PLC Structure and Hardware

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02_PLC_1_Structure.doc/hardware

Programmable Logic Controller

Handout

INDUSTRIAL AUTOMATION AND NETWORKING SECTION

Programmable Logic Controller_1

==== PPLLCC SSTTRRUUCCTTUURREE//HHAARRDDWWAARREE ====

1. BASIC COMPONENTS

Basic components of a PLC:

� Power supply

� Central Processing Unit (CPU)

� Inputs Modules

� Outputs Modules

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The general structure of programmable logic controller is as follow:

Power

Supply

Processor

Program Memory

Data Memory

Inputs

and

Outputs

I/O Bus

External Power Supply I

Central Processing Unit

O

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1.1 - Power Supply Module

There are several types of power supply module:

� 240V ac

� 110V ac

� 24V dc

Function:

Provides all the voltage level needed to operate the PLC. Converts 110V ac or 240V ac

into the dc voltage required by the CPU, memory, I/O interface.

Example of a power supply module

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Power supply wiring/installation guide:

1.2 - Central Processing Unit (CPU)

Consist of:

� Processor

� Memory

Function:

Receives information from input interface, process according to the program stored, update

the output information.

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Example of a CPU module

Memory

Memory location is an address in RAM or ROM where a group of bits (binary digits) can be

stored.

Grouping of bits depending on PLCs:

8 bits per word

16 bits per word

32 bits per word

LSB : Least Significant Bit - Digit that represents the smallest value

MSB : Most Significant Bit - Digit that represents the largest value

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Example:

The memory size of PLC is 884 words. How many bits of information can be stored?

Solution: 8 bits PLC : 884 x 8 = 7 072 bits 16 bits PLC : 884 x 16 = 14 144 bits 32 bits PLC : 884 x 32 = 28 288 bits

Types of memory:

� Volatile

� information lose when power lost

� easy to alter

� suitable when supported by battery backup

� Non-volatile

� no information lose when power lost

RAM (Random Access Memory)

� Volatile memory

� Read and write easily

� PLCs usually has CMOS-RAM with battery backup

ROM (Read Only Memory)

� Non-volatile memory

� Read only

� Information stored by manufacturer for internal use of the PLC

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EPROM (Erasable Programmable Read Only Memory)


� Can be reprogrammed after being entirely erased with the use of an UV light

source

EEPROM (Electrically Erasable Programmable Read Only Memory)


� Same programming flexibility as RAM. Provides permanent storage of the

program but can be easily changed using standard programming devices.

Memory Organisation or Memory Map:

Memory are divided into 3 areas:

� Data table area

Stores information for user program:

� Input table

� Output table

� Internal relay

� Function blocks current and pre-set value (timer / counter)

If signal is ON/OFF: information is stored in unique bit location (bit address)

If signal is number or code: information is stored in unique register or word location

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� User program area

Stores the programmed ladder logic/sequential function chart.

Most instructions require one word of memory & occupy the largest area of

memory

� System area

Memory Areas Used With CQM1H

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� Programming a PLC

� Using a PC

� Point-to-point connection

� Connection through network

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� Using a hand-held programming terminal

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1.3 – Inputs Module

Function:

To receive and convert field signals from pushbuttons, sensors or switches into a form that

can be used by the CPU.

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1.4 – Outputs Module

Function:

Takes signal from the CPU and translates them into forms that are appropriate to

produce control actions by external devices such as indicator lights, solenoids or motor

starters.

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1.5 – Inputs / Outputs Signal Processing

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2. TYPES OF PLC

There are two types of PLC:

� Compact

� Modular

� Compact PLC

The compact/ micro has the power supply and I/O on-board. It can be used on smaller,

stand-alone applications such as elevators, car washes, or mixing machines. It can also

be used on more complex industrial applications such as bottling and packaging

machines.

Siemens

Allen Bradley MicroLog

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� Modular PLC

The modular PLCs are used in more complex applications that support a greater number

of I/O points. They are are expandable. The power supply and I/O consist of separate

modules connected to the CPU. Choosing of modular PLCs depends on the complexity

of the task and possible future expansion.

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PLC Structure and Hardware

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