Specifications

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Programmable logic controllers (PLCs)
Overview
Overview
Overview
Programmable logic controllers
(PLCs) have been an integral part of
factory automation and industrial
process control for decades. PLCs
control a wide array of applications
from simple lighting functions to
environmental systems to chemical
processing plants. These systems
perform many functions, providing
a variety of analog and digital
input and output interfaces; signal
processing; data conversion; and
various communication protocols.
All of the PLC’s components and
functions are centered around the
controller, which is programmed for
a specific task.
The basic PLC module must be suffi-
ciently flexible and configurable to
meet the diverse needs of different
factories and applications. Input
stimuli (either analog or digital) are
received from machines, sensors,
or process events in the form of
voltage or current. The PLC must
accurately interpret and convert
the stimulus for the CPU which, in
turn, defines a set of instructions
to the output systems that control
actuators on the factory floor or in
another industrial environment.
Modern PLCs were introduced in the
1960s, and for decades the general
function and signal-path flow
changed little. However, twenty-first-
century process control is placing
new and tougher demands on a PLC:
higher performance, smaller form
factor, and greater functional flexibil-
ity. There must be built-in protection
against the potentially damaging
electrostatic discharge (ESD), elec-
tromagnetic interference and radio
frequency interference (RFI/EMI),
and high-amplitude transient pulses
found in the harsh industrial setting.
Robust design
PLCs are expected to work flawlessly
for years in industrial environments
that are hazardous to the very
microelectronic components that
give modern PLCs their excellent
flexibility and precision. No mixed-
signal IC company understands
this better than Maxim. Since our
inception, we have led the industry
with exceptional product reliability
and innovative approaches to
protect high-performance electronics
from real environmental dangers,
including high levels of ESD, large
transient voltage swings, and EMI/
RFI. Designers have long endorsed
Maxim’s products because they
solve difficult analog and mixed-
signal design problems and
continue solving those problems
year after year.
Higher integration
PLCs have from four to hundreds of
input/output (I/O) channels in a wide
variety of form factors, so size and
power can be as important as system
accuracy and reliability. Maxim leads
the industry in integrating the right
features into ICs, thereby reducing
the overall system footprint and
power demands and making designs
more compact. Maxim has hundreds
of low-power, high-precision IC’s
in the smallest available footprints,
so the system designer can create
precision products that meet strict
space and power requirements.
Factory automation,
a short history
Assembly lines are a relatively new
invention in human history. There
have likely been many parallel inven-
tions in many countries, but here we
will mention just a few highlights
from the U.S.
Samuel Colt, the U.S. gun manufac-
turer, demonstrated interchangeable
parts in the mid-1800s. Previously
each gun was assembled with indi-
vidually made pieces that were filed
to fit. To automate that assembly
process, Mr. Colt placed all the pieces
for ten guns in separate bins and
then assembled a gun by randomly
pulling pieces from the bins. Early
in the twentieth century Henry Ford
expanded mass-production tech-
niques. He designed fixed-assembly
stations with cars moving between
positions. Each employee learned
just a few assembly tasks and
performed those tasks for days on
end. In 1954 George Devol applied
for U.S. Patent 2,988,237, which
enabled the first industrial robot
named Unimate. By the late 1960s
General Motors® used a PLC to
assemble automobile automatic
transmissions. Dick Morley, known as
the “father” of the PLC, was involved
with the production of the first PLC
for GM®, the Modicon. Morley’s U.S.
Patent 3,761,893 is the basis of many
PLCs today. (For more information on
the above four inventors, please see:
www.wikipedia.org/; for their
patents, search: http://patft.uspto.
gov/netahtml/PTO/srchnum.htm).
Basic PLC operation
How simple can process control be?
Consider a common household
space heater.
The heater’s components are
enclosed inside one container, which