A Programmable Logic Controller (PLC) is a
digital computer used for automation of
electromechanical processes, such as control of
machinery on factory assembly lines, control of
amusement rides, or control of lighting fixtures.
PLC was introduced in late 1960’s to remove relay’s based
First commercial & successful Programmable Logic
Controllers was designed and developed by Modicon as a
relay replacer for General Motors.
Earlier, it was a machine with thousands of electronic
Later ,in late 1970’s,the microprocessor became reality &
greatly enhanced the role of PLC permitting it to evolve
form simply relay to the sophisticated system as it is today.
Major Components of a Common PLC
Major Components of PLC
Provides the voltage needed to run the primary PLC components
Provides signal conversion and isolation between the internal
logic level signals inside the PLC and the field’s high level signal.
Provides intelligence to command and govern the activities of
the entire PLC systems.
Used to enter the desired program that will determine the
sequence of operation and control of process equipment or
Programming a PLC
Functional block diagram
Structural text(similar to Pascal)
Instruction list(similar to assembly language)
Why PLC ?....
• The controller is designed in modular form, so that sub-
assemblies could be removed easily for replacement or
• A PLC can work in the harsh and extreme conditions of the
industry where other systems such as a Micro-Controller
• The system becomes reusable.
• The method used to program the controller is simple, so
that it can be easily understood by plant personnel.
PAC stands for Programmable Automation Controller.
PLC is best suited for machine control but has limited
memory and a focus on discrete I/O with on/off
On the other hand , a PAC is geared more toward
complex automation system architectures composed
of a number of PC-based software applications
Features of PAC
Composed of a number of PC-based software
Including HMI (human machine interface) functions
Architecture based on Ladder logic
Provide greater flexibility in programming
Larger memory capacity
Advantages of PACs over PLCs
More open architecture
More analog I/O options
Better process control capabilities
More built in communication ports
Programming over USB ports
Tag based programming (one software package
capable of programming multiple models )