Specifications

3PLCs
Introduction
Programmable logic controllers (PLCs)
have been an integral part of factory
automation and industrial process
control for decades. PLCs are dedicated
computers that have an architecture
designed to accommodate control
functions. They control a wide array
of applications from simple lighting
functions to environmental systems
to complete 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 communications protocols.
The architecture of a PLC must be
suciently exible and congurable
to meet the diverse needs of a wide
variety of control applications. Inputs
(either analog or digital) are received
from human operators, machines,
sensors, and process events in the
form of voltage or current signals.
The PLC must accurately receive and
interpret these inputs so they can be
presented to the control application
running on the PLC, which will then
determine a set of outputs. These
outputs are sent to control actuators in
order to maintain control or to cause
desired modications in the process.
Modern PLCs were introduced in the
1960s, and for decades the general
function and signal-path ow changed
little. However, 21st century process
control is placing new and tougher
demands on PLCs: higher performance
(higher speed, greater precision),
smaller form factor, more functional
exibility, and built-in protection against
potentially damaging electrostatic
discharge (ESD), electromagnetic
interference and radio frequency
interference (EMI/RFI), and high-
amplitude transient pulses found
in the harsh industrial setting.
Today, the PLC role has been expanded
to include more targeted PLCs for
specic applications. Safety PLCs include
extra safety features and redundancy
for safety-critical applications. Mini- and
micro-PLCs automate simpler systems.
Standard PLCs are also improving and
are being integrated with other systems
such as programmable automation
controllers (PACs). PACs typically add
motion control, machine vision, easier
programming, more communications
functions, etc. Doing so allows the
PAC to be used in more complex
manufacturing operations, but the
boundaries between PACs and PLCs are
blurring as PLCs add PAC functionality.
Throughout this chapter, PLC will be
the generic term used for all types
of PLCs and PACs. Maxim’s products
can be used in all of these systems.
Overview