor less than equal to it .It is 0 denotes that there is no request that is unattended at a floor less than equal to it . 2. xr: It is 1 denotes that there is at least one unattended request for a floor greater than equal to it . It is 0 denotes that there is no request that is unattended at a floor greater than equal to it.

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