Avionics News February 2020 - 64
series
B Y
T H O M A S
I N M A N
Logic
O
ne thing in short supply these days is logic.
To pass ASTM's NCATT Aircraft Electronics
Technician certification examination, a student
must understand basic digital logic gates, symbols used and
truth tables. I have studied maintenance manuals and found
logic gate symbology used, even when the system was not
electronic. Fortunately, these gates are easy to understand.
Logic gates are the building blocks of digital circuitry
and can be defined as a digital device with an input and an
output determined by a rule. Engineers and technicians use
symbols to represent the devices. Since these are digital,
they will use binary - only zeros and ones. Electronically,
the zeros and ones can be represented in different ways, for
example, a very common form of digital circuitry uses zero
volts for a binary zero, and 5 volts for a binary one. The rule
used is known as a logic function, which will express the
relationship between the inputs, or variables, and the output.
For each type of logic gate, we can use a table, known as a
truth table, to show all possible variable combinations and
the outputs they produce.
Figure 1 shows an AND gate. The AND gate symbol
will be flat on the input side and have a smooth curve on
the output side. The AND gate shown has two inputs. It's
possible to have three or more inputs to an AND gate. The
inputs will be at binary logic levels and will be defined here
as zeros and ones. The rule, or logic function, of an AND
gate is in order to produce a one on the output; all inputs
must be one. Next to the AND gate is a truth table, which
shows the inputs (labeled A and B) and the output (labeled
X). As the truth table shows, the only way to get a one on the
output is when A and B are both one. Imagine this gate in a
landing-gear circuit for the nose gear. The nose gear must be
down (1) and locked (1) before the nose gear green light is
illuminated (1).
A variation to the AND gate is the NAND gate. The rule
for the NAND gate is "Not AND." Figure 2 shows a NAND
gate symbol and its truth table. Take note of the circle at the
output of the symbol. We call these negation bubbles. The
negation bubble takes the output and changes it to the other
state. The output of the NAND gate is the opposite of an
AND gate. As you can see in Figure 2, the output will remain
at one until both inputs are one, and then the output goes
to zero. This gate would also work in a nose-gear landing
circuit, but instead of providing voltage for a light, it provides
a ground for a light.
Figure 3 shows an OR gate. The symbol for this gate is
Figure 1: The symbol and truth table for an AND gate
Figure 2: The symbol and truth table for a NAND gate
64
avionics news
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february
2020
Avionics News February 2020
Table of Contents for the Digital Edition of Avionics News February 2020
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Avionics News February 2020 - Intro
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Avionics News February 2020 - Cover2
Avionics News February 2020 - 1
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Avionics News February 2020 - Cover3
Avionics News February 2020 - Cover4
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