components of a Programmable Controller (PLC). It
details the architecture and basic instruction set common
to all PLC’s. Basic programming techniques and logic
designs are coered. This training describes the
operating features of the PLC! the adantages of the
PLC oer hard"wired control s#stems! practical
applications! troubleshooting and maintenance of PLC’s.
 
3
Objectives
 $t the end of the training the participants should be able
to%
 
 
Advantages of PLCs
/ Less wiring. / 0iring between deices and rela# contacts are done in
the PLC program. / 1asier and faster to ma2e changes. /  Trouble shooting aids ma2e programming easier and
reduce downtime. / *eliable components ma2e these li2el# to operate for
#ears before failure.
 
/" &eeloped to replace rela#s in the late 3456s
/" Costs dropped and became popular b# 3476s
/" 8ow used in man# industrial designs
 
Corporation specified the design criteria for the first
programmable controller in 1,-. 
Their primar# goal
o e&i)inate t/e /ig/ costs associated 0it/ inf&eib&e
re&a(contro&&ed s(ste)s.
 
3457 Programmable concept deeloped
3454 ardware CP+ controller! with logic
  instructions! 3 9 of memor# and 3:7 I,
  points
  PLC " timers and counters= arithmetic
  operations= 3: 9 of memor#
  and 36:< I, points
34;; icroprocessors " based PLC introduced
 
1nhanced communications facilities
1nhanced software features
programming aids
347? on  8etwor2ing of all leels of PLC! computer
and machine using -C$&$ software.
 
1
Progra))ab&e Logic Contro&&ers 5 Definition according to N6+A standard IC#31,$.7
 $ digitall# operating electronic apparatus which uses a
programming memor# for the internal storage of instructions
for implementing specific functions such as logic!
sequencing! timing! counting and arithmetic to control
through digital or analog modules! arious t#pes of machines
or process.
:. @ould odicon
>. TeAas Instruments
<. @eneral 1lectric
PLC #i8e
3. -$LL " it coers units with up to 3:7 I,’s and
memories up to : 9b#tes.
  " these PLC’s are capable of proiding
simple to adance leels or machine
controls.
:. 1&I+ " hae up to :6<7 I,’s and memories up
to >: 9b#tes.
famil#. The# hae up to 734: I,’s and
memories up to ;?6 9b#tes.
" can control indiidual production
processes or entire plant.
 
an' "sed to +i 0o Li:uids
 $ tan2 is used to miA two liquids. The control circuit operates
as follows%
3. 0hen the start button is pressed! solenoids $ and B
energiEe. This permits the two liquids to begin filling the tan2.
:. 0hen the tan2 is filled! the float switch trips. This de"
energiEes solenoids $ and B and starts the motor used to
miA the liquids together.
>. The motor is permitted to run for one minute. $fter one
minute has elapsed! the motor turns off and solenoid C
energiEes to drain the tan2.
 
solenoid C.
?. $ stop button can be used to stop the process at an#
point.
5. If the motor becomes oerloaded! the action of the entire
circuit will stop.
;. nce the circuit has been energiEed it will continue to
operate until it is manuall# stopped.
1$
 
P!OC6##O!
  PO<6!
PO<6! #"PPL=
Proides the oltage needed to run the primar# PLC
components
Proides signal conersion and isolation between the
internal logic" leel signals inside the PLC and the field’s
high leel signal.
P!OC6##O!
Proides intelligence to command and goern the actiities
of the entire PLC s#stems.
P!O*!A++IN* D6>IC6
used to enter the desired program that will determine the
sequence of operation and control of process equipment or
drien machine.
 Programming Panel ( @ould odicon )
 
 &es2top t#pe with a C*T displa#
 Compatible computer terminal
/ The I, interface section of a PLC connects it to
eAternal field deices.
/ The main purpose of the I, interface is to condition the
arious signals receied from or sent to the eAternal input
and output deices.
/ Input modules conerts signals from discrete or analog
input deices to logic leels acceptable to PLC’s processor.
/ utput modules conerts signal from the processor to
leels capable of driing the connected discrete or analog
output deices.
transients from damaging the
processor. •elps reduce the
FLT$@1
transients from
damaging the
I8P+T T &C $8&
&*P- T1 FLT$@1
T L@IC L1F1L
OPO-
transients from
damaging the
3. Pilot &ut# utputs
utputs of this t#pe t#picall# are used to drie high"current
electromagnetic loads such as solenoids! rela#s! ales! and
motor starters.
These loads are highl# inductie and eAhibit a large inrush
current.
Pilot dut# outputs should be capable of withstanding an
inrush current of 36 times the rated load for a short period of
time without failure.
These are usuall# low" oltage and low"current and are used
to drie indicating lights and other non"inductie loads. 8oise
suppression ma# or ma# not be included on this t#pes of
modules.
>. &iscrete Inputs
Circuits of this t#pe are used to sense the status of limit
switches! push buttons! and other discrete sensors. 8oise
suppression is of great importance in preenting false
 
I@O Circuits
<. $nalog I,
Circuits of this t#pe sense or drie analog signals.
 $nalog inputs come from deices! such as thermocouples!
strain gages! or pressure sensors! that proide a signal
oltage or current that is deried from the process ariable.
-tandard $nalog Input signals% <":6m$= 6"36F
 $nalog outputs can be used to drie deices such as
oltmeters! H"G recorders! seromotor dries! and ales
through the use of transducers.
-tandard $nalog utput signals% <":6m$= 6"?F= 6"36F
 
?. -pecial " Purpose I,
Circuits of this t#pe are used to interface PLCs to er# specific
t#pes of circuits such as seromotors! stepping motors PI&
(proportional plus integral plus deriatie) loops! high"speed
pulse counting! resoler and decoder inputs! multipleAed
displa#s! and 2e#boards.
This module allows for limited access to timer and counter
presets and other PLC ariables without requiring a program
loader.
 
&iscrete Input
 $ discrete input also referred as digital input is an input that is
either 8 or are connected to the PLC digital input. In the
8 condition it is referred to as logic 3 or a logic high and in the
condition ma#be referred to as logic o or logic low.
 N#*+%% O&n P!'!""#n
 N#*+%% C%#'&$ P!'!""#n
+odu&e Tan2
Leel Transmitter 
 $n analog input is an input signal that has a continuous
signal. T#pical inputs ma# ar# from 6 to :6m$! < to :6m$
or 6 to36F. Below! a leel transmitter monitors the leel of
liquid in the tan2. &epending on the leel TA! the signal to the
PLC can either increase or decrease as the leel increases
or decreases.
 $ discrete output is either in an 8 or condition. -olenoids!
contactors coils! lamps are eAample of deices connected to the
&iscrete or digital outputs. Below! the lamp can be turned 8 or
b# the PLC output it is connected to.
&igital utput
+odu&e
 $n analog output is an output signal that has a continuous
signal. T#pical outputs ma# ar# from 6 to :6m$! < to :6m$
or 6 to36F.
to 1>
The processor module contains the PLC’s microprocessor!
its supporting circuitr#! and its memor# s#stem.
The main function of the microprocessor is to anal#Ee data
coming from field sensors through input modules! ma2e
decisions based on the user’s defined control program and
return signal bac2 through output modules to the field
deices. ield sensors% switches! flow! leel! pressure! temp.
transmitters! etc. ield output deices% motors! ales!
solenoids! lamps! or audible deices.
The memor# s#stem in the processor module has two parts%
a system memory  and an application memory .
 
composed of permanentl#"stored programs that direct all s#stem
actiities! such as eAecution of the users control program!
communication with peripheral deices! and other s#stem
actiities. /The s#stem memor# also contains the routines that implement
the PLC’s instruction set! which is composed of specific control
functions such as logic! sequencing! timing! counting! and
arithmetic. /-#stem memor# is generall# built from read"onl# memor#
deices.
APPLICAION
/The application memor# is diided into the data table area and
user program area. /The data table stores an# data associated with the user’s control
program! such as s#stem input and output status data! and an#
stored constants! ariables! or preset alues. The data table is where data is monitored! manipulated! and changed for control
purposes. /The user program area is where the programmed instructions
entered b# the user are stored as an application control program.
/&ata Table /+ser Program
when power is remoed.
1en momentar# losses of power will erase an# information
stored or programmed on a olatile memor# chip.
Common T#pe of Folatile emor#
!A+? !andom Access +emor#(*ead,0rite)
*ead,write indicates that the information stored in the
memor# can be retrieed or read! while write indicates that
 
The words rando) access  refer to the abilit# of an#
location (address) in the memor# to be accessed or used.
*am memor# is used for both the user memor# (ladder
diagrams) and storage memor# in man# PLC’s.
*$ memor# must hae batter# bac2up to retain or protect
the stored program.
3.-
#emiconductor) is probabl# one of the most popular. C-"
*$ is popular because it has a er# low current drain when
not being accessed (3?microamps.)! and the information
 
as the abilit# to retain stored information when power is
remoed! accidentall# or intentionall#. These memories do not
require batter# bac2"up.
!O+ !ead Onl# +emor#
can be read onl# and cannot be changed. Information in *
is placed there b# the manufacturer for the internal use and
operation of the PLC.
P!O+ Programmable !ead Onl# +emor#
 $llows initial and,or additional information to be written into
the chip.
P* ma# be written into onl# once after being receied
from the PLC manufacturer= programming is accomplish b#
pulses of current.
The current melts the fusible lin2s in the deice! preenting it
from being reprogrammed. This t#pe of memor# is used to
preent unauthoriEed program changes.
 
Ideall# suited when program storage is to be semi"
permanent or additional securit# is needed to preent
unauthoriEed program changes.
The 1P* chip has a quartE window oer a silicon
material that contains the electronic integrated circuits. This
window normall# is coered b# an opaque material! but
when the opaque material is remoed and the circuitr#
eAposed to ultra iolet light! the memor# content can be
erased.
 
Onl# +emor#
 $lso referred to as 1:P*! is a chip that can be
programmed using a standard programming deice and can
be erased b# the proper signal being applied to the erase pin.
11P* is used primaril# as a non"olatile bac2up for the
normal *$ memor#. If the program in *$ is lost or erased!
a cop# of the program stored on an 11P* chip can be
down loaded into the *$.
Basic unction of a T#pical PLC
*ead all field input deices ia the input interfaces! eAecute
the user program stored in application memor#! then! based
on whateer control scheme has been programmed b# the
user! turn the field output deices on or off! or perform
whateer control is necessar# for the process application.
This process of sequentiall# reading the inputs! eAecuting
the program in memor#! and updating the outputs is 2nown
as scanning.
0hile the PLC is running! the scanning process includes the
following four phases! which are repeated continuousl# as
indiidual c#cles of operation%
P%A#6 2
P%A#6 1 Input #tatus scan
•  $ PLC scan c#cle begins with the CP+ reading the status
of its inputs.
the inputs
•  nce the program is eAecuted! the CP+ performs
diagnostics and communication tas2s
/ $n output status scan is then performed! whereb# the
stored output alues are sent to actuators and other field
 
9-
 $s soon as Phase < are completed! the entire c#cle begins
again with Phase 3 input scan.
The time it ta2es to implement a scan c#cle is called -C$8
TI1. The scan time composed of the program scan time!
which is the time required for soling the control program! and
the I, update time! or time required to read inputs and
update outputs. The program scan time generall# depends on
the amount of memor# ta2en b# the control program and t#pe
of instructions used in the program. The time to ma2e a single
scan can ar# from 3 ms to 366 ms.
 
 Changing resident PLC programs " uploading,downloading
from a superisor# controller (Laptop or des2top computer).
   orcing I, points and memor# elements from a remote
terminal.
   Lin2ing a PLC into a control hierarch# containing seeral
siEes of PLC and computer.
onitoring data and alarms! etc. ia printers or perator
Interface +nits (I+s).
transmission of information.
 +sed in short"distance computer communications! with the
ma'orit# of computer hardware and peripherals.  as a maAimum effectie distance of approA. >6 m at 4566
baud.
Loca& Area Net0or' 5LAN7
Local $rea 8etwor2 proides a ph#sical lin2 between all
deices plus proiding oerall data eAchange management or
protocol! ensuring that each deice can tal2J to other
machines and understand data receied from them.
L$8s proide the common! high"speed data communications
bus which interconnects an# or all deices within the local
area.
 
 +sed for longer"distance lin2s! often between seeral PCs
in a distributed s#stem. *- <7? can hae a maAimum
distance of about 3666 meters.
 
+anufacturer Net0or'
.itsubishi .elsec"81T
-quare & -G,81T
-2
#pecifications
-eeral factors are used for ealuating the qualit# and
performance of programmable controllers when selecting a
unit for a particular application. These are listed below.
8+B1* I , P*T-
This specifies the number of I, deices that can be
connected to the controller. There should be sufficient I,
ports to meet present requirements with enough spares to
proide for moderate future eApansion.
 
/ emor#
/ -can Time
1? +nderstand the process
 
1ach output port should be capable of suppl#ing sufficient
oltage and current to drie the output peripheral connected
to it.
-C$8 TI1
This is the speed at which the controller eAecutes the rela#"
ladder logic program. This ariable is usuall# specified as the
scan time per 3666 logic nodes and t#picall# ranges from 3 to
:66 milliseconds.
The amount of memor# required for a particular application is
related to the length of the program and the compleAit# of the
control s#stem. -imple applications haing 'ust a few rela#s
do not require significant amount of memor#. Program length
tend to eApand after the s#stem hae been used for a while. It
is adantageous to a acquire a controller that has more
memor# than is presentl# needed.
 
:. PLC status lights
*+8 " the PLC thin2s it is 9 (and probabl# is)
1*** " a ph#sical problem has occurred with the PLC
>. Indicator lights on I, cards and sensors
<. Consult the manuals! or use software if aailable.
?. +se programming terminal , laptop.
1. Programming Terminal " laptop or des2top PC.
:. PLC -oftware. PLC manufacturers hae
their own specific software and license 2e#.
>. Communication cable for connection from Laptop
  to PLC.
<. Bac2up cop# of the ladder program (on dis2ette! C&*!
  hard dis2! flash memor#). If none! upload it from the PLC.
?. &ocumentation" (PLC manual! -oftware manual! drawings!
  ladder program printout! and -eq. of perations manual.)
 
1. $llen"Bradle# K *oc2well -oftware *-LogiA?66
:. odicon " odsoft
>. mron " -#swin
?. -quare &" PowerLogic
5. Telemecanique K odicon T-H icro
 
Power flows through these contacts when the# are closed. The
normall# open (8) is true when the input or output status bit
controlling the contact is 3. The normall# closed (8C) is true
 
72
C#)%'
Coils represent rela#s that are energiEed when power flows to
them. 0hen a coil is energiEed it causes a corresponding
output to turn on b# changing the state of the status bit controlling
the output to 3. That same output status bit ma#be used to control
 
1Aecuted when power flows to the boA. -ome of these
unctions are timers! counters and math operations.
 
1ach rung or networ2 on a ladder program represents
a logic operation. In the rung aboe! both inputs $ and B
must be true (3) in order for the output C to be true (3).
*ung
* P1*$TI8
In the rung aboe! it can be seen that either input $ or B
is be true (3)! or both are true! then the output C is true (3).
*ung
A
 B
C
8T P1*$TI8
In the rung aboe! it can be seen that if input $ is be true (3)!
then the output C is true (6) or when $ is (6)! output C is 3.
*ung