Specifications

30 Control and Automation Solutions Guide
Overview
The CPU functions for a PLC include the
processor, memory, and support circuitry
required to execute the programmed
instructions and to communicate
with the various I/O functions.
The CPU controls all the PLC activity.
The input/output system provides an
interface between the CPU and eld
devices like sensors and switches on
the input side and controllable devices
like drivers on the output side.
The PLC control program was historically
developed in ladder logic, which is a
graphical, diagram-based construct
used when relays were the primary
logic elements controlling an industrial
process. But modern development tools
allow PLC programmers to use high-
level programming environments and
to create structured software. If desired,
within these sophisticated programming
environments PLC programming can
still be done using ladder logic.
Field devices provide the input data via
the input interface to the CPU, which,
in a deterministic fashion, executes the
control program. Based on the result,
the connected controllable devices
are adjusted via the output interface,
and data is communicated over the
communications interface to other PLCs
for process coordination and up to the
higher levels of the factory process-
control system for reporting and data
logging. The control process, called
scanning, continues until a change has
been made to the control program.
Many of today’s PLC applications require
faster scan times that, in turn, depend
on a high-performance CPU. The range
of PLC types has proliferated: micro-PLCs
and mini-PLCs at the low end of CPU
performance needs; motion-control PLCs
with very fast processing capabilities;
and safety-PLCs at the high end of
performance capabilities. The safety-
PLCs may require CPU redundancy and
a host of safety-related features in order
to meet safety integrity levels (SILs) in
potentially hazardous plan operations.
Self-monitoring functions are performed
by the CPU’s power-supply voltage
monitors, watchdog timers, and reset
circuits. Various security functions can be
incorporated to ensure the integrity of
the CPU’s code and its proper execution.
Thermal sensors are used on critical
devices and known hot spots where
high power dissipation is likely, such
as on output drivers or on the CPU.
The CPU module also includes
components to enable communications
with attached expansion modules,
other PLCs, industrial PCs, and the
built-in HMI control panel on the PLC.
Functions such as switch debounce,
display and backlight driving, and audio
functions may be needed. Universal
asynchronous receiver-transmitters
(UARTs) dene the eldbus data rates,
ensure data integrity, and interface
to either the RS-485 or PROFIBUS
transceivers on the eldbus module.
Isolated power supplies, hot-swap
controllers, and battery backup
combine for power management
on the CPU module.
CPU Functions
µP
THERMAL
MANAGEMENT
1-Wire
®
INTERFACE
AND
AUTHENTICATION
DISPLAY
DRIVERS
SECURE µP
VOLTAGE
MONITORS
CLOCK
WATCHDOG
TIMER
RESET ICs
BACKPLANE
TO DISPLAY
AUDIO OUTPUT
HOT-SWAP
CONTROLLER
POWER SUPPLY
TO ALL
BATTERY
BACKUP
SWITCH
DEBOUNCER
BACKLIGHT
AUDIO AMPLIFIER
SECURITY MANAGER
USB
MEMORY
= MAXIM SOLUTION
Block diagram of PLC CPU functions.