# Digital Logic Exam

Sample Digital Logic Exam
Items in green have not been covered as of 11/7, and will not be included on
the midterm. Of course they will be fair game for the final.
Section 1. For the following problems, one or more of the statements may be correct.
Circle all that are correct, don’t circle any that are incorrect.
1)
(4) Which of the following statements are true?
a. Outputs of combinational circuits do not depend on previous events.
b. Outputs of sequential circuits do not depend on previous events.
c. Sequential logic devices contain memory or state storage information.
d. Sequential logic devices’ outputs are dependent on current inputs only.
2)
(4) Which of these devices are sequential?
a. Exclusive-Or gate
b. Multiplexer
c. Decoder
d. Ripple counter
3)
(4) Why do we make it a general practice to debounce switches?
a. Because it provides a faster clock signal than without debouncing.
b. Because contacts on switches often produce electrical glitches when
switching from one state to another, resulting in multiple spurious
transitions.
c. Because a manual clock switch which is not debounced can lead to
unpredictable results.
d. Because the rubber insulator on switches may leak current.
4)
(4) State machines:
a. Are composed of both sequential circuits and combinational circuits.
b. Are composed of only sequential circuits.
c. Are composed of only combinational circuits.
d. Are a way of controlling a sequence of events based on the status of
various inputs.
5)
(4) Regarding bits, there are
a. 4 in a nibble
b. 2 in a nibble
c. 4 in a byte
d. 16 in a byte
6)
(4) Which of these Boolean equations are correct?
a. A  1  A
b. A 1  A
c. A 1  A
d. AB  A  B
7)
(4) Which of these combinational logic devices are considered to be
“universal” gates?
a. Multiplexers
b. NANDs
c. XORs
d. NORs
8)
(4) Which of these statements are true?
a. A minterm equation is an unreduced sum-of-products Boolean expression.
b. A minterm equation requires the least number of logic gates when
implemented.
c. A maxterm equation is an unreduced sum-of-products Boolean expression.
d. A maxterm equation is an unreduced product-of-sums Boolean expression.
9)
(4) Gray codes
a. are used to express negative Boolean numbers.
b. are used to order the cells in a Karnaugh map.
c. change only in 1 bit in sequential numbers.
d. make a halftone image when printed.
10)
(4) Shift registers may be used to
a. convert a serial data stream to parallel.
b. convert a parallel data stream to serial.
c. multiply a binary number by 2.
d. divide a binary number by 2.
Section 2. In the space provided, answer the following questions. Put a box around your
11)
(4) What is 110101112 in decimal? (show your work)
12)
13)
(4) What is 9110 in binary? (show your work)
14)
(4) Give the negative (2’s complement) of this 8-bit signed quantity:
Section 3. In the space provided, answer the following questions.
15)
(4) In order to assure that a signal is stable at the input to a sequential circuit,
it is often necessary to delay a clock signal slightly. What is a simple way to
accomplish this delay with digital logic components?
16)
(10) Write a minimal Boolean equation for this Karnough map.
17)
AB
CD
00
01
11
00
1
1
X
0
01
0
1
1
0
11
0
0
0
1
10
X
1
0
1
10
(10) Write a minimal Boolean equation for this 3-bit system: (hint: make the
Karnough map)
1 for {0,1,2,4,6 }
F ( A, B, C )  {
0 otherwise
18)
(10) Give a truth table for
A
X
B
C
Y
19)
(10) Give the transition table and the state output table for the following state
machine.
X
clk
D
SET
CLR
Q
Q
Z