Top PLC courses in Lahore

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A programmable logic controller (PLC) or programmable controller is an industrial computer. Burraq engineering solution provides free PLC courses in Lahore. That has been durable and adapted to control production processes such as assembly lines, machines, robotic devices, or any activity that requires high reliability, easy programming, and diagnosis of process errors.


PLCs can range from small modular devices with dozens of inputs and outputs (I/O), in a case integrated with the processor, to large modular rack-mounted devices with thousands of I/Os, often interconnected with other PLC and SCADA systems. They can be designed for many digital and analog layouts.

Harsh environments

PLCs were first developed in the automotive industry to provide flexible, robust and easily programmable controllers to replace hard-wired relay logic systems. Since then, they have been widely accepted as highly reliable automation controllers suitable for harsh environments.


The hard-wired nature made it difficult for designers to change the automation process. Changes would require new involvement and careful updating of the documentation. If even one wire was out of place or one relay failed. Technicians often spent hours troubleshooting by examining schematics and comparing them to existing wiring.

General purpose

When general-purpose computers became available, they were soon used to control logic in industrial processes. These early computers were unreliable[5] and required specialized programmers and strict control of operating conditions such as temperature, cleanliness, and power quality.

Several advantages

The PLC provided several advantages over earlier automation systems. It tolerated industrial environments better than computers and was more reliable, more compact, and required less maintenance than relay systems. It was easily expandable with additional I/O modules, while relay systems required complicated hardware changes in case of reconfiguration.

Manufacturing process

This made it easier to repeat the design of the manufacturing process. With a simple programming language focused on logic and switching operations, it was more user-friendly than computers using general-purpose programming languages. It also enabled monitoring of its operation.

Ladder logic

Early PLCs were programmed in ladder logic that strongly resembled the schematic diagram of relay logic. This program enrollment was chosen to reduce training requirements for existing technicians. Other PLCs used a form of instruction list programming based on a stack-based logic solver.


In 1968, GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposals for an electronic replacement for hardwired relay systems based on a white paper written by engineer Edward R. Clark. The winning proposal came from Bedford Associates of Bedford, Massachusetts.


Bedford Associates formed a company to develop, manufacture, sell and service this new product, which they named Modicon (which stands for Modular Digital Controller). One of the people who worked on this project was Dick Morley, who is considered the “father” of PLC.


The Modicon brand was sold in 1977 to Gould Electronics and later to Schneider Electric, the current owner.[11] Around the same time, Modicon created Modbus, the data communication protocol used with its PLCs. Modbus has since become a standard open protocol commonly used to connect many industrial electrical devices.

Electric plant

One of the first 084s produced is now on display at the Schneider Electric plant in North Andover, Massachusetts. It was introduced to Modicon GM when the unit was retired after nearly twenty years of continuous service. Modicon used the 84 monikers at the end of its product line until the 984 appeared.


In a parallel development, Odo Josef Stranger is also sometimes referred to as the “father of the programmable logic automaton”. He co-invented the Allen-Bradley programmable logic controller and is credited with inventing the initialism PLC. Allen-Bradley (now a brand owned by Rockwell Automation) became the main PLC.

Graphical representation

Many early PLCs were not capable of graphical representation of logic, so it was instead represented as a series of logical expressions in some type of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more common to use ladder logic as it was a familiar format used for electromechanical control panels.

Function block

There are newer formats such as state logic and function block (which are similar to how logic is represented when using digital ICs), but they are still not as popular as ladder logic. The primary reason is that the PLC solves logic in a predictable and repetitive sequence.

Programming panels

Until the mid-1990s, PLCs were programmed using proprietary programming panels or special programming terminals, which often had dedicated function keys representing the various logic elements of the PLC programs. Some proprietary programming terminals displayed elements of PLC programs as graphic symbols.

Cassette tapes

The programs were stored on cassette tapes. Printing and documentation facilities were minimal due to a lack of memory capacity. The earliest PLCs used magnetic core non-volatile memory. Modular PLC with EtherNet/IP module, discrete and analog I/O, with some slots empty.

Mechanical design

There are two types of mechanical design for PLC systems. A single box or cube is a small programmable controller that fits all units and interfaces into one compact housing, although additional expansion modules for inputs and outputs are usually available. Several racks can be managed by a single processor and can have thousands of inputs and outputs.

Communication method

Either a special high-speed serial I/O connection or a comparable communication method is used so that the racks can be distributed outside the processor, reducing cabling costs for large plants. There are also options for mounting I/O points directly on the machine and using quick disconnect cables to sensors and valves, saving time on wiring and component replacement.

Discrete signals

Discrete (digital) signals can only take on or off values ​​(1 or 0, true or false). Examples of devices providing a discrete signal include limit switches, photoelectric sensors, and encoders. Discrete signals are sent using voltage or current, with specific extreme ranges marked as on and off. For example, a controller may use a 24 Vdc input with values ​​above 22 Vdc representing on, values ​​below 2 Vdc representing off, and intermediate values ​​are undefined.

Analog signals

Analog signals can use a voltage or current that is proportional to the size of the monitored quantity and can take any value within their scale. Pressure, temperature, flow and mass are often represented by analog signals. These are usually interpreted as integer values ​​with different ranges of precision depending on the device and the number of bits available to store the data.

Proper integration

The PLC will take this value and transpose it into the desired units of the process so the operator or program can read it. Proper integration will also include filter times to reduce noise as well as high and low limits to report faults. Current inputs are less sensitive to electrical noise than voltage inputs.

Short compared

Distance from the device and the controller is also a concern as the maximum traveling distance of a good quality 0-10 V signal is very short compared to the 4-20 mA signal.[citation needed] The 4-20 mA signal can also report if the wire is disconnected along the path as an mA signal would indicate an error.


Special processes

Some special processes need to work permanently with minimum unwanted downtime. Therefore, it is necessary to design a system that is fault-tolerant and capable of handling the process with faulty modules. In such cases to increase the system availability in the event of hardware component failure.

Programming developed

Programmable logic controllers are intended to be used by engineers without a programming background. For this reason, a graphical programming language called Ladder Diagram (LD, LAD) was first developed. It resembles the schematic diagram of a system built with electromechanical relays and was adopted by many manufacturers and later standardized.

Majority of PLC

As of 2015, the majority of PLC systems adhere to the IEC 61131-3 standard that defines 2 textual programming languages: Structured Text (ST; similar to Pascal) and Instruction List (IL); as well as 3 graphical languages: Ladder Diagram, Function Block Diagram (FBD) and Sequential Function Chart (SFC). Instruction List (IL) was deprecated in the third edition of the standard.

Fundamental concepts

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization, and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.

PLC programs

PLC programs are typically written in a programming device, which can take the form of a desktop console, special software on a personal computer, or a handheld programming device.[26] Then, the program is downloaded to the PLC directly or over a network. It is stored either in non-volatile flash memory or battery-backed-up RAM.


Incorrectly programmed PLC can result in lost productivity and dangerous conditions. Testing the project in simulation improves its quality, increases the level of safety associated with equipment, and can save costly downtime during the installation and commissioning of automated control applications since many scenarios can be tried and tested before the system is activated.

Programming style.

Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which, in a sufficiently complex circuit, may either pass current left-to-right or right-to-left, depending on the configuration of surrounding contacts. The elimination of these “sneak paths” is either a bug or a feature, depending on the programming style.

Manipulating variables

More advanced instructions of the PLC may be implemented as functional blocks, which carry out some operation when enabled by a logical input and which produce outputs to signal, for example, completion or errors, while manipulating variables internally that may not correspond to discrete logic.

Vendor specific

PLCs use built-in ports, such as USB, and Ethernet, to communicate with external devices (sensors, actuators) and systems (programming software, SCADA, HMI). Communication is carried over various industrial network protocols, like Modbus, or EtherNet/IP. Many of these protocols are vendor specific.

Communication link

PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to coordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.


PLCs may need to interact with people for the purpose of configuration, alarm reporting, or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interfaces (GUIs). A simple system may use buttons and lights to interact with the user.


The program follows the sequence of instructions. It typically takes a time span of tens of milliseconds for the processor to evaluate all the instructions and update the status of all outputs.[30] If the system contains remote I/O—for example, an external rack with I/O modules—then that introduces additional uncertainty in the response time of the PLC system.

Ladder execution

As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented. This enhanced programming could be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from those parts required to operate at a higher speed.


Special-purpose I/O modules may be used where the scan time of the PLC is too long to allow predictable performance. Precision timing modules, or counter modules for use with shaft encoders, are used where the scan time would be too long to reliably count pulses or detect the sense of rotation of an encoder.

Parr pointed

In his book from 1998, E. A. Parr pointed out that even though most programmable controllers require physical keys and passwords, the lack of strict access control and version control systems, as well as an easy-to-understand programming language make it likely that unauthorized changes to programs will happen and remain unnoticed.

Microsoft Windows

Prior to the discovery of the Stuxnet computer worm in June 2010, the security of PLCs received little attention. Modern programmable controllers generally contain a real-time operating system, which can be vulnerable to exploits in a similar way to desktop operating systems, like Microsoft Windows. PLCs can also be attacked by gaining control of a computer they communicate with.

Rockwell Automation

In February 2021, Rockwell Automation publicly disclosed a critical vulnerability affecting its Logix controllers family. The secret cryptographic key used to verify communication between the PLC and workstation can be extracted from Studio 5000 Logix Designer programming software and used in the IEC control systems programming standard. As of 2015, it is still widely used, thanks to its simplicity.


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