MASSACHVSETTS INSTITVTE OF TECHNOLOGY 6.004 ...

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page 4 of 8 Problem Set #2ABThe circuit diagramof an intermittent button.When the button is pressedA is connected to B.B. Draw a circuit diagram for a circuit that records the press of the elevator lobby button(i.e. remembers the press of the button until reset by the controller). Output a 1 whenthe button is pressed, 0 otherwise. We can use the outputs of these button circuits toconstruct the FBUT input to your controller which has 4 bits, one for each floor. The 4RESET lines are collected into a new output for the controller, FRESET.C. In the next sections it will prove handy to have a set of lines which tell us the floor thatthe elevator is on: CFLOOR (the i-th bit of CFLOOR equals 1 if and only if the elevatoris on the ith floor). CFLOOR is similar to FLOOR. How can you computeCFLOOR from FLOOR using Boolean logic?Let’s design a simple scheme to get the Helevator moving in the right direction. For thetime being we will ignore the buttons in the elevator and concentrate only on the elevatorlobby buttons. We will construct a Boolean expression to drive the MOVE and U/D inputsso that the Helevator begins to move in the correct direction when a button is pushed. Donot worry about the state of the doors for now.To solve this problem (and the rest in the problem set) we will find it easier to use an overloadedset of Boolean Algebraic operations. Assume that AND, OR and NOT can operatenot just on 1 bit Boolean variables but also on N bit boolean variables (in our case 4 bitBoolean words). So FOO+BAR is the OR of each of the bits of FOO and BAR separately.OR returns an N bit word (i.e. the 2nd bit of the result is the OR of the 2nd bit of FOO andthe 2nd BAR). In addition the you may use a new unary Boolean function called ZERO?which returns a 1 if all of the bits in its operand is zero (i.e. (ZERO? FBUT) returns a 1 ifand only if any of the floor buttons have been pressed). You can still write expressions thatinvolve a single bit of a word by referring to it with a subscript (e.g. FBUT 1 ).One possible solution is:MOVE = (NOT (ZERO? FBUT))(The elevator will move whenever one of the buttons is pressed)U/D = (ZERO? (AND FLOOR FBUT))((AND FLOOR FBUT) will be non-zero whenever a button has been pressed on a floorbelow the current floor, in that case we should head down.)D. Draw a diagram of a simple circuit that can implement ZERO? on 4 bit operands.2/11/97 <strong>6.004</strong> Spring 1996
Problem Set #2 page 5 of 8E. The design spec states 1) if no button is pressed the elevator should sit at some floorwith its doors wide open and 2) the elevator should not move if the doors are open. Toensure this behavior you will need to create a new expression for O/C and possiblymodify the expressions for MOVE and U/D. (HINT: You will need to use the FRESETlines to solve this problem.)(Special Note: The Ophus helevator will never stop between floors. As a result we willnot need to worry about stopping the elevator precisely on the floor.)F. While the current controller gets the elevator moving in the right direction it does notstop at the calling floor. Modify the expressions for MOVE, U/D, and O/C so that theelevator will stop at all the floors where the lobby button has been pressed and opensits doors to accept passengers.G. If we are not careful we are going to end up with a lot of unhappy people on our elevator.Currently the floor buttons inside the elevator are not hooked up. Suggest a simplecircuit that can be used to modify the current controller so that it will drop people offon the floor they selected. The existing controller need not change too much.The design is nearly complete, it needs only to be implemented. Take a minute to thinkabout the importance of hazards in this design. Assume that the worse case glitches are10ns. Since our device is a large physical system a tiny glitch should be unnoticeable.H. On the last page of the problem set is a diagram of the PLA that you will use to implementthe Helevator controller (see section 5.2 of the text for a description of PLAs).Some of the inputs and outputs of the PLA are already labelled. Note that among theseare the CFLOOR inputs. You do not need to compute these from FLOOR in the PLA ithas already been done for you. Also note that some of the inputs are labelled BUT--assume that your solution to Part G is used to compute these inputs. There are a fewextra input and output lines that you can use as you see fit. If it makes your design anysimpler, you may invert invert any output of the PLA.Remember that your design will consist of a set of PULLDOWNs in the AND PLANEand a set of PULLUPs in the OR PLANE. In the text these are shown as AND and ORgates. Don’t worry about drawing these pictures. A single blackened dot will suffice.Be sure to add some documentation to vertical lines in the PLA (these correspond toproducts in your controller). In many cases a single comment will suffice for manylines.Note: The Remainder of the Problem Set is to be done without collaboration.<strong>6.004</strong> Spring 1997 2/11/97
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