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Programmable Logic
Controllers (PLC’s)
Definition
According to National Electrical Manufacturers’ Association
a PLC is:
A digital electronic device using a programmable memory
to store instructions and to implement specific functions
such as logic, sequence, counting, timing and arithmetic to
control machines and processes through digital or analog
input/output modules.
What is a PLC?

A special computer for logic controls
PLC History
In the late 1960's PLCs were first introduced
Bedford Associates (Bedford, MA) proposed something called
a Modular Digital Controller (MODICON) to a major US car
manufacturer
In the mid70's the dominant PLC technologies were sequencer statemachines and the bit-slice based CPU. The AMD 2901 and 2903 were
quite popular in Modicon and A-B PLCs
Communications abilities began to appear in approximately 1973.
The first such system was Modicon's Modbus. The PLC could now
talk to other PLCs and they could be far away from the actual
machine they were controlling
The 80's saw an attempt to standardize communications with General
Motor's manufacturing automation protocol(MAP
The 90's have seen a gradual reduction in the introduction of new protocols
Basic PLC Operation
PLCs consist of input modules or
points, a Central Processing
Unit (CPU), and output modules
or points.
Pushbuttons (sensors), in this
simple example, connected to
PLC inputs, can be used to start
and stop a motor connected to
a PLC through a motor starter
(actuator).
PLC ARCHITECTURE
Programmable controllers replace most of the relay panel wiring
by software programming.
It is consists of the
following modules:
Power supply module
CPU module
Digital input module
Digital output module
Analog input module
Analog output
PLC COMPONENTS
1. Processor
Microprocessor based, may allow
arithmetic operations, logic operators,
block memory, computer interface, local
area network, functions, etc.
2. Memory
Measured in words.
ROM (Read Only Memory),
RAM (Random Access Memory),
PROM (Programmable Read Only Memory),
EEPROM (Electric Erasable Programmable
ROM),
EPROM (Erasable Programmable Read Only
Memory),
PLC COMPONENTS
3. I/O
Modular plug-in periphery
AC voltage input and output,
DC voltage input and output,
Low level analog input,
High level analog input and output,
Special purpose modules, e.g., high speed
timers,
Stepping motor controllers, etc. PID, Motion
4. Power supply AC power
5. Peripheral
hand-held programmer (HHP)
printer
simulator
EPROM loader
graphics processor
network communication interface
modular PC
Discrete I/O :
-AC Voltage I/O or
Discrete Input:
Selector switches,
Pushbuttons,
Photoelectric Eyes,
Limit switches,
Circuit breakers,
proximity switches,
Level switch,
Pressure switch,
Temperature switches,
Motor starter contacts,
Relays contacts and
Thumbwheel switch
-DC Voltage I/O
Non-locking
Normally
open
Multiple
through
SPDT
locking
Normally closed
Multiple
pole
DPST
RELAYS
A switch whose operation is activated by an electromagnet is called
a "relay" . Coil circuit controls many contact points
contact
Relay coil
R1
coil
input
Output contact normally
open
Output contact normally
closed
R1
R1
Relays and Schematic
Symbols
input coil
OR
normally
closed
normally
open
OR
Logical Control with Relays
115VA C
wall plug
relay logic
input B
(normally open)
input A
(normally closed)
A
B
output C
(normally open)
C
ladder logic
Relay Logic In a PLC
push buttons
power
supply
+24V
com.
PLC
inputs
ladder
logic
A
B
C
outputs
115Vac
AC power
neut.
light
Discrete Output
Alarms,
Control relays,
Fans, Lights, Horns,
Valves,
motor starters, and
Solenoids
Analog I/O
Analog inputs:
-Flow sensor,
Pressure sensors,
Potentiometers,
Humidity sensors,
Temperature sensors,
load cell transducers
Analog outputs:
Analog meters
Analog valves
Actuators
Chart recorders
DC and AC motor drives
Some Special I/O
Thermocouple input
Low level analog signal, filtered, amplified, and digitized
before sending to the processor through I/O bus.
Fast input
50 to 100 microsecond pulse signal detection.
ASCII I/O
Communicates with ASCII devices.
Stepper motor output
Provide directly control of a stepper motor.
Servo interface
Control DC servo motor for point-to-point control and axis
positioning.
PID control
The Proportional Integral Derivative is used for closed
loop process control.
Network module
LADDER DIAGRAM
A ladder diagram (also called contact symbology) is a
means of graphically representing the logic required in a
relay logic system.
Rail
start
PB1
emergency stop
PB2
R1
Rung
R1
R1
A
LADDER DIAGRAM
The left vertical line of a ladder
logic diagram represents the
power or energized conductor.
The output element or instruction
represents the neutral or return
path of the circuit.
The right vertical line, which
represents the return path on a
hard-wired control line diagram,
is omitted. Ladder logic diagrams
are read from left-to-right, topto-bottom. Rungs are sometimes
referred to as networks.
A network may have several
control elements, but only one
output coil.
Reading Ladder Logic
HOT
NEUTRAL
A
B
X
C
D
G
E
F
H
INPUTS
Note: Power needs to flow through some combination of the inputs
(A,B,C,D,E,F,G,H) to turn on outputs (X,Y).
Y
OUTPUTS
A Ladder Logic Example
A
B
B
Note: When A is pushed, the output B will turn on, and
the input B will also turn on and keep B on perma
nently - until power is removed.
Note: The line on the right is being left of
and is implied in these diagrams.
f intentionally
Statement list
A statement list (STL) provides another view of a set of instructions. The
operation, what is to be done, is shown on the left. The operand, the
item to be operated on by the operation, is shown on the right. The set
of instructions in this statement list perform the same task as the ladder
diagram.
00000
00001
00002
00003
00004
00005
00006
00007
00008
LDN
LD
AND
LD
LD
AND
OR
ST
END
A
B
the mnemonic code is equivalent to
the ladder logic below
C
D
X
A
B
C
D
X
END
Note: The notation shown above is
not standard Allen-Bradley
notation. The program to the
right would be the A-B equiva
lent.
SOR
BST
XIC A
XIO B
NXB
XIO C
XIO D
BND
OTE X
EOR
END
Function Block Diagrams
Function Block Diagrams (FBD) provide another view of a set
of instructions. Each function has a name to designate its
specific task. Functions are indicated by a rectangle. Inputs
are shown on the left-hand side of the rectangle and outputs
are shown on the right-hand side.
Standard PLC scan cycle
PLC Scan
The PLC program is executed as part of a repetitive process referred
to as a scan. A PLC scan starts with the CPU reading the status of
inputs. The application program is executed using the status of the
inputs. Once the program is completed, the CPU performs internal
diagnostics and communication tasks.
The scan cycle ends by updating the outputs, then starts over. The
cycle time depends on the size of the program, the number of I/Os,
and the amount of communication required.
Software
Software is any information in a form that a
computer or PLC can use. Software includes the
instructions or programs that direct hardware.
Hardware
Hardware is the actual equipment. The PLC, the programming device,
and the connecting cable are examples of hardware.
Basic Requirements
In order to create or change a program, the following items are
needed:
• PLC
• Programming Device
• Programming Software
• Connector Cable
Programming Devices
The program is created in a programming device (PG) and then
transferred to the PLC. The program for the S7-200 can be
created using a dedicated Siemens SIMATIC S7 programming
device, such as a PG 720 (not shown) or PG 740, if STEP 7
Micro/WIN software is installed.
A personal computer (PC), with STEP 7 Micro/WIN installed,
can also be used as a programming device with the S7-200.
Software
A software program is required in order to tell the PLC what instructions it
must follow. Programming software is typically PLC specific. A software
package for one PLC, or one family of PLCs, such as the S7 family, would
not be useful on other PLCs. The S7-200 uses a Windows based software
program called STEP 7-Micro/WIN32. The PG 720 and PG 740 have STEP
7 software pre-installed. Micro/WIN32 is installed on a personal computer
in a similar manner to any other computer software.
Connector Cables PPI
(Point-to-Point Interface)
Connector cables are required to transfer data from the (Point-to-Point
Interface) programming device to the PLC. Communication can only take
place when the two devices speak the same language or protocol.
Communication between a Siemens programming device and the S7-200 is
referred to as PPI protocol (pointto- point interface). An appropriate cable is
required for a programming device such as a PG 720 or PG 740. The S7-200
uses a 9-pin, D-connector. This is a straight-through serial device that is
compatible with Siemens programming devices (MPI port) and is a standard
connector for other serial interfaces.
Connector Cables PPI
A special cable is needed when a personal computer is used as a
programming device. Two versions of this cable are available.
One version, called an RS-232/PPI Multi-Master Cable, connects a
personal computer’s RS-232 interface to the PLC’s RS-485 connector.
The other version, called a USB/PPI Multi-Master Cable, connects a
personal computer’s USB interface to the PLC’s RS-485 connector.
S7-200 Micro PLCs
The S7-200 Micro PLC is the smallest member of the SIMATIC S7 family
of programmable controllers.
The central processing unit (CPU) is internal to the PLC. Inputs and
outputs (I/O) are the system control points. Inputs monitor field devices,
such as switches and sensors. Outputs control other devices, such as
motors and pumps. The programming port is the connection to the
programming device.
S7-200 Models
There are five S7-200 CPU types: CPU 221, CPU 222, CPU 224,
CPU 224XP, and CPU 226 and two power supply configurations
for each type.
S7-200 Features
The S7-200 family includes a wide variety of CPUs and features. This
variety provides a range of features to aid in designing a cost-effective
automation solution. The following table provides a summary of the
major features, many of which will be covered in this course.
Mode Switch and
Analog Adjustment
When the mode switch is in the RUN position the CPU is in the run
mode and executing the program. When the mode switch is in the STOP
position the CPU is stopped. When the mode switch is in the TERM
position the programming device can select the operating mode.
The analog adjustment is used to increase or decrease values stored in
special memory. These values can be used to update the value of a timer
or counter, or can be used to set limits.
Optional Cartridge
The S7-200 supports an optional memory cartridge that provides a
portable EEPROM storage for your program. The cartridge can be
used to copy a program from one S7-200 PLC to a like S7-200 PLC.
Expansion Modules
The S7-200 PLCs are expandable. Expansion modules contain additional
inputs and outputs. These are connected to the base unit using a ribbon
connector.
The ribbon connector is protected by a cover on the base unit. Sideby-side mounting completely encloses and protects the ribbon
connector.
The ribbon connector is protected by a cover on the base unit. Sideby-side mounting completely encloses and protects the ribbon
connector.
External Power Supply
An S7-200 AC/DC/Relay would be connected to a 120 or
230 VAC power supply.
I/O Numbering
S7-200 inputs and outputs are labeled at the wiring terminations and
next to the status indicators. These alphanumeric symbols identify
the I/O address to which a device is connected. This address is used
by the CPU to determine which input is present and which output
needs to be turned on or off. I designates a discrete input and Q
designates a discrete output. The first number identifies the byte,
the second number identifies the bit. Input I0.0, for example, is byte
0, bit 0.
Inputs
Outputs
Freeport Mode
The programming port has a mode
called freeport mode.
Freeport mode allows connectivity to
various intelligent sensing devices
such as a bar code reader.
Printer
Freeport mode can also be used to
connect to a non-SIMATIC printer.
Interconnection
It is possible to use one
programming device to address
multiple S7-200 devices on the
same communication cable. A total
of 31 units can be interconnected
without a repeater.
Symbols
Contacts
Coils
Coils represent relays that are energized when power flows to them.
When a coil is energized, it causes a corresponding output to turn on
by changing the state of the status bit controlling that output to 1.
That same output status bit may be used to control normally open and
normally closed contacts elsewhere in the program.
Boxes
Boxes represent various instructions or functions that are executed when
power flows to the box. Typical box functions are timers, counters, and
math operations.
Entering Elements
An AND Operation
PLC HARD WIRING DIAGRAM
External switches with stored program ladder
diagram and output devices can be presented using
The shown PLC hard wired diagrams
A
Input
01
B
Output
PLC
01
02
20
11
12
02
20
03
20
11
External
switches
Stored program
C
•
•
•
•
PLC
Programming Device
Programming Software
Connector Cable
Input Card Example
PLC Input Card
24V AC
normally open push-button
24 V AC Hot
Power
Supply
Neut.
00
01
02
03
04
normally open
temperature switch
05
06
07
COM
Pushbutton (bob:3:I.Data.1)
it is in rack "bob"
slot 3
Te mpsensor (bob:3:I.Data.3)
Note: inputs are normally high impedance. This means that they will
use very little current.
Output Card Example (Transistors)
24 V DC
Output Card
120 V AC
Power
Supply
Neut.
00
01
Relay
02
03
Motor
04
05
24 V Lamp
06
07
COM
rack "sue"
slot 2
+24 V DC
Power
Supply
COM
Motor (sue:2.O.Data.3)
Lamp (sue:2.O.Data.3)
Output Card Example (Relays)
120 V AC/DC
Output Card
24 V DC
Power
Supply
00
01
02
03
Relay
04
05
06
Motor
07
in rack 01
I/O group 2
24 V lamp
120 V AC
Power
Supply
Some Design Issues
• DC voltages are usually lower, and therefore safer (e.g.,
24V).
• DC inputs are very fast, AC inputs require a longer on-time.
• DC voltages can be connected to more electrical systems.
• AC signals are more immune to noise than DC.
• AC power is easier and less expensive to supply to
equipment.
• AC signals are very common.
Relays
Contactor - Special relays for switching large current loads.
Motor Starter - Basically a contactor in series with an overload relay
to cut off when too much current is drawn.
Arc Suppression - when any relay is opened or closed an arc will
jump. This becomes a major problem with large relays. On relays
switching AC this problem can be overcome by opening the relay
when the voltage goes to zero (while crossing between negative and
positive). When switching DC loads this problem can be minimized by
blowing pressurized gas across during opening to suppress the arc
formation.
AC coils - If a normal relay coil is driven by AC power the contacts will
vibrate open and close at the frequency of the AC power. This
problem is overcome by adding a shading pole to the relay.
Problem: You are planning a project that will be controlled by a PLC.
Before ordering parts you decide to plan the basic wiring and select
appropriate input and output cards. The devices that we will use for
inputs are 2 contact switches, a push button and a thermal switch. The
output will be for a 24Vdc solenoid valve, a 110Vac light bulb, and a
220Vac 50HP motor. Sketch the basic wiring below including PLC cards.
L1
N
010
stop
start
CR1
MCR
020
CR1
030
L1
PLC
N
90-1
040
CR1
I:0/0
PB1
I:0/1
LS1
I:0/2
O:0/0
090
R
050
060
100-1
O:0/1
110-1
O:0/2
PB2
070
I:0/3
100
110
120
CR2
ac com
080
CR1
090
90-1
035
100
100-1
050
110
110-1
060
120-1
070
CR2
130
L2
G
S1
120-1
O:0/3
120
L1
Drill S tation
L1
N
Wiring Symbols
disconnect
(3 phase AC)
normally open
limit switch
normally open
push-button
circuit interrupter
(3 phase AC)
normally closed
limit switch
breaker (3 phase AC)
normally closed
push-button
double pole
push-button
mushroom head
push-button
Input Connections
AC
AC input module allows
interfacing of input
devices which provide
AC output. Converter is
used to covert incoming
AC voltage to a DC logic
signal to be used by the
processor
DC
The Dc input module
allows interfacing of
input devices which
provide a DC output
voltage. The range of
input voltage for DC
input module varies
between 5 VDC and 30
VDC
TTL
TTL input interfaces allow
the controller to accept
signals from TTL compatible
devices including solid state
controls and sensing
instrumentation. TTL inputs
are used for interfacing with
some 5 VDC level control
devices and several types of
photoelectric sensors
Output Connections
AC
AC output module
allows interfacing of
output devices which
provide AC input.
DC
DC output module
interface is used to
control DC loads by
switching On and Off.
TTL
The TTL output
interface allows
the controller to
drive output
devices that are
TTL compatible
such as seven
segment LED
display
DC Inputs
The difference between the two types is whether the load (in our
case, the plc is the load) is switched to ground or positive voltage.
An NPN type sensor has the load switched to ground whereas a
PNP device has the load switched to positive voltage.
DC Inputs
Inside the sensor, the transistor is just acting as a switch. The sensors
internal circuit tells the output transistor to turn on when a target is
present. The transistor then closes the circuit between the 2 connections
shown above. (V+ and plc input).
AC Inputs
AC input modules are less common these days than dc input modules
sensors typically have transistor outputs
Relay Outputs
A relay is non-polarized and typically it can switch either AC or DC.
Here the common is connected to one end of our power supply and
the other end of the supply is connected to the load. The other half
of our load gets connected to the actual plc output you have
designated within your ladder program.
Transistor Outputs
Shown above is how we typically connect our output device to the
transistor output. Please note that this is an NPN type
transistor. If it were a PNP type, the common terminal would most
likely be connected to V+ and V- would connect to one end of our
load. Note that since this is a DC type output we must always
observe proper polarity for the output. One end of the load is
connected directly to V+ as shown above.
Typical I/O connections with PLC:
The shown
diagram is a
typical PLC
with input
and output
module to
control
pneumatic
actuator
Advantages of PLCs:









Increase flexibility,
Faster implementation of changes and
correction,
Lower cost,
Increased visual observations,
Increased operation speed,
Increased reliability and maintainability,
Increased security,
Reprogramming capability,
Elimination of wiring.
Summary
This sequence covered the following
topics
1-PLC’S definition
2-PLC’s Components
3-PLC’S advantages
5-PLC’S I/O connections
