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2.1.1Define PLC
PROGRAMMABLE LOGIC CONTROLLER (PLC)
is an industrial computer control system that
continuously monitors the state of input devices
and makes decisions based upon a custom
program to control the state of output devices

FUNCTIONS OF CONTROLLERS
1) on-off control,
2) sequential control,
3) feedback control,
4) motion control.

ADVANTAGES OF USING PLC:
 Easier modification
 Design easily changed using software ( changes and addition to
specifications can be processed by software.
 A wide range of control application
 Easy maintenance.
 High Reliability
 Able to withstand Harsh plant/process environments (Operate
normally under severe conditions of temperature, humidity, voltage
fluctuations and noises).

i. Compact PLC
ii. Modular PLC
iii. PLC Plug-in Card
Manufacturer: OMRON, Allen Bradley,
Mitsubishi, NAIS, Siemens,
Toshiba, Festo etc.
2.1.4 Types of PLC Construction:

2.1.4 Types of PLC Construction:
i. Compact PLC:
Compact PLC – MELSEC FX3U
Figure 1: Compact PLC

i. Modular PLC
Figure 2: Modular PLC

iii. PLC Plug-in Card
Figure 3: Plug-in Card PLC

3.0 PLC HARDWARE DESIGN
Microproce
ssor/ CPU
ROM RAM
Input
Output
Data BUS
Control Bus
Address Bus
3.1 Processor Architecture for PLC:
Figure 4: Design of Basic Microcomputer

3.1.1 FUNCTION OF EACH BLOCK:
a. Central Processing Unit (CPU)
 The CPU controls, monitors and supervises all
operations within PLC.
 It is also caries out programmed instructions stored in
the memory.
 An internal communications highway also known as a
bus system, carries information to and from the CPU,
memory and I/O units under the control of the CPU.

b. Memory Unit
• For storage of programs.
• The user’s ladder logic program, the state of I/O in the memory
of PLC.
• The main program and the other programs necessary for the
operation of PLC.
• The organization of the data and information in the memory is
called memory map.
2 types of Memory
RA
M
ROM

c. Input/ Output (I/O) Unit
 Input Unit:
Function as a medium that connects the external
input devices
(Switch, sensor & timer)
 Output Unit:
Function as a medium that connects the external
output devices to the CPU within PLC.
( Lamp, motor & solenoid )

INPUT DEVICES:
Level SW
Flow SW
Thumbwheel
SW
Limit Switch
Push Button

Motor
Solenoid
LED Display
Heater Coil
Lamp
INPUT DEVICES:

d. Display and Indicator Unit
Referring to PLC standard display Internal Relay:
 Console’s LCD Display
if user use Mnemonic Code Programming.
 Monitor Screen
if user use software programming method.
LCD Display
Monitor Screen

h. Power Supply Unit:
Supplies DC power to the Central Processing Unit, Input Unit and
Output Unit.
i. Printing Unit
Used to print control system programming that controlled by PLC
wether graphically or text.

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