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CS302 Midterm Solved MCQ’s &
Subjective by Haya
Question: How can a D flip-flop can be made to toggle?
Answer: A D flip-flop can be made to toggle by connecting Q' to D.
Question: What is the difference between a counter and shift
Answer: A counter has a specified sequence of states, but a shift
register does not.
Question: How many outputs and inputs GAL22V10 have?
Answer: The GAL22V10 has 22 inputs and 10 outputs. V=variable
Question: What is an equivalent representation for the Boolean
expression A' + 1
Answer: From the Boolean property A + 1 = 1, let A = A'=> A' + 1=1
Question: What is K-map and why we used it?
Answer: A Karnaugh map provides a pictorial method of grouping
Expressions with common factors and therefore eliminating unwanted
variables. The
Karnaugh map can also be described as a special arrangement of a
truth table.
Question: Each stage in a shift register represents how much
storage capacity?
Answer: one bit
Question: what are PLD's? How are they classified.
Answer: The programmable logic devices (PLD's) are used in a lot of
These replaced SSI (Small Scale Integration) and MSI (Medium
Scale Integration) circuits,
due the space saving and reduce the number of devices in a certain
design. A PLD is
Made of a matrix of AND and OR gates, that can be programmed to
obtain certain logic functions. There are four types of devices that can
be classified as PLD's:
a)The Programmable Read-Only Memory, PROM.
b)The Programmable Logic Array , PLA.
c)The Programmable Array Logic, PAL.
d)The Generic Array Logic, GAL.(same as PAL with OR gate fixed)
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Question: What are Flip-flops?
Answer: The memory elements in a sequential circuit are called flip-
flops. A flip-flop
circuit has two outputs, one for the normal value and one for the
complement value of
the stored bit.
Question: If an S-R latch has a 0 on the S input and a 1 on the R
input and then
the R input goes to 0, then what the latch will be?
Answer: The latch will be in reset condition. See the table.
Question: In a 4-bit Johnson counter sequence there are a total
of how many states, or bit patterns?
Answer: eight bit patterns.
Question: Explain the truth table and timing diagram of Gated S-
R latch and
Gated D latch in detail.
Answer: The logic symbol for the S-R flip-flop is shown here and its
outlined in Table below.
Now we examine the output waveforms from the S-R flip-flop given
the inputs. Assume
that Q is HIGH initially.
The logic symbol for the D flip-flop is also shown below and its
operation outlined in the
Table. Notice that this flip-flop only has one input in addition to the
clock called the Dinput.
Note that whatever is on the D-input when the trigger occurs is output
at Q.
Notice that a D flip flop can be made from a S-R flip flop by ensuring
that the S and R
outputs are the complement of each other at all times.
Question: What is the difference between asynchoronous and
Answer: Synchronous refers to the situation when all the interrelated
devices have
some common and fixed time relationship. Whereas in Asynchronous
refers to the
situation when the situation is opposite.
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In Synchronous counters all the flip-flops have same clock pulse and
in Asynchronous
counters flip-flops does not change state at the exactly same time
because they don't
have common clock pulse.
Question: What is meant by D in gated D latch and what is the
fuction of this D
input. What is the basic difference between latchs and flip-flops?
Answer: The 'D' in 'Gated D Latch' stands for 'Data'.Unlike 'S-R Latch'
Gated D Latch
has only one input ,which is D(data) Input. Whcih will give the output
of the latch
depending on the 'EN' (enable) state of the latch. To understand
latches and flip-flops lets
consider a basic fact about the whole DLD
In the same way that gates are the building blocks of combinatorial
circuits, latches and
flip-flops are the building blocks of sequential circuits. While gates
had to be built
directly from transistors, latches can be built from gates, and flip-flops
can be built from
Both latches and flip-flops are circuit elements whose output depends
not only on the
current inputs, but also on previous inputs and outputs. The
difference between a latch
and a flip-flop is that
a latch does not have a clock signal, whereas a flip-flop always does
Latches are asynchronous, which means that the output changes
very soon after the
input changes. A flip-flop is a synchronous version of the latch.
Question: I cannot understand the timing diagram for the master
slave flip flop.
Answer: A master-slave flip-flop is constructed from two separate flip-
flops. One
circuit serves as a master and the other as a slave. The logic diagram
of an SR flip-flop is
shown here. The master flip-flop is enabled on the positive edge of
the clock pulse CP and
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the slave flip-flop is disabled by the inverter. The information at the
external R and S
inputs is transmitted to the master flip-flop. When the pulse returns to
0, the master flipflop
is disabled and the slave flip-flop is enabled. The slave flip-flop then
goes to the same
state as the master flip-flop.
Logic diagram of a master-slave flip-flop
The timing relationship is also shown here and is assumed that the
flip-flop is in the
clear state prior to the occurrence of the clock pulse. The output state
of the master-slave
flip-flop occurs on the negative transition of the clock pulse. Some
master-slave flip-flops
change output state on the positive transition of the clock pulse by
having an additional
inverter between the CP terminal and the input of the master.
Timing relationship in a master slave flip-flop.
Question: I am not able to understand the truth table and timing
diagram of " S-R
Edge-trigged flip-flop, D edge-trigged flip-flop and J-K edge-
trigged flip-flop kindly
explain it in detail.
Answer: An edge-triggered flip-flop changes states either at the
positive edge (rising
edge) or at the negative edge (falling edge) of the clock pulse on the
control input.
The S-R, J-K and D inputs are called synchronous inputs because
data on these inputs
are transferred to the flip-flop's output only on the triggering edge of
the clock pulse. On
the other hand, the direct set (SET) and clear (CLR) inputs are called
inputs, as they are inputs that affect the state of the flip-flop
independent of the clock.
For the synchronous operations to work properly, these
asynchronous inputs must both
be kept LOW.
The basic operation of Edge-triggered S-R flip-flop is illustrated
below, along with the
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Truth table for this type of flip-flop. The operation and truth table for a
negative edge share triggered flip-flop are the same as those for a
positive except that the falling edge of the clock pulse is the triggering
edge.Note that the S and R inputs can be changed at any time when
the clock input is LOW or
HIGH (except for a very short interval around the triggering transition
of the clock)
Without affecting the output. This is illustrated in the timing diagram
While an Edge-triggered J-K flip-flop works very similar to S-R flip-
flop. The only
difference is that this flip-flop has NO invalid state. The outputs toggle
(change to the
opposite state) when both J and K inputs are HIGH. The truth table is
shown below.
The operations of an Edge-triggered D flip-flop are much simpler. It
has only one
input addition to the clock. It is very useful when a single data bit (0 or
1) is to be stored.
If there is a HIGH on the D input when a clock pulse is applied, the
flip-flop SETs and
stores a 1. If there is a LOW on the D input when a clock pulse is
applied, the flip-flop
RESETs and stores a 0. The truth table below summarize the
operations of the positive
edge-triggered D flip-flop. As before, the negative edge-triggered flip-
flop works the same
except that the falling edge of the clock pulse is the triggering edge.
Question: What is Multiplexer and what are its applications and
simplification using Multiplexer?
Answer: Multiplexer is a digital circuit with multiple signal inputs, one
of which is
selected by separate address inputs to be sent to the single output.
The multiplexer
circuit is typically used to combine two or more digital signals onto a
single line, by
placing them there at different times. Technically, this is known as
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Input A is the addressing input, which controls which of the two data
inputs, X0 or X1,
will be transmitted to the output. If the A input switches back and forth
at a frequency
more than double the frequency of either digital signal, both signals
will be accurately
reproduced, and can be separated again by a demultiplexer circuit
synchronized to the
This is not as difficult as it may seem at first glance; the telephone
network combines
multiple audio signals onto a single pair of wires using exactly this
technique, and is
readily able to separate many telephone conversations so that
everyone's voice goes only
to the intended recipient. With the growth of the Internet and the
World Wide Web, most
people have heard about T1 telephone lines. A T1 line can transmit
up to 24 individual
telephone conversations by multiplexing them in this manner.
A very common application for this type of circuit is found in
computers, where
dynamic memory uses the same address lines for both row and
column addressing. A set
of multiplexers is used to first select the row address to the memory,
then switch to the
column address. This scheme allows large amounts of memory to be
incorporated into the computer while limiting the number of copper
traces required connecting that
memory to the rest of the computer circuitry. In such an application,
this circuit is
commonly called a data selector. Multiplexers are not limited to two
data inputs. If we
use two addressing inputs, we can multiplex up to four data signals.
With three
addressing inputs, we can multiplex eight signals.
Question: Explain S-R Latch? what do you mean by bi-stable
Answer: A bi-stable multivibrator has two stable states, as indicated
by the prefix bi
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in its name. Typically, one state is referred to as set and the other as
reset. The simplest
bi-stable device, therefore, is known as a set-reset, or S-R, latch.
The Q and not-Q outputs are supposed to be in opposite states. I say
"supposed to"
because making both the S and R inputs equal to 1 results in both Q
and not-Q being 0.
For this reason, having both S and R equal to 1 is called an invalid or
illegal state for the
S-R multivibrator. Otherwise, making S=1 and R=0 "sets" the
multivibrator so that Q=1
and not-Q=0. Conversely, making R=1 and S=0 "resets" the
multivibrator in the opposite
state. When S and R are both equal to 0, the multivibrator's outputs
"latch" in their prior
By definition, a condition of Q=1 and not-Q=0 is set. A condition of
Q=0 and not-Q=1 is
reset. These terms are universal in describing the output states of
any multivibrator
circuit. So A bistable multivibrator is one with two stable output states.
In a bistable
multivibrator, the condition of Q=1 and not-Q=0 is defined as set. A
condition of Q=0 and
not-Q=1 is conversely defined as reset. If Q and not-Q happen to be
forced to the same
state (both 0 or both 1), that state is referred to as invalid. In an S-R
latch, activation of
the S input sets the circuit, while activation of the R input resets the
circuit. If both S
and R inputs are activated simultaneously, the circuit will be in an
invalid condition. A
race condition is a state in a sequential system where two mutually-
exclusive events are
simultaneously initiated by a single cause.
Question: What is meant by triggering or triggering edge of
clock pulse and
synchronous? also what is trigging transition of clock?
Answer: Generally the term 'synchronous' means "Moving or
changing at the same
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time". In our senario this term also holds the same meaning.
Here the two things which will change at the same time will be "Clock
(CLK or C )" and
the "output of the device". Means changes in the output occur with
synchronization with
Edge-Triggered devices changes staes either at the positive
edge(rising edge) or the
negative edge (falling edge) of the clock pulse and is sensative to its
inputs only at the
these two (negative or positive) edges,which in technical terms is
called 'Transition of the
By examining the picture below you will understand it completly.
Question: How to up and down the clock in J K flops plz explain
the example?
Answer: In J-K filp-flops the clock moves normaly as in other cases
no difference.The
clock pulse will change its state after the specified intervals(usually
defined in 'nano
seconds'(ns) ) to either UP i.e '1' or DOWN i.e '0'.No.11
Question: For BCD numbers that add up to an invalid BCD
number or generate a
carry, the number 6 (0110) is added to the invalid number, why ?
Answer: These binary numbers are not allowed in the BCD code:
1010, 1011, 1100,
1101, 1110, 1111
Then, if the addition produces a carry and/or creates an invalid BCD
number, an
adjustment is required to correct the sum. The correction method is to
add 6 to the sum
in any digit position that has caused an error.
For example,
15 + 9 = 24
0001 0101 = 15
+ 0000 1001 = 9
0001 1110 = 1? (invalid 1110)
0001 1110 = 1? (invalid)
+ 0000 0110 = 6 (adjustment)
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0010 0100 = 24
Question: Why do we use +0V and +5V instead of +0V and +1V in
DLD, when it is
always '0' and '1' ?
Answer: In DLD, the circuits of logic gates (embedded in IC's) are
operated with +5
Volts input. That is why we refer to +5 V for these logic inputs. It is
considered as binary
1 when the +5V are applied to the logic gate, and binary 0 when 0 V
are applied to the
logic gate.
Question: What is BCD and how do we write them?
Answer: BCD (Binary-Coded Decimal) is a system for encoding
Decimal Numbers in
binary form to avoid rounding and conversion errors. In BCD coding,
each digit of a
decimal number is coded separately as a binary numeral. Each of the
decimal digits 0
through 9 is coded in four bits and for ease of reading, each group of
four bits is
separated by a space. This format, also called 8-4-2-1 after the
weights of the four bit
positions, uses the following codes:
0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
1001 = 9
Thus, the decimal number 12 is 0001 0010 in binary-coded decimal
Question: Where do we use Caveman Number System ?
Answer: Caveman Number System was introduced in old ages as
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representation of decimal number system. You do not need to study it
in detail, as it is
also mentioned that this system is not used anywhere now a
Question: What is Gray Code and how do we write them?
Answer: Gray Code is a binary sequence with the property that an
ordering of 2n
binary numbers such that only one bit changes from one entry to the
next. Gray codes
are useful in mechanical encoders since a slight change in location
only affects one bit.
Using a typical binary code, up to n bits could change, and slight
misalignments between
reading elements could cause wildly incorrect readings.
It is a number code where consecutive numbers are represented by
binary patterns that
differ in one bit position only.
Here you can see, for each number, there is a difference of 1
(addition or elimination of 1)
0000 =0
0001 =1
0011 =2 ,1 is added
0010 =3 , again change of 1, elimination of 1
0110 =4 ,addition of 1
0111 =5 ,again addition of 1
0101 =6 ,elimination of 1
0100 =7 ,elimination of 1
1100 =8 ,addition of 1
1101 =9 ,addition of 1
One way to construct a Gray code for n bits is to take a Gray code for
n-1 bits with each
code prefixed by 0 (for the first half of the code) and append the n-1
Gray code reversed
with each code prefixed by 1 (for the second half). This is called a
"binary-reflected Gray
code". Here is an example of creating a 3-bit Gray code from a 2-bit
Gray code. 00 01 11
A Gray code for 2 bits
000 001 011 010 the 2-bit code with "0" prefixes
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10 11 01 00 the 2-bit code in reverse order
110 111 101 100 the reversed code with "1" prefixes
000 001 011 010 110 111 101 100 A Gray code for 3 bits
Glossary (Updated Version)
ABEL : Advanced Boolean Expression Language; a software
compiler language for
SPLD programming; a type of hardware description language (HDL)
Adder : A digital circuit which forms the sum and carry of two or more
address : The location of a given storage cell or group of cells in a
memory; a unique
memory location containing one byte.
address bus : Generally, a one-way group of conductors from the
microprocessor to
memory, containing the address information.
Analog : A signal which is continuously variable and, unlike a digital
signal, does not
have discrete levels. (A slide rule is analog in function.)
Analog Computer : Computer which represents numerical quantities
as electrical
and physical variables. Solutions to mathematical problems are
accomplished by
manipulating these variables.
AND Gate : A basic logic gate that outputs a 1 only if both inputs are
a 1 , otherwise
outputs a 0. See also, NAND, NOR and OR.
Binary : The binary number system has only two digits - 0 and
Binary Code : A code in which each element may be either of two
distinct values (eg
the presence or absence of a pulse).
Binary Coded Decimal (BCD) : A coding system in which each
decimal digit from 0 to 9 is
represented by four bits.
Bit : A single digit of a binary number. A bit is either a one
represented by a voltage or
a zero represented by no voltage. The number 5 represented in 4 and
8 bit binary would
be 0101 and 00000101 respectively.
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Boolean Algebra : The algebra of logic named for George Boole.
Similar in form to
ordinary algebra, but with classes, propositions, yes/no criteria, etc
for variables rather
than numeric quantities. It includes the operators AND, OR, NOT, IF,
Cascade : To connect 'end-to-end' as when several counters are
connected from the
terminal count output of one counter to the enable input of the next
Clock : The device in a digital system which provides the continuous
train of pulses
used to synchronize the transfer of data. Sometimes referred to as
"the heartbeat."
CMOS : (Complementary Metal Oxide Semiconductor) An advanced
manufacturing process technology characterized by high integration,
low cost, low power
and high performance. CMOS is the preferred process for today's
high density ICs.
Combinational Logic : Logic circuits whose outputs depend only on
the present logic
inputs. These do not have any storage element.
Comparator : A digital circuit that compares the magnitudes of two
quantities and
produces an output indicating the relationship of the quantities.
Counter: A digital circuit capable of counting electronic events, such
as pulses, by
progressing through a sequence of binary states.
Data selector : A circuit that selects data from several inputs one at a
sequence and
places them on the output: also called a multiplexer.
Decoder : A logic function that uses a binary value, or address, to
select between a
number of outputs and to assert the selected output by placing it in its
active state.
Digital System : A system in which information is transmitted in a
series of pulses.
The source is periodically sampled, analyzed, and converted or
coded into numerical
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values and transmitted. Digital transmissions typically use the binary
coding used by
computers so most data is in appropriate form, but verbal and visual
must be converted. Many satellite transmissions use digital formats
because noise will
not interfere with the quality of the end product, producing clear and
Emitter: One of the three regions in a bipolar junction transistor.
Encoder: A digital circuit (device) that converts information to a coded
Even parity : The condition of having an even number of 1s in every
group of bits.
Exponent: The part of floating point number that represents the
number of places that
the decimal point (or binary point) is to be moved.
Fan in : The number of logic inputs into a logic gate.
Fan out : The number of logic inputs that can be driven by the output
of a logic gate.
flip-flop : A basic digital building block that, at its simplest, uses two
gates crosscoupled
so that the output of one gate serves as the input of the other. It is
capable of
changing from one state to another on application of a control signal,
but can remain in
that state after the signal is removed. It thus serves as a basic
storage element. Most flipflops
contain additional features to make them more versatile. Many digital
circuits, such
as registers and counters, are a number of flip flops connected
GAL: Generic array logic; an SPLD with a reprogrammable AND
array, a fixed OR array,
and programmable Output Logic Macro Cells No.14
Gate: The control terminal of a MOSFET, or alternately a basic digital
logic element, for
example an AND Gate, See also, OR, NAND, NOR.
Gate Array : An integrated circuit made up of digital logic gates that
are not yet
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connected. Typically gate arrays are fabricated up to the metal layers
and then a custom
metal mask is designed for a customer and used to connect the gates
into a customer
specific circuit.
Gray code: The mirror image of the binary counting code which
changes one bit at a
time when increasing or decreasing by one.
half-adder : A digital circuit that adds two bits and produces a sum
and output carry. It
cannot handle input carries.
High: A digital logic state corresponding to a binary "l."
High logic: In digital logic, the more positive of the two logic levels in a
binary system.
Normally, a high logic level is used to represent a binary 1 or true
IC : (Integrated Circuit) A single piece of silicon on which thousands
or millions of
transistors are combined. ICs are the major building blocks of modern
Inverter: In logic, a digital circuit which inverts the input signal, as for
changing a 1 to a 0. This is equivalent logically to the NOT function.
An inverter may also
serve as a buffer amplifier.
JK flip-flop: A type of flip-flop that can operate in the SET, RESET,
no-change, and
toggle modes.
Karnaugh map : An arrangement of cells representing the
combinations of literals in a
Boolean expression and used for a systematic simplification of the
Latch: A bi-stable digital circuit used for storing a bit.
LED: Light-Emitting Diode (component) Abbreviated LED. A
semiconductor diode,
generally made from gallium arsenide, that can serve as an infrared
or visible light
source when voltage is applied continuously or in pulses.
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Logic: One of the three major classes of ICs in most digital electronic
microprocessors, memory, and logic. Logic is used for data
manipulation and control
functions that require higher speed than a microprocessor can
Low: A logic state corresponding to a binary "0". Satellite imagery is
displayed on a
computer monitor by a combination of highs and lows.
Low logic : In digital logic, the more negative of the two logic levels in
a binary system.
In positive logic, a low-logic level is used to represent a logic 0, or a
not-true, condition.
Mantissa: The magnitude of a floating-point number.
MSI: Medium-scale integration' a level of fixed-function IC complexity
in which there
are 12 to 99 equivalent gates per chip.
Multiplexer: An electronic device normally used to scan a number of
input terminals and
receive data from, or send data to, the same. Multiplexers are
normally one of two types:
The cyclic type which continually and sequentially looks at each
input for a request to
send or receive data.
The random type which waits in a "rest" position until other circuitry
notifies it of a
request to receive or send data.
NAND gate : A logic circuit in which a LOW output occurs only if all
the inputs are HIGH.
NOR gate : A logic circuit which performs the OR function and then
inverts the result. A
NOT-OR gate.
NOT : The logical operator having that property which if P is a
statement, then the not of
P (P) is true if P is false, and the not of P (P) is false if P is true.
octal : Describes a number system with a base of eight.No.15
odd parity : The condition of having an odd number of 1s in every
group of bits.
OR gate : A multiple-input gate circuit whose output is energized
when any one or
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more of the inputs is in a prescribed state. Used in digital logic
overflow : The condition that occurs when the number of bits in a sum
exceeds the
number of bits in each of the numbers added.
PAL : Programmable array logic; an SPLD with a programmable AND
array and a fixed
OR array with programmable output logic.
parity : In relation to binary codes, the condition of evenness or
oddness of the
number of 1s in a code group.
parity bit : A bit attached to each group of information bits to make the
total number of
1s in a code group.
PLA : Plogrammable logic array; an SPLD with programmable AND
and OR arrays.
queue : A high-speed memory that stores instructions or data.
register : A digital circuit capable of storing and shifting binary
information; typically
used as a temporary storage device.
Shift : To move information serially right or left in a register(s).
Information shifted out of
a register may be lost, or it may be re-entered at the other end of the
Shift register : A shift register is an electronic device which can
contain several bits
of information. Shift registers are normally used to collect variable
input data and send
this data out in a predetermined pattern.
Sign bit : Computers generally indicate whether a number is positive
or negative by a
sign bit, which is usually located adjacent to the most significant
numerical digit. Usually
zero (0) is used for positive (+) and one (1) for negative (-).
Significant digit : A digit that contributes to the preciseness of a
number. The number
of significant digits is counted beginning with the digit contributing the
most value,
called the most significant digit, and ending with the one contributing
the least value,
called the least significant digit.
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toggle : The action of flip-flop when it changes state on each clock
Truth Table : A table that defines a logic function by listing all
combinations of
input values, and indicating for each combination the true output
TTL : Transistor-transistor logic; a class of integrated logic circuits
that uses bipolar
junction transistors.
Universal gate : Either a NAND or a NOR gate; The term universal
refers to the
property of a gate that permits any logic function to be implemented
by that gate or by a
combination of gates of that kind.
up/down counter : A counter that can progress in either direction
through a certain
VLSI : Very large-scale integration; a level of IC complexity in which
there are 10,000 to
99,000 equivalent gates per chip.
volatile : A term that describes a memory that loses stored data when
the power is
Weight: The value of digit in a number based on its position in the
Decoders: Decoder : A logic function that uses a binary value, or
address, to select
between a number of outputs and to assert the selected output by
placing it in its active
Multiplexer: An electronic device normally used to scan a number of
terminals and receive data from, or send data to, the same.
Multiplexers are normally
one of two types:
The cyclic type which continually and sequentially looks at each input
for a request to
send or receive data.
The random type which waits in a "rest" position until other circuitry
notifies it of a
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request to receive or send data.
Demultiplexer : A Multiplexer has several inputs. It selects one of the
inputs and routes
the data at the selected input to the single output. Demultiplexer has
an opposite
function to that of the Multiplexer. It has a single input and several
outputs. The
Demultiplexer selects one of the several outputs and routes the data
at the single input
to the selected output. A demultiplexer is also known as a Data
Some commonly used combinational functional devices are
Comparators, Decoders,
Encoders, Multiplexers and Demultiplexers.
Sequential Circuits:
Sequential logic and implementation Digital systems are used in vast
variety of industrial
applications and house hold electronic gadgets. Many of these digital
circuits generate an
output that is not only dependent on the current input but also some
previously saved
information which is used by the digital circuit.
Consider the example of a digital counter which is used by many
digital applications
where a count value or the time of the day has to be displayed. The
digital counter which
counts downwards from 10 to 0 is initialized to the value 10. When
the counter receives
an external signal in the form of a pulse the counter decrements the
count value to 9. On
receiving successive pulses the counter decrements the currently
stored count value by
one, until the counter has been decremented to 0. On reaching the
count value zero, the
counter could switch off a washing machine, a microwave oven or
switch on an airconditioning
Why S and R input of NAND based latch should not be at logic high
at same time?
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Thus, with S and R inputs both set to logic 1, the previous output
state is maintained.
If initially, the Q andQare at logic 1 and 0 respectively, setting S=1
and R=1 maintains
the same outputs. Similarly, if initially Q and Q are at logic 0 and 1
setting S=1 and R=1 maintains the same outputs.
2 input 4 bit multiplexer function table 3 marks
The MSI, 74X157 is a 2-input, 4-bit Multiplexer. This multiplexer has
two sets of 4-bit
inputs. It also has 4-bit outputs. The single select input line allows the
first set of four
inputs or the second set of 4-inputs to be connected to the output.
Thus four-bits of data from two sources are routed to the output. The
function table and the circuit of
the multiplexer are shown. table 18.1, figure 18.1
The multiplexer has two sets of 4-bit active-high inputs 1A, 2A, 3A,
4A and 1B, 2B,
3B, 4B respectively. The multiplexer has 4-bit active-high outputs 1Y,
2Y, 3Y 4Y. The
single select input allows either the 4-bit input A or the 4-bit input B to
be connected
to the 4-bit output Y.
The G active-low pin enables or disables the Multiplexer.
BCD to decimal conversion of three BCDs codes 3marks
Half adder explanation its function table Boolean expression and
circuit diagram
5 marks
Explain S-R latch in your own words
Mid-Term Past Papers (Updated Version)
Short Question (Set-1)
Question No: 18 ( Marks: 2 )
Provide some of the inputs for which the adjacent 1s detector circuit
have active high
The Adjacent 1s Detector accepts 4-bit inputs.
If two adjacent 1s are detected in the input, the output is set to high.
Some input combinations will be
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1. 0011,
2. 0110,
3. 0111,
4. 1011,
5. 1100,
6. 1101,
7. 1110 and
8. 1111
The output function is a 1.
Question No: 19 ( Marks: 2 )
Draw the Truth-Table of NOR based S-R Latch
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Circuit Diagram of NOR based S-R Latch.
NAND Based S-R Latch
Function Table:
Circuit Diagram:
Question No: 20 ( Marks: 3 )
For a two bit comparator circuit specify the inputs for which A >
1. 01 00,
2. 10 00,
3. 10 01,
4. 11 00,
5. 11 01
6. 11 10
Write a note on COMPARATOR
A comparator circuit compares two numbers and sets one of its three
outputs to 1
indicating the result of the comparison operation. A Comparator
circuit has multiple
inputs and three outputs.
A 2-bit Comparator circuit compares two 2-bit numbers A and B. The
comparator circuit
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has three outputs. It sets the A>B output to 1 if A>B. It sets the A=B
output to 1 if A=B
and sets A<B output to 1 if A < B.
• The output A>B is set to 1 when the input combinations are 01 00,
10 00, 10 01, 11
00, 11 01 and 11 10
• The output A=B is set to 1 when the input combinations are 00 00,
01 01, 10 10 and
11 11
• The output A<B is set to 1 when the input combinations are 00 01,
00 10, 00 11, 01
10, 01 11 and 10 11
The circuit has 4-bit input, 2-bits represent A and 2-bits represent B
and a 3-bit output
representing A>B, A=B and A<B. To represent the function of a
Comparator circuit, three
function tables are required for each of the three outputs. A single
function table is
drawn with three outputs. Table 12.1.Page No.20
Question No: 22 ( Marks: 5 )
One of the ABEL entry methods uses logic equations; explain it with
at least a single
In ABEL any letter or combination of letters and numbers can be used
to identify
ABEL however is case sensitive, thus variable „A‟ is treated
separately from variable „a‟.
All ABEL equations must end with „;
Boolean expression F = AB' + AC +(BD)' is written in ABEL as
F = A & !B # A & C # !B & !D;
Question No: 23 ( Marks: 5 )
Explain Carry propagation in Parallel binary adder?
Parralel binary adder:
A binary adder circuit is described using dynamic transistor logic in
which for high speed
carry propagation the adder stages are grouped in pairs or larger
numbers and additional
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dynamic logic means is provided in each group to control a single
transistor connected in
series in the carry propagation path over the group.
The transistors used in the specific embodiments are MOS
transistors, but some or all of
these could be replaced by junction FET's or bipolar transistors
Short Question (Set-2)
Question No: 18 ( Marks: 1 )
Name any two modes in which PALs are programmed.
PAL devices are programmed by blowing the fuses permanently
using over voltage. PALs
typically have 8 or more inputs to the AND array and 8 or less outputs
from the fixed OR
array. Some PALs have combined inputs and outputs that can be
programmed as either
inputs or outputs
Question No: 19 ( Marks: 2 )
Explain Combinational Function Devices?
XOR,XNOR,NAND,NOR are combinational function devices.
Question No: 20 ( Marks: 3 )
Differentiate between hexadecimal and octal number system
Octal - base 8
Hexadecimal - base 16
Octal and hex are used to represent numbers instead of decimal
because there is a very
easy and direct way to convert from the "real" way that computers
store numbers (binary)
to something easier for humans to handle (fewer symbols). To
translate a binary number
to octal, simply group the binary digits three at a time and convert
each group. For
hex, group the binary digits four at a time.
Question No: 21 ( Marks: 5 )
Explain “Sum-of-Weights Method” for Hexadecimal to Decimal
Conversion with at least
one example ?
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The hexadecimal (Hex) numbering system provides even shorter
notation than octal.
Hexadecimal uses a base of 16. It employs 16 digits: number 0
through 9, and letters A
through F, with A through F substituted for numbers 10 to 15,
Hexadecimal numbers can be expressed as their decimal equivalents
by using the sum of
weights method, as shown in the following example:
Weight 2 1 0
Hex. Number 1 B 7
7 x 160 = 7 x 1 = 7
11 x 161 = 11x 16 = 176
1 x 162 = 1 x 256 = 256
Sum of products 43910
Like octal numbers, hexadecimal numbers can easily be converted to
binary or vise
versa. Conversion is accomplished by writing the 4-bit binary
equivalent of the hex digit
for each position, as illustrated in the following example:
Hex. Number 1 B 7
0001 1011 0111 Binary number
Hexadecimal Binary Decimal
0 0000 0
1 0001 1
2 0010 2
3 0011 3
4 0100 4
5 0101 5
6 0110 6
7 0111 7
8 1000 8
9 1001 9
A 1010 10
B 1011 11
C 1100 12
D 1101 13
E 1110 14
F 1111 15
Question No: 22 ( Marks: 10 )
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Draw the function table of two-bit comparator circuit, map it to K-Map
and derive the
expression for (A > B)
The circuit has inputs X1X0 and Y1Y0 and outputs X > Y, the
expression for > is
X1 Y1 X0 Y1 Y0 X1 X0 Y0
X1 X0 Y1 Y0 X<Y X=Y X>Y
0 0 0 0 0 1 0
0 0 0 1 1 0 0
0 0 1 0 1 0 0
0 0 1 1 1 0 0
0 1 0 0 0 0 1
0 1 0 1 0 1 0
0 1 1 0 1 0 0
0 1 1 1 1 0 0
1 0 0 0 0 0 1
1 0 0 1 0 0 1
1 0 1 0 0 1 0
1 0 1 1 1 0 0
1 1 0 0 0 0 1
1 1 0 1 0 0 1
1 1 1 0 0 0 1
1 1 1 1 0 1 0
Short Question (Set-3)
How SOP is converted into POS ? 3mark
Converting Standard SOP into Standard POS
The binary values of the product terms in a given standard SOP
expression are not
present in the equivalent standard POS expression. Also, the binary
values that are not
represented in the SOP expression are present in the equivalent POS
S-R latch Diagram 5mark
Two NAND base S-R Latch (Set-Reset)
Two NOR base S-R Latch (Set-Reset)
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Write NOR gate table 3mark
Write NAND gate table 3mark
Write XOR gate table 3mark
8 to 3 bit encoder 5mark
An Encoder functional device performs an operation which is the
opposite of the Decoder
function. The Encoder accepts an active level at one of its inputs and
at its output
generates a BCD or Binary output representing the selected input.
There are various
types of Encoders that are used in Combinational Logic Circuits.
Binary Encoder
The simplest of the Encoders are the 2n-to-n Encoders. The
functional table and the
circuit diagram of an 8-to-3 Binary Encoder are shown in table 17.2
and figure 17.6
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Tri-stuff diagram 3mark
The output of the OR gate from the OR gate Array is shown to be
connected to a tri-state
buffer input. The tri-state buffer can be activated or deactivated
through the control line
shown connected to its side. The Combinational Output for an SOP
function is
implemented by activating the tri-state buffer which allows the output
of the OR gate to
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be inverted by the tri-state buffer and passed to the output of the PAL
device. An activehigh
output can be obtained if the PAL device has active-high output tri-
state buffers.
Short Question (Set-4)
Question No: 17 ( Marks: 2 )
For what values of A, B, C and D, value of the expression given
below will be logic 1.
Explain at least one combination.
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CS302 Digital Logic & Design_ Muhammad Ishfaq Page No.26
Write the uses of multiplexer. 2 marks question
The Multiplexers are used to route the contents of any two registers
to the ALU inputs.
Many Audio signals in telephone network.
Computer use Dynamic Memory addressing using same address line
for row and column
addressing to access data.
Write any two advantages of Boolean expressions. 2 marks question
Boolean expressions which represent Boolean functions help in two
ways. The function
and operation of a Logic Circuit can be determined by Boolean
expressions without
implementing the Logic Circuit. Secondly, Logic circuits can be very
large and complex.
Such large circuits having many gates can be simplified and
implemented using fewer
gates. Determining a simpler Logic circuit having fewer gates which is
identical to the
original logic circuit in terms of the function it performs can be easily
done by evaluating
and simplifying Boolean expressions.
Draw the diagram of odd parity generator circuit. 2 marks question
2 XOR and 1 XNOR gate is used.
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What does a 8-bit adder/subtracter circuit do"? 3 marks question
An 8-bit Adder/Subtracter Unit
Two 4-bit 74LS283 chips can be cascaded together to form an 8-bit
Parallel Adder Unit.
Each of the two 74LS283 ICs is connected to the 1s Complement
circuitry that allows
either the un-complemented form for addition or the complemented
form for subtraction
to be applied at the B inputs of the two 74LS283s. Figure 15.8 The 8-
Adder/Subtracter Circuit is similar to the 4-bit Adder/Subtracter
Circuit. Two sets of
AND-OR based circuit that allows complemented and un-
complemented B input to be
applied at the B inputs of the two 4-bit Adders. The Add/Subtract
function select input
are tied together. The Carry In of the 1st 4-bit Adder circuit is
connected to the
Add/Subtract function select input. The Carry Out of the 1st 4-bit
Adder circuit is
connected to the Carry In of the 2nd 4-bit Adder circuit No.27
Draw the function table of 3 to 8 decoder. 3 marks question
The 3-to-8, 74XX138 Decoder is also commonly used in logical
circuits. Similar, to the 2-
to-4 Decoder, the 3-to-8 Decoder has active-low outputs and three
extra NOT gates
connected at the three inputs to reduce the four unit load to a single
unit load. The 3-to-
8 Decoder has three enable inputs, one of the three enable inputs is
active-high and the
remaining two are active-low. All three enable inputs have to be
activated for the Decoder
to work. The function table of the 3-to-8 decoder is presented. Table
Describe 16 bit ALU". 5 Marks
Implementing 16-bit ALU
16-bit ALU can be implemented by cascading together four 4-bit
ALUs. These 4-bit ALUs
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have built in Look-Ahead Carry Generator circuits that eliminate the
delay caused by
carry bit propagating through the Parallel Adder circuit within the 4-bit
ALU circut.
However, when a number of such units are cascaded together to
implement large 16-bit
and 32-bit ALU, the carry propagating between one unit to the next
gets delayed due to
the Carry rippling
through multiple 4-bit units. For large 32-bit ALUs, the Carry
propagates through 8, 4-
bit units delaying the Carry out from the last most significant unit by a
factor of 8.The
74XX181 and 74XX381 circumvent the problem by having Group-
Carry Look-Ahead.
Describe in your own words about latches". 5 Marks
See the answer above S-R latch using NAND and NOR
Short Question (Set-5)
Question No: 17 ( Marks: 2 )
Why a 2-bit comparator is called parallel comparator?
The 2-bit Comparator discussed earlier is considered to be a Parallel
Comparator as all
the bits are compared simultaneously. External Logic has to be used
to Cascade together
two such Comparators to form a 4-bit Comparator.
Question No: 18 ( Marks: 2 )
Explain at least two advantages of the circuit having low power
Power Dissipation
Logic Gates and Logic circuits consume varying amount of power
during their operation.
Ideally, logic gates and logic circuit should consume minimal power.
Advantages of low
power consumption are circuits that can be run from batteries instead
of mains power
supplies. Thus portable devices that run on batteries use Integrated
circuits that have
low power dissipation. Secondly, low power consumption means less
heat is dissipated
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by the logic devices; this means that logic gates can be tightly packed
to reduce the
circuit size without having to worry about dissipating the access heat
generated by the
logic devices.
Microprocessors for example generate considerable heat which has
to be dissipated by
mounting small fans. Generally, the Power dissipation of TTL devices
remains constant
throughout their operation. CMOS device on the other hand dissipate
varying amount
power depending upon the frequency of operation.
Question No: 19 ( Marks: 2 )
Name the four OLMC configurations
A typical GAL has eight or more inputs to the reprogrammable AND
array and 8 or more
input/outputs from its „Output Logic Macro Cells‟ OLMCs. The
OLMCs can be
programmed to Combinational Logic or Registered Logic.
Combinational Logic is used for
combinational circuits, where as Registered Logic is based on
Sequential circuits.
The four OLMC configurations are
• Combination Mode with active-low output
• Combinational Mode with active-high output
• Registered Mode with active-low output
• Registered Mode with active-high output
Question No: 20 ( Marks: 3 )
Explain “Test Vector” in context of ABEL
Test Vectors
Once the Logic circuit design has been entered its operation can be
verified by using „test
vectors‟. A „test vector‟ specifies the inputs and the corresponding
outputs. The software
simulates the operation of the logic circuit by applying the test vectors
and checking the
Test vectors are essentially the same as Truth Tables. Thus the Test
Vector for testing the
2-bit comparator circuit is the same as its truth table. No.29
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Question No: 21 ( Marks: 3 )
For a two bit comparator circuit specify the inputs for which the output
A < B is set to 1
This question is answer above in detail.
Question No: 23 ( Marks: 5 )
Explain the Operation of Odd-Parity Generator Circuit with the help of
timing diagram
Operation of Odd-Parity Generator Circuit
The timing diagram shows the operation of the Odd-Parity generator
circuit. Figure 14.3.
The A, B, C and D timing diagrams represent the changing 4-bit data
values. During time
interval t0 the 4-bit data value is 0000, during time interval t1, the
data value changes to
Similarly during time intervals t2, t3, t4 up to t8 the data values
change to 0010, 0011,
0100 and 1000 respectively. During interval t0 the output of the two
XOR gates is 0 and
0, therefore the output of the XNOR gate is 1. At interval t1, the
outputs of the two XOR
gates is 1 and 0, therefore the output of the XNOR gate is 0. The
output P can similarly
be traced for intervals t2 to t8.
Short Question (Set-6)
Question No: 17 ( Marks: 1 )
How can a PLD be programmed?
PLDs are programmed with the help of computer which runs the
programming software.
The computer is connected to a programmer socket in which the PLD
is inserted for
programming. PLDs can also be programmed when they are installed
on a circuit board
Question No: 18 ( Marks: 1 )
How many input and output bits do a Half-Adder contain?
The Half-Adder has a 2-bit input and a 2-bit output.
Question No: 19 ( Marks: 2 )
Explain the difference between 1-to-4 Demultiplexer 2-to-4
Binary Decoder?No.0
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The circuit of the 1-to-4 Demultiplexer is similar to the 2-to-4 Binary
Decoder described
earlier figure 16.9. The only difference between the two is the addition
of the Data Input
line, which is used as enable line in the 2-to-4 Decoder circuit figure
Question No: 20 ( Marks: 3 )
Name the three declarations that are included in “declaration section”
of the module that
is created when an Input (source) file is created in ABEL.
Device declaration,
Pin declarations
Set declarations
Question No: 21 ( Marks: 5 )
Explain with example how noise affects Operation of a CMOS
AND Gate circuit.
Two CMOS 5 volt series AND gates are connected together. Figure
7.3 The first AND gate
has both its inputs connected to logic high, therefore the output of the
gate is
guaranteed to be logic high. The logic high voltage output of the first
AND gate is
assumed to be 4.6 volts well within the valid VOH range of 5-4.4
volts. Assume the same
noise signal (as described earlier) is added to the output signal
of the first AND gate.
Question No: 22 ( Marks: 10 )
Explain the SOP based implementation of the Adjacent 1s Detector
The Adjacent 1s Detector accepts 4-bit inputs. If two adjacent 1s are
detected in The
input, the output is set to high. The operation of the Adjacent 1s
Detector is represented
by the function table. Table 13.6. In the function table, for the input
combinations 0011,
0110, 0111, 1011, 1100, 1101, 1110 and 1111 the output function is
a 1.
Implementing the circuit directly from the function table based on the
SOP form requires
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