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Unformatted text preview: CITY UNIVERSITY OF HONG KONG Course code & title : EE20200 Logic Circuit Design
Session : Semester A, 199899
Time Allowed : Two hours This paper has EIGHT pages. (Including this page and one page of Appendix) Course, code & title : EE20200 Logic Circuit Design Instructions to candidates:
1. This paper contains TWO parts. 2. Students MUST answer all the questions in Part A. And choose TWO questions
from Part B. 3. Start a NEW page for EACH QUESTION. E— Materials, aids & instruments to be used during examination: Nonprogrammable portable battery operated calculator PART A (Answer all questions in this part  total of 40 marks) Question 1
Use Boolean algebraic techniques to simplify the following Boolean equation.
(a®b+55+ab)(5+abc+5c+513)
[6 marks] Question 2 Figure Q2 shows a circuit which implements the function f (A2, AHAO) = Y. Tabulate the
truth table of the circuit and ﬁnd the minimum SOP expression of Y. [8 marks]
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Figure Q2 Question 3 Find the minimum SOP expression of the following Boolean function and use PLA to design
the circuit. F(a, b, c, d)=HM(1, 4, 5, 6, 9,11)+ 2d(10,13, 14,15)
[9 marks] Question 4 Use implication table method to obtain a reduced state table with minimum number of states
from the following state table. NS/Output "U
(/2 mommoouw [9 marks] Question 5 Figure Q5a shows a sequential circuit with an input x and output Q2. Assuming that the
initial conditions for x, Q and Q2 are all Os, complete the timing diagram shown in Figure
QSb for the given input x. (You do not need to consider the propagation delay time. You
can use the timing diagram given in Appendix A to answer this question and attach it to your
answer book.) [8 marks] Figure Q5b PART B (Choose two questions to answer  30 marks for each question)
Question 6 Figure 6 shows a block diagram for a motor control system. Switch S 1 controls the
lubrication pump motor, M1. Switch S; controls the conveyor motor, M2. Switch 83 controls
the band saw motor, M3. Switch 84 controls the cross—cut saw motor, M4. The motor that provides for lubrication of the conveyor (M1) must be on when the conveyor
is on. The motor that drives the conveyor (M2) is on only when both switches SI and 82 are
on. The band saw and the crosscut saw motors require no lubrication, but they must never
be on at the same time. If switches 83 and S4 are both turned on at the same time, the system
must be completely shut down, including the conveyor and lubrication motors. Also, the
cross—cut saw and the conveyor cannot be on at the same time. The control logic controls the motors to prevent any of the unallowed conditions from occurring. A logic 1 represents a ON
condition of both S,~ and M. Motors S1
Motor Conveyor
From S2 drive
switches interface Band saw Crosscut
aw Figure Q6. (a) Tabulate the tmth table of the control unit.
[6 marks] (b) Find the Boolean expression of M1, M2, M3 and M4 . Your result should be in
minimum SOP form. [8 marks] (c) Design the control logic of M1 using NAND gates only. Draw the circuit.
[5 marks] (d) Use a 4to1 lines mutliplexer and suitable logic gates to implement the control logic
of M1. Draw the circuit. [6 marks] (e) The input of an inverter (74805) with opencollector output is used to connect to M .
A LED circuit is required to connect to the output of 74805 to indicate a ON
condition of M. For a supply voltage of 5V, ﬁnd the minimum value of a pull—up
resistor and draw the LED circuit. The electrical speciﬁcations of 74805 and LED are
given in Table Q6. [5 marks] Question 7 Figure Q7 shows a bidirectional universal shift register and its functional table is given in
Table Q7. Figure Q7 Table Q7 a. The above register can be used to perform serial data to parallel data conversion.
Complete the timing diagram given in Appendix A according to the following steps for
each serialtoparallel conversion. (You must consider the propagation delay time
when completing the timing diagram.) i. Clear the output;
ii. Start to shift the 4 bits serial data to the register;
iii. Hold the data for one clock cycle;
iv. Repeat step i.
[10 marks] b. Draw the circuit diagrams of a basic ring counter and a Johnson counter using the above
register and suitable logic gates. Brieﬂy describe the initialization steps for both
counters. (You must show all the pin connections including external inputs.) [10 marks] c. i. Draw a circuit diagram of a 4bit serial adder based on the above registers, a full
adder module and other logic components. Brieﬂy describe the functions of the
components used in your design. [7 marks]
ii. Brieﬂy describe the setting of control signals and their associated functions. [3 marks] Question 8 A serial data line is connected to an input of a synchronous sequential circuit. This circuit is
used to count the number of Is of the latest three consecutive bits received from the serial
input x. This circuit has two output bits yLyo to represent the number of 1s. Table Q8 shows
the state assignment of this counting circuit. The threebit state assignment represents the
three consecutive bits received and the leftmost digit represents the latest binary data received
from the serial input x. State State Assignment Assignment S4 100
SS 101
S6 110
S7 111
Table Q8
a. Use Moore machine to design a state diagram that describe the operation of the counting
circuit.
[7 marks]
b. Tabulate a state table from (a).
[3 marks] c. Use JK ﬂip—ﬂop(s) and other necessary logic gates to design the counting circuit. Draw the circuit.
[16 marks] d. Devise a simple circuit from the output circuit of part (c) to produce an even parity bit according to the received three consecutive bits.
[4 marks] ***** END ***** Agpendix A Student N 0: Programme: Timing diagram for Question 5: Mode SO
Control
Inputs S1 clear
Serial Ri Data
Inputs 03 '
Outputs 02
Q1 L.' ...
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 Spring '07
 VanCWTing

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