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MKT2802 Logic Circuits
Mechatronics Engineering Department

Spring : 2023

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Logic Circuits

1

1’s-Complement Form
The 1’s complement of a binary number is obtained by changing each 0 to a
1 and each 1 to a 0
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The process is shown below
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The process is illustrated below for (101101)2 = (45)10
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 A logic gate has one output terminal and one or more input terminals
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 The logic gates can be used to design digital systems that will evaluate
digital input levels and produce a specific output response based on that
particular logic circuit design
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 The NAND, NOR, Exclusive-OR and Exclusive- NOR gates are
constructed using the basic logic gates
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Logic Circuits

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NOT Gate

A NOT gate is a one-input, one-output logic circuit whose output is always
the complement of the input
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OR Gate (Inclusive OR Gate)

An OR gate performs an ORing operation on two or more than two logic
variables
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An OR gate is a logic circuit with two or more inputs and one output
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For all
other possible input combinations, the output is HIGH
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The output of a NOR gate is a logic ‘1’ when all its
inputs are logic ‘0’
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Logic Circuits

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NOR Gate

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AND Gate

An AND gate is a logic circuit having two or more inputs and one output
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In all other cases, the output is LOW
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The output of a NAND gate is a
logic ‘0’ when all its inputs are a logic ‘1’
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NAND Gate

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EXCLUSIVE-OR (EXOR) Gate
The EXCLUSIVE-OR gate, commonly written as EX-OR gate, is a two-input, oneoutput gate
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Although EX-OR gates are available in integrated circuit
form only as two-input gates, unlike other gates which are available in multiple inputs
also, multiple-input EX-OR logic functions can be implemented using more than one
two-input gates
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e
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Logic Circuits

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EXCLUSIVE-NOT-OR (EXNOR) Gate

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Digital Buffer
In digital electronic circuits we need to isolate logic gates from each other or have
them drive or switch higher than normal loads, such as relays, solenoids and lamps
without the need for inversion
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Three-State Buffer
A Tri-state Buffer can be thought of as an input controlled switch with an output that
can be electronically turned “ON” or “OFF” by means of an external “Control” or
“Enable” ( EN ) signal input
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Digital Buffers and Tri-state Buffers can provide current amplification in a
digital circuit to drive output loads

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Logic Circuits

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Three-State Buffer
Tri-state Buffers are available in integrated form as quad, hex or octal buffer/drivers in
both uni-directional and bi-directional forms, with the more common being the TTL
74240, the TTL 74244 and the TTL 74245

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Pull-up and Pull-down Resistors
One final point to remember, when connecting together digital logic gates to produce
logic circuits, any “unused” inputs to the gates must be connected directly to
either a logic level “1” or a logic level “0” by means of a suitable “Pull-up” or
“Pull-down” resistor ( for example 1kΩ resistor ) to produce a fixed logic signal
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Logic Circuits

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Pull-up and Pull-down Resistors
As well as using pull-up or pull-down resistors to prevent unused logic gates from
floating about, spare inputs to gates and latches can also be connected together or
connected to left-over or spare gates within a single IC package as shown
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This standard uses a
rectangular symbol for all
devices instead of a different
symbol shape for each
device

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