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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.