Post on 06-Apr-2018
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` Programmable logiccontrollers, also calledprogrammable controllers orPLCs, are solid-statemembers of the computerfamily, using integratedcircuits instead ofelectromechanical devices toimplement control functions.
` PLCs can be thought of insimple terms as industrialcomputers with speciallydesigned architecture in boththeir central units (the PLCitself) and their interfacingcircuitry to field devices(input/output connections tothe real world).
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Honeywell
Allen
Bradley
(Rockwell
Mitsubishi
Yokogawa
ABB
Siemens
SIMATIC
S7-400
SIMATIC
S7-300
SIMATIC
S7-200
SiemensLogo
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` The central processing unit
` The input/output interface
system
` The central processing unit
(CPU) governs all PLC
activities.
` The following three
components, form the CPU:
` The processor
` The memory system
` The system power supply
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` During its operation, the CPU
completes three processes:
` (1) it reads, or accepts, the
input data from the field
devices via the input
interfaces,` (2) it executes, or performs,
the control program stored in
the memory system, and
` (3) it writes, or updates, the
output devices via the output
interfaces.
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` Programming device,
usually a personal
computer or a
manufacturers mini
programmer
unit, is required to enterthe control program into
memory
` The programming device
must be connected to the
controller when entering
or monitoring the controlprogram.
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` Micro PLCs
` Small PLCs
` Medium PLCs
` Large PLCs
` Very large PLCs
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` The ladder diagram has and
continues to be the
traditional way of
representing electrical
sequences of operations.+
` These diagrams representthe interconnection of field
devices in such a way that
the activation, or turning ON,
of one device will turn ON
another device according to a
predetermined sequence ofevents.
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` The CPU forms what can be
considered to be the brain
of the system. The three
components of the CPU are:
` The processor
` The memory system
` The power supply
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` Very small microprocessors (ormicros)integrated circuitswith tremendous computing andcontrol capabilityprovide theintelligence of todaysprogrammable controllers.
` The CPU of a PLC system maycontain more than one processor(or micro) to execute the systemsduties and/or communications,because extra processorsincrease the speed of theseoperations.
` Standard word lengths are 8, 16,and 32 bits. This word lengthaffects the speed at which theprocessor performs mostoperations.
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` The basic function of a
programmable controller
is to read all of the field
input devices and then
execute the control
program, whichaccording to the logic
programmed, will turn
the field output devices
ON or OFF.
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` What occurs during the
scanning operation of a
programmable controller
if the signal(s) from an
input field device behave
as shownin Figures a and b?
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` The common scan
method of monitoring
the inputs at the end of
each scan may be
inadequate for reading
certain extremely fastinputs.
` Some PLCs provide
software instructions
that allow the
interruption of the
continuous programscan to receive an input
or to update an output
immediately.
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` Referencing Figure
illustrate how, in one
scan, (a) an immediate
instruction will respond to
an interrupt input and (b)
the same inputinstruction can update an
immediate output field
device, like a solenoid
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` The immediate instruction will
interrupt the control program to
read the input signal. It will then
evaluate the signal and return to
the control program, where it will
resume program execution and
update outputs.
` Immediate update of an output.
As in part (a), the immediate
instruction interrupts the control
program to read and evaluate the
input signal. However, the output
is updated before normalprogram execution resumes
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` The PLCs processorconstantly communicates withlocal and remote subsystemsorracks as they may also becalled I/O interfaces connectthese subsystems to fielddevices located either close
to the main CPU or at remotelocations
` Error-checking techniquesare also incorporated in thecontinuous communicationbetween the processor andits subsystems. Thesetechniques confirm the
validity of the data transmittedand received..
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` The processor uses error-checkingtechniques to monitor thefunctional status of both thememory and the communicationlinks between subsystems andperipherals, as well as its own
operation. Common error-checkingtechniques include parity andchecksum.
` Parity or vertical redundancycheck (VRC).
` Checksum
` Cyclic redundancy check (CRC)` The most common error-detecting
and error-correcting code is theHamming code.
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` The processor is responsible for
detecting communication failures, as
well as other failures, that may occur
during system operation.
` Typical diagnostics include memory
OK, processor OK, battery OK, andpower supply OK.
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` The system power supply
plays a major role in the total
system operation. In fact, it
can be considered the first-
line manager of system
reliability and integrity.` Require a 120 VAC or 220
VAC power source, while a
few controllers will accept 24
VDC.
` Constant Voltage
Transformers
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` Undercurrent conditions cancause unpredictable operation ofthe I/O system.
` Undercurrent conditionsometimes arises in applicationswhere an excessive number of
special purpose I/O modules areused.
` When power supply loadinglimits have been exceeded andoverload occurs, the normalremedy is to either add an
auxiliary power supply or toobtain a supply with a largercurrent capability.