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Chapter 3 (part 1)
Basic Logic Gates
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Introduction
• Logic gates are the basic building blocks for
forming digital electronic circuitry.
• A logic gate has
– One output terminal; One or more input terminals
– The outputs is either HIGH(1) or LOW(0)
depending on the digital levels at the input
terminals and the property of the gate
– Five basic logic gates
• AND, OR NAND, NOR and inverter
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3-1 The AND Gate
• The output, X, is HIGH if input A AND input B
are both HIGH.
– If A = 1 and B = 1, then X = 1.
– If A = 0 or B = 0, then X = 0.
Schematic symbol of two-input AND gate
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Truth table for AND Gate
All possible input combinations
Resultant output
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Figure 3-2 Use AND gate to activate
a burglar alarm
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Figure 3.3
Electrical analogy for an AND gate
(a) using manual switches;
(b) using transistor switches.
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Boolean equation for AND Gate
• Boolean Equation: X = A•B or X = AB
– Read as “X equals A AND B”
– Can have more than two inputs
• Equation 3-1
Number of combinations in truth table = 2N
where N = number of inputs
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Figure 3-4 Multiple-input AND gate
symbols
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3-2 The OR Gate
• The output at X will be HIGH whenever input
A OR input B is HIGH or both are HIGH
– If A = 1 and/or B = 1, then X = 1
– If A = 0 and B = 0, then X = 0
Schematic symbol of two-input OR gate
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Truth table for the OR Gate
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Figure 3.6
Electrical analogy for an OR gate
(a) using manual switches;
(b) using transistor switches.
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Boolean equation for the OR Gate
• Boolean Equation: X = A + B
– Read as, “ X = A or B”
• Can have more than two inputs
• Number of combinations = 2N
– N = number of input bits
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Figure 3-7 Multiple-input OR gate
symbols
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Figure 3.8 eight-input OR gate symbol
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3-3 Timing Analysis
• Timing diagrams are used to analyze the
output response to varying inputs
• Oscilloscope
– Display plots voltage versus time
– Dual-trace can display two waveforms at a time.
• Logic analyzer
– Can display state tables that show binary levels
on signal lines
– Can display up to 16 waveforms at a time
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3-4 Enable and Disable Functions
• AND and OR gates can be used to enable or
disable a waveform from being transmitted
from one point to another.
• Example: Use a 1-MHz clock oscillator to
transmit only four pulses
– Solution: Enable four clock pulses to be
transmitted, and then disable the transmission
from then on.
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Enable function using AND gate
Figure 3-16
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Disable function using OR gate
Figure 3-17
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3-5 Using IC Logic Gates
• AND and OR gates are available as ICs.
• Data Manual from manufacturer includes
– IC pin layout, Logic gate type, Technical
specifications
• Example ICs
– 7408(74LS08,74HC08): quad two-input AND gate
– 7411(74LS11,74HC11):dual four-input AND gate
– 7432(74LS32,74HC32):quad two-input OR gate
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Common prefixes and suffixes
DIP
• Family designation
– LS – Lower power TTL family
– HC – High-speed CMOS family
– (74LS11,74HC11):dual four-input AND gate
• Manufacturer
– SN: Texas Instruments. E.g., SN7400
• Package style (suffix)
SOIC
– N: plastic dual-in-line package (P-DIP), SN7400N
– M: Small-outline Integrated Circuit (SOIC)
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Figure 3.19 The 7408 quad two-input
AND gate IC pin configuration.
The top side typically has
Indented circle / notch cut
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Figure 3.20 Using the 7408 TTL IC in
the clock enable circuit
of Figure 3–17.
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Figure 3.21 Pin configurations for
other popular TTL and CMOS AND and
OR gate ICs:
(a) 7411 (74HC11);
(b) 7421 (74HC21);
(c) 7432 (74HC32).
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3-6 Introduction to Troubleshooting
Techniques
• Troubleshooting
– The procedure used to find the fault or trouble in
a circuit.
• Tools to test ICs and digital circuits
– Logic Probe shows digital levels: Low, High, and
floating - open circuit, neither high nor low
– Logic Pulser provides digital pulses to a circuit
being tested.
– Both tools allow you to tell if the pulse signal is
getting through the IC as expected.
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Gate 2
Gate 3
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Gate 1
Gate 4
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