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Chapter 4
PLC Hardware
PLC Development • Programmable Logic
Controllers • PLC Sections • PLC Input Sections •
PLC Output Sections • PLC Power Supplies • PLC
Central Processing Units • PLC Programming
Devices • PLC Classifications • Form Factors • PLC
Memory • Random Access Memory (RAM) •
Electrically Erasable Programmable Read-Only
Memory (EEPROM) • PLC Operating Cycle
Chapter 4 — PLC Hardware
PLCs require less space and are
easier to modify than hardwired
control components.
Chapter 4 — PLC Hardware
Regardless of the form
factor, all PLCs have five
common sections. These
sections are the input
section, output section,
power supply, central
processing unit (CPU),
and programming device
or port.
Chapter 4 — PLC Hardware
PLC input sections have LEDs that indicate when an
input device is sending a signal to a specific address
of the input section.
Chapter 4 — PLC Hardware
The internal circuitry of a
PLC input section converts
signals to the voltage level
required by the CPU, and
the internal circuitry of an
output section converts
signals from the CPU to
the voltage level required
by output devices.
Chapter 4 — PLC Hardware
PLC output sections have LEDs that indicate when
the CPU sends a signal to a specific address of the
output section to turn ON an output component.
Chapter 4 — PLC Hardware
Power supplies convert
the supplied voltage to a
voltage level the CPU
and other electronic
circuits can use.
Chapter 4 — PLC Hardware
The CPU compares
input signals to the
stored PLC program
in order to turn output
components ON and
OFF.
Chapter 4 — PLC Hardware
Integrated programming
devices provide a
convenient way to access
a PLC program without
additional hardware;
however, making any
programming changes
with an integrated keypad
can be time-consuming.
Chapter 4 — PLC Hardware
Handheld programming
devices can copy, store,
and transfer PLC
programs to multiple
PLCs that have the same
type of programming;
however, viewing and
editing programs is still
limited.
Chapter 4 — PLC Hardware
Operator interface
panels allow access
to PLC programs and
operation similar to
PC access but with
slightly limited
capabilities.
Chapter 4 — PLC Hardware
PCs are the most
common programming
devices used with PLCs
but require specific
interface cables and
connectors specific to a
PLC family and
manufacturer.
Chapter 4 — PLC Hardware
The size of a PLC is
determined by the
number of input and
output terminals on the
PLC.
Chapter 4 — PLC Hardware
Typically, fixed PLCs are used in small applications
that require only a couple of sensors or switches, and
where space is limited.
Chapter 4 — PLC Hardware
Modular PLCs are made up of individual sections that
can be added to a PLC to expand the PLC’s I/O and
power capabilities for almost any application.
Chapter 4 — PLC Hardware
Memory in a PLC is used for
program files that contain the
user PLC program and data
table files, which contain the
status of inputs and outputs,
timer preset and
accumulated values, counter
preset and accumulated
values, and other program
instruction values.
Chapter 4 — PLC Hardware
The standard unit of
memory for a PLC is the
16-bit word. A bit is the
smallest unit of memory.
Chapter 4 — PLC Hardware
The operating cycle of a
PLC runs continuously
and sequentially while
data tables are updated
as inputs and outputs
change state.