In todays world, PLC is heart and brain of all industries no matter which segment it is. It has changed the face of technology and way of work. Hear i am sharing just the basic and the evolution of PLC.

1. Prepared By
Ujjwal Kumar Choubey
ujjwalchoubey80@gmail.com

2. A programmable logic controller (PLC) is an
industrial digital computer which has been
adapted for the control of manufacturing
processes, such as assembly lines, robotic devices,
or any activity that requires high reliability and
process fault diagnosis.

3. Before PLC
Before the days of PLC the only way to control machinery
was through the use of relays. Relays work by utilizing a
coil that, when energized, creates a magnetic force to
effectively pull a switch to the ON or OFF position. When
the relay is de-energized, the switch releases and returns
the device to its standard ON or OFF position.
For example, if I wanted to control a motor ON or OFF, I
could attach a relay between the power source and the
motor. Then I could control when the motor is getting
power by either energizing or de-energizing the relay. This
type of relay is known as a power relay. There could be
several motors in one factory that need to be controlled, so
one add lots of power relays. So the factories started with a
mass electrical cabinets full of power relays.
Also, to switch the coils in the power relays ON and OFF we
need to add more relays. These relays are known as control
relays.

4. Problem with Relays
All the relays had to be hardwired in a very
specific order for the machine to work
properly, and if one relay would have an
issue, the system as a whole would not work.
Troubleshooting would take hours, and
because coils would fail and contacts would
wear out, there was need for lots of
troubleshooting.
These machines had to follow a strict
maintenance schedule and they took up a lot
of space.

5. Birth of PLC and Evolution
The engineers at General Motors were struggling with
same every day. Fortunately, at that time, the concept of
computer control had started to make its way into
conversations as General Motors. They identify the
following requirements for computer control to replace
hardwired panels.
 A solid-state system that was flexible like a computer
but priced like kind relay logic system.
 Easily maintained and programmed
 It had to work in an industrial harsh environment with
all its dirt, moisture, electromagnetism and vibration.
 It had to be modular in form to allow for easy
exchange of components and expandability.

6. Birth of PLC and Evolution
The first PLCs had the ability to work with input
and output signals, relay coil/contact internal
logic, timers and counters.
Timers and counters made use of word size
internal registers. The PLC continued to evolve
with the addition of one-shots, analog input and
output signals, enhanced timers and counters,
floating point math, drum sequencers and
mathematic functions.
Having built-in PID (Proportional-Integral-
Derivative) functionality was a huge advantage
for PLCs being used in the process industry.
Common sets of instructions evolved into fill-in-
the-blank data boxes that have made
programming more efficient.

7. Birth of PLC and Evolution
The ability to use meaningful Tag Names in place
of non-descriptive labels has allowed the end
user to more clearly define their application, and
the ability to import/export the Tag Names to
other devices eliminates errors that result when
entering information into each device by hand.
The first programming devices were dedicated,
but unfortunately the size of suitcases.
Later, handheld programming devices came into
the picture, but soon were replaced with
proprietary programming software running on a
personal computer.
AutomationDirect’s DirectSOFT, developed by
Host Engineering, was the first Windows-based
PLC programming software package.

8. Birth of PLC and Evolution
Having a PC communicating with a PLC provided the
ability to not only program, but also allowed easier
testing and troubleshooting.
Communications started with the MODBUS protocol
using RS-232 serial communications.
The addition of various automation protocols
communicating over RS-485, DeviceNet, Profibus,
and other serial communication architectures have
followed.
The use of serial communications and the various
PLC protocols also allowed PLCs to be networked
with other PLCs, motor drives, and human to
machine interfaces (HMI).
Most recently EtherNet and protocols such
as EtherNet/IP (for Industrial Protocol) have gained
tremendous popularity.

9. PLC Hardwares
 Power supply
 Processing unit (CPU)
 Memory
 Input/ Output Modules
 Programing device

10. Power Supply
The Power Supply is a module located in the
PLC system module rack. The DC power
(voltage and current) it provides power the
other modules in the rack, such as the CPU,
Co-processor Modules, and I/O Modules.

11. Processing unit (CPU)
The function of the CPU is to store and run the PLC
software programs. It also interfaces with the Co-
Processor Modules, the I/O Modules, the peripheral
device, and runs diagnostics. It is essentially the "brains"
of the PLC.
The items shown inside the CPU and their basic
functions are as follows:
 The microprocessor codes, decodes, and computes
data.
 The memory (ROM, PROM/EEPROM/UVPROM, and
RAM) stores both the control program and the data
from the field devices.
 The I/O Interface adapter connects the Co-Processor
Modules, the I/O Modules and the Peripheral Device
to the CPU.

12. Memory
System (ROM) give permanent storage for the
operating system and the fixed data used by the
CPU.
RAM for data, this is where information is stored
on the status of input and output devices and the
values of timers and counters and other internal
devices.
EPROM for ROM’s that can be programmed and
then the program made permanent.

13. Input/ Output Modules
Input modules provide the electrical connection
between field devices (pushbuttons, limit switches,
photoeyes) and internal process of the PLC.
They differ in voltages and types of signals produced
such as ON, OFF or a variable voltage.
Output modules are the devices that the PLC uses to
send changes out to the field.
These are the actuator the PLC can change to adjust or
control the process - motors, lights, relays, pumps, etc.

14. Programming Device
The programming device is to enter the required
program into the memory of the processor.
The program is developed in the programming device
and then transferred to the memory unit of the PLC.

15. PLC Software
It is a software technology designed to turn an
embedded computer into a fully functional and
programmable logic controller, also known as a
PLC.
It combines PLCs’ discrete, PID and analog I/O
control with high performing computer
networking, data handling, and computational
capabilities.
As such, software PLCs offer dependable
operation, exceptionally fast and deterministic
program scan durations, generate instructions
set, clear data table memory, unlimited user
programs, and also an open platform that allows
users to connect to a wide range of I/O systems
and networks among other devices.

16. PLC Language
IEC 61131-3 is the international standard for programmable controller programming
languages. The following is a list of programming languages specified by this
standard:
 Ladder diagram (LD)
 Sequential Function Charts (SFC)
 Function Block Diagram (FBD)
 Structured Text (ST)
 Instruction List (IL)

17. Ladder Diagram (LD)
Ladder logic is the main programming method
used for PLC's. Ladder logic has been developed
to mimic relay logic.
The first PLC was programmed with a technique
that was based on relay logic wiring schematics.
This eliminated the need to teach the electricians,
technicians and engineers how to program - so
this programming method has stuck and it is the
most common technique for programming in
today's PLC.

18. Sequential Function Charts (SFC)
SFC was developed to accommodate the
programming of more advanced systems. These
are similar to flowcharts, but much more
powerful.
This method is much different from flowcharts
because it does not have to follow a single path
through the flowchart.

19. Function Block Diagram (FBD)
FBD is another graphical programming language.
The main concept is the data flow that start from
inputs and passes in block(s) and generate the
output.

20. Structured Text (ST)
Programming has been developed as a more modern
programming language. It is quite similar to languages
such as BASIC and Pascal.
Structured Text (ST) is a high level textual language that
is a Pascal like language. It is very flexible and intuitive
for writing control algorithms.

21. Instruction List (IL)
ILs are not a graphical programming language.
Instead, they most resemble assembly language
programming.
As the name implies, a program is a series of
instructions, listed in much the same way as an
assembly program.

22. Advantages
 Small physical size & shorter project time.
 Cost effective for controlling complex system.
 Reliability.
 Less and simple wiring.
 Faster response.
 Remote control capability.
 More flexibility.
 Ease of maintenance / troubleshooting

23. Disadvantages
 Fixed circuit operation.
 PLCs manufacturers offer only closed loop
architecture.
 PLCs are propitiatory, which means software and
parts one manufacturer can’t be easily used in
combination with part of another manufacturer.
 Number of optional modules must be added to
maximize flexibility and performance.