‫بسم هللا الرحمن الرحيم‬

‫‪PLC Course‬‬
‫مقرر الحاكمات المنطقية المبرمجة‬

‫‪Lec 3‬‬

‫‪DR. M. Arafa‬‬ ‫‪1/3/2023‬‬

 Output Devices

➢ Control relays (CRs)

➢ Motor

➢ Solenoids

➢ Valves
➢ Alarms

➢ Fans

➢ Lights
 Output Devices

➢ Solenoids
• A solenoid is an electromechanical device which allows for an electrical
device to control the flow of a gas or liquid (valves).

• The electrical device causes a current to flow through a coil located on the
solenoid valve.

• This current flow in turn results in a magnetic field which causes the
displacement of a metal actuator.

• Electromagnetic solenoids are used in hotel door locks, water-pressure

valves and in air conditioning systems.
Solenoid basic wired coil
Solenoid Valve
 Output Devices

➢ Control Relay (Electromagnetic Relay):

• An electromagnetic relay is a magnetic switch. It uses electromagnetism

to switch contacts.

• A relay will usually have only one coil but may have any number of
different contacts.
 Output Devices

➢ Control Relay (Electromagnetic Relay):

N.C (Normally
Closed Contact)

N.O (Normally
(Common)
Opened Contact)

Electromagnetic Relay
Relay Operation

(1) With no current flow through

the coil (the coil is de-energized),
the armature is held away from the
core by spring tension.

(2) When the coil is energized, the

electromagnetic field moves the
armature towards the core causing the
contact points of the relay to open or
close.
Physical Relay with different contacts
 Relay coil power source
5v/6v/12v/24v/48v/100v

N.O
N.C
common

Relay Connection with A 220v Device/Load

Example1: write the truth table for the following relay circuit

A B C ON

0 0
OFF
0 1

1 0

1 1

OFF
C
Solution:

ON

OFF

C = A/ B
OFF
C
Example2: writ the truth table for the following relay circuit

A B C ON
0 0

0 1 OFF

1 0

1 1

ON
C
Solution:

ON

OFF

C =(A . B) /
NAND gate ON
C
Example 3:
For the Relay in the following figure, explain the number of contacts, the type
and pins number for each contact. How many devices can be controlled using this
relay?

1 2 3 4 5 6
10

11
9 8 7
Solution
➢ The number of contacts = 6

• 3 Normally Open (NO) contacts with the following pins numbers:

(1 , 9) & (3 , 8) & (5 , 7)

• 3 Normally Closed (NC) contacts with the following pins numbers:

(2 , 9) & (4 , 8) & (6 , 7)

➢ The number of devices that can be controlled using this relay is 6

1 2 3 4 5 6
10

11
9 8 7
Relay Logic Diagram
&
PLC Ladder Diagram
 (1) Relay Logic Diagram

➢ Like ladder diagram, shows the logical relationship between I/O devices
(shows how to control outputs based on input conditions).

➢ Certain symbols were used to represent different input and output devices.

➢ It is also called hardwired ladder diagram.

➢ The PLC power supply will be the voltage between the two vertical lines,
labeled L1 and L2. In general, L1 represents the supply terminal and L2
represents the common (neutral) terminal.

➢ Relay logic diagrams use rungs (one or more) to accomplish the control of
outputs based on input conditions.
 (1) Relay Logic Diagram

➢ Any number of inputs and only one output appear in a rung of the relay
logic diagram.

➢ The last symbol used in a rung (from left to right) must be for an output.

➢ power flows through any rung from left to right such that there exist
a continuous path (logic continuity = rung is true) through this rung.
➢ Current in the below relay logic diagram can flow to the electric motor or motor
starter (turn on the Motor) when all three input switches are closed.

Inputs

Output

Rung 1

A continuous path is required in a rung to

flow power from left to right (rung is true)

Simple Relay Logic diagram

➢ The following figure illustrates how a relay coil and contacts of a control relay
(CR) are used in a relay logic diagram to control a solenoid (SOL).

▪ When the pushbutton switch (PB) is pressed, the relay coil (CR) is energized, and
its contact CR is closed.

▪ Closing the contact CR causes the solenoid (SOL) to be energized. When the
normally closed (NC) limit switch (LS1) is opened, the relay coil (CR) is
de-energized and its contact CR is opened.

▪ Opening the CR contact causes the solenoid (SOL) to be de-energized.

Rung 1

Rung 2

Use of a relay coil and contact in

the relay logic diagram
PLC I/O Physical Connection
Symbols used in Relay Logic Diagram
Temperature Switch
(NO)
 (2) PLC Ladder Diagram (Ladder diagram)
➢ Ladder diagram is a graphical programming method shows how to control
outputs and perform functional operations based on input conditions.

➢ Normally open (NO) or normally closed (NC) contacts are used for inputs
while coils of control relays are used for outputs.

➢ The contact symbol is the same for any input switches (toggle, push button,
limit, liquid level, relay contact).

➢ The coil symbol is the same for any outputs (Solenoid, Lamp, Motor,
Control relay).

➢ The PLC power supply will be the voltage between the two vertical lines
(rails or bars), labeled L1 and L2. In general, L1 represents the supply
terminal and L2 represents the common (neutral) terminal.
 (2) PLC Ladder Diagram (Ladder diagram)

➢ Ladder diagram uses rungs (one or more) to accomplish the control of

outputs based on input conditions.

➢ Any number of inputs and only one output appear in a rung of the ladder
diagram.

➢ The last symbol used in a rung (from left to right) must be for an output.

➢ power flows through any rung from left to right such that there exist
a continuous path (logic continuity = rung is true) through this rung.
EX1:

Rung 1

Relay Logic Ladder Diagram

Rung 1

A continuous path is required in a rung to

flow power from left to right (rung is true)

PLC Ladder Diagram

EX2:

L1 L2
EX3:

Relay Logic Ladder Diagram

PLC Ladder Diagram

Symbols used in Ladder Diagram
Guidelines For Using Symbols
In PLC Ladder Diagram
 Guidelines For Using Symbols In PLC Ladder Diagram

1) Normally Open Contact:

When evaluated by the program, this symbol is examined for a 1 to close

the contact; therefore, the signal referenced by the symbol must be ON,
CLOSED, activated, high, …. etc. (it is called examine-ON instruction)

2) Normally Closed Contact:

When evaluated by the program, this symbol is examined for a 0 to keep

the contact closed; thus, the signal referenced by the symbol must be OFF,
OPEN, deactivated, low, ….. etc. (it is called examine-OFF instruction)
3) Output:

An output of a given rung will be energized if any left to right path has all
contacts closed (continuity path), with the exception of power flow going in
reverse before continuing to the right. An output can control either
a connected device or an internal output used exclusively within the
program. An internal output does not control a field device. Rather,
it provides interlocking functions within the PLC.

4) Input:

The contact symbol can represent input signals sent from connected inputs
(like digital inputs switches), or contacts from internal outputs, or contacts
from connected outputs (like contacts of a control relay CR).
5) Contact addresses:

Each program symbol is referenced by an address. If the symbol refers to

a connected input/output device, then the address is determined by the point
where the input/output device is connected. The same thing is done for the
outputs.

6) Repeated use of contacts:

Contacts of a given input, output, or internal output can be used throughout

the program as many times as required.

7) Logic format:

Contacts can be programmed in series or in parallel, depending on the

required control logic for the output. The number of series contacts or
parallel branches allowed in a rung depends on the PLC type.
 Memory

➢ The total memory in a PLC is actually composed of two different

memories:
1) Executive Memory (System Memory)

2) Application Memory (User Memory)

➢ Data Table Area

➢ User Program Area

 ➢ stores the status of digital inputs connected to
1
the PLC’s input interface.

1
➢ Stores the status of control actions of digital
2
2 output devices that are connected to the
3 PLC’s output interface.

➢ contains storage bits that are referred to as

either internal outputs, internal coils, internal
(control relays), or internals.
➢ These internals provide an output, for
3 interlocking purposes, of ladder sequences in
the control program.
➢ Internal outputs do not directly control
output devices because they are stored in
Application Memory addresses that do not map the output table
and, therefore, any output devices.
 Normally
open

Held
closed

(a) NO contact with a logic 0 reference address

(b) NO contact with a logic 1 reference address
 Normally
closed

Held
open

(a) NC contact with a logic 0 reference address

(b) NC contact with a logic 1 reference address
(a) An Output coil with a logic 0 reference address
(b) An Output coil with a logic 1 reference address
Basic Logic Operations in
Ladder Diagram
 (1) Logical AND Operation in Ladder Diagram
➢ The logical AND function is constructed by series combinations (two or
more) of a normally opened or closed contacts. These contacts are used to
represent input signals sent from connected inputs (like digital inputs
switches), or contacts from internal outputs, or contacts from connected
outputs (like contacts of control relay CR).
L1 L2
For Example:

➢ Y1 = A B

➢ Y2 = A B C
 (2) Logical OR Operation in Ladder Diagram

➢ The logical OR function is constructed by parallel combinations of a

normally opened or closed contacts.

For Example:

➢ Y=A+B+C
Rung 1
 (3) Logical NOT Operation in Ladder Diagram

➢ The logical NOT function is constructed by referencing the input signal

with a normally closed contact.

For Example:

➢ Y1 = A

➢ Y2 = A' Rung 1

Rung 2
Example 1:
➢ Draw the ladder diagram to implement the following logic expression:

Y = (A + C ) B
Solution
L1 L2

Rung 1
Example 2:
➢ Draw the ladder diagram to implement the following logic expression:

Y = (A B ) + C

Solution Z = Y'

Rung 1

Rung 2
Example 3:

➢ Draw the ladder diagram to implement the following logic expression:

Y1 = (A B +D' E ) C

Y2 = F (G +K') H
Solution
Y1 = (A B + D' E ) C
Y2 = F (G +K' ) H

Rung 1

Rung 2
Example 4:

➢ Draw the equivalent ladder diagram of the following logic circuit:

Solution

The Boolean logic expression is:

Y = A B C' + D + E'

Rung 1