Practical Programmable Logic Controllers (PLCs) for Automation and Process Control

Practical Programmable Logic Controllers (PLCs) for
Automation and Process Control
Technology www.idc-online.com/slideshare Technology TTrraaiinniinngg tthhaatt Wwoorrkkss

Objectives
In this chapter, we will
learn the following:
• Introduction to the PLC
• Basic block diagram of the PLC
• Size of the PLC system
• Components of the PLC system
• PLC and process interaction
• Number system and codes

Introduction to the PLC
“PLC” means “Programmable Logic Controller”. The word
“Programmable” differentiates it from the conventional
hard-wired relay logic. It can be easily programmed or
changed as per the application’s requirement. The PLC also
surpassed the hazard of changing the wiring.
The PLC as a unit consists of a processor to execute the
control action on the field data provided by input and output
modules.
In a programming device, the PLC control logic is first
developed and then transferred to the PLC.

What can a PLC do?
• It can perform relay-switching tasks.
• It can conduct counting, calculation and comparison of analog process
values.
• It offers flexibility to modify the control logic, whenever required, in
the shortest time.
• It responds to the changes in process parameters within fraction of
seconds.
• It improves the overall control system reliability.
• It is cost effective for controlling complex systems.
• Trouble-shooting becomes simpler and faster.
• An operator can easily interact with the process with the help of the
HMI (Human-Machine Interface) computer.

Basic block diagram of the PLC

Size of the PLC system
PLCs are classified on basis of their sizes:
• A small system is one with less than 500 analog and digital
I/Os.
• A medium system has I/Os ranging from 500 to 5,000.
• A system with over 5,000 I/Os is considered large.

Components of the PLC system

PLC Configuration

CPU or processor
The main processor (Central Processing Unit or CPU) is a
microprocessor-based system that executes the control
program after reading the status of field inputs and then
sends commands to field outputs. It is easy to perform
arithmetic functions, manipulate data and calculate Boolean
logic.
The PLC’s memory contains the manufacturer’s operating
system and housekeeping functions. It also has program
written by the user and data stored by the user related to the
process or equipment being controlled.

I/O section
The I/O section consists of a rack and individual I/O modules,
which are plugged into the rack and a DC power supply. A
standard approach is to connect to the main processor rack
with communication cables to a series of other I/O racks.
I/O modules act as “Real Data Interface” between field and
PLC CPU. The PLC knows the real status of field devices,
and controls the field devices by means of to the relevant
I/O cards.

Programming device
A CPU card can be connected with
a programming device through a
communication link via a
programming port on the CPU.
Thus, it is possible to transfer
programs from the programming
device to the CPU, monitor the
CPU’s program online and make
necessary changes in the CPU’s
program.

Operating station
• An operating station is commonly used to provide an
"Operating Window" to the process. It is usually a
separate device (generally a PC), loaded with HMI
(Human Machine Software).
• This operating station can change any process set point,
observe all process parameters, process alarms, etc.

PLC and process interaction
The field devices or field data are broadly classified as follows:
• Digital or discrete or on/off type field devices
• Analog or continuous type field devices

Digital I/O and PLC

Digital I/O and PLC
• Discrete or digital inputs are the most common types of
field inputs. Selector switches, pushbuttons, limit switches,
temperature switches, level switches, flow switches, etc., are
common examples. All types of switches that permit digital
contact are included in this class.
• Depending on the field device contacts, they are normally
connected to 110VAC, 230V AC and, 24VDC types of
digital input cards installed in PLC rack.

Analog I/O and PLC

Analog I/O and PLC
• Analog or continuous devices are generally used for
getting feedback of the process parameter control. For
example, temperature transducers (RTDs, thermocouples),
level, pressure, flow, etc., transmitters.
• Depending on field transducer devices, they are normally
connected to 0-20 or 4-20 mA DC, 0-10VDC, RTDs,
thermocouples, etc., type of analog input card, installed in
the PLC rack.
• Analog output actuators, commonly used in the field,
include continuous actuators, I/P converter for valves,
reference for drives, etc.

Number systems and codes
The commonly used number systems can be listed as:
• Decimal number system
• Binary number system
• Octal number system
• Hexadecimal (Hex) number system

Number systems and codes
The following definitions / terms are commonly encountered:
• Bit: A single binary digit that can have either value 0 or 1.
• Base: This denotes the total number of digits used by the number
system.
• LSB (Least Significant Bit): This is the bit that represents the
smallest value.
• MSB (Most Significant Bit): This is the bit that represents the
largest value.
• Byte: A group of 8 bits.
• Nibble: A group of 4 bits.
• Word: A group of 16 bits

Decimal number system
The decimal number system has a base of ‘10’. Base ‘10’ means it uses ten unique
numbers (0 to 9) for the entire number system. A specific weight value is allocated
to each digit from the right to the left. Therefore, the value of a decimal number
depends on the digit as well as its location.
For example:
If we take ‘2312’ as a decimal number, its weight can be shown as,
2 4 1 3
3 x 10 exp. 0 = 3x1 = 3
1 x 10 exp. 1 = 1x10 = 10
4 x 10 exp.2 = 4x100 = 400
2 x 10 exp.3 = 2x1000 = 2000
= 2413 (Decimal)

Binary number system
The weight of the digits is calculated in terms of base ‘2’
like for example:
1 0 0 1 1
1 x 2 exp. 0 = 1x1 = 1
1 x 2 exp .1 = 1x2 = 2
0 x 2 exp .2 = 0x4 = 0
0 x 2 exp .3 = 0x8 = 0
1 x 2 exp .4 = 1x16 = 16
= 19 (Decimal)

Octal number system
The binary system uses eight digits – from zero (0) to
seven (7) to represent all the numbers.
For example, the octal number 24 is represented as,
2 4
4 x 8 exp.0 = 4x1 = 4
2 x 8 exp.1 = 2x8 = 16
= 20 (Decimal)

Hexadecimal (Hex) number system
• The base of this number system is ‘16’. It provides a shorter
notation of the numbers, compared octal system. It is very
popular and was introduced by IBM. The hexadecimal
number system uses 16 digits. These are:
• Numbers ‘0’ to ‘9’, and it uses letters A, B, C, D, E, and F
to represent the decimal equivalents of 10, 11, 12, 13, 14
and 15.

Hexadecimal (Hex) number system
For example, the hex number 2F is represented as:
0 0 2 F
F x 16 exp.0 = Fx1 = 15
2 x 16 exp. 1 = 2x16 = 32
= 47 (Decimal)

BCD (Binary Coded Decimal)
• Binary coded decimal uses binary numbers in coded
format to represent the decimal numbers (unlike the
normal way; discussed in the earlier section).
• It is also called as 8421 BCD code. It basically
employs four binary bits, with the weights 1, 2, 4
and 8 assigned to it.

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Practical Programmable Logic Controllers (PLCs) for Automation and Process Control

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