Ladder Logic Programming Instructions – Part 1

A PLC or Programmable Logic Controller is a special type of digital computer without a monitor and keyboard. This is basically used in industrial automation such as manufacturing, automotive, food and beverage, chemical processing, and more to automate systems. This tiny computer receives data through input and sends operating instructions as output.

As the name suggests, it's a programmable device. The main languages that are used to program a PLC are 'Ladder Logic' and 'C'. Ladder Logic is the most used programming for PLCs.

What is Ladder Logic?

A Boolean takes a single bit in the memory, can be set to 0 or 1, and is used in most basic PLC instructions.

Ladder logic diagram is graphical programming language which executes through real time input. It has two vertical lines, which is called as rails, the left rail supplies power to the circuit, then it passes through each rung. Each rung has switches and output coil. Switches can perform OR, AND and NOT operation, and through this basic logic operation we can make any programming logic in PLC. A ladder logic diagram executes from left to right and top to bottom. When all the condition of a rung met the connected output coil get energized. then it activates the real-world system, such as turning on a motor or turning off a motor. This process is continuously scanned and repeated by the PLC and control system to ensure the automation of operating system.

Normally Open Contact (NO) Symbol

A normally open switch or contact is open when not actuated. It closes the circuit when pressed or activated, allowing current flow. In ladder logic diagrams, a normally open contact is shown as an open gap and becomes a solid line when activated.

Normally Close Contact (NC) Symbol

Normally closed switch/contact is closed when not actuated. Activation of the switch opens the circuit, stopping current flow. In ladder logic diagrams, it's represented as a solid line, becoming an open gap when activated.

Output/Coil

Output devices are such as motor, valve, indicator, lights etc. but in LLD it is represented by vertical line with a label representing the output device.

Advantages of Ladder Logic

Ladder logic is easy to understand and write and does not require extensive programming knowledge. Because it is a graphical language, the logic diagrams can be easily interpreted by operators and maintenance personnel, and it can also make it easier to troubleshoot problems with the control system. Another benefit of ladder logic is that it is flexible and can be easily modified. Changes to the control system can be made by adding, deleting, or modifying rungs in the ladder diagram. This can help to reduce downtime and increase productivity in the manufacturing process. Ladder logic is also a cost-effective solution for control systems.

Disadvantages of Ladder Logic

Despite its benefits, ladder logic does have some limitations.

Ladder logic can become cluttered and difficult to read as more complex control strategies are implemented. It is limited in its ability to handle complex mathematical and branch logic operations. Additionally, ladder logic is not well-suited for systems that require high-speed processing or real-time control.

Examples of Ladder Logic Programming

1. Creating AND gate Ladder logic diagram

Control Behavior: The light should be on when switch A is on (i.e., closed) and switch B is on (closed). Otherwise, it should be off

Truth Table

Ladder Logic

2. Creating OR gate Ladder logic diagram

Control Behavior: The light should be on when switch A is on (i.e., closed) or switch B is on (closed). Otherwise, it should be off

Truth table

Ladder diagram

Append above to the leading two rungs of relay ladder logic diagram Switch A and Switch B are connected to discrete input channels of the PLC Light is connected to discrete output channel (actuator) of the PLC

3. Creating NAND gate Ladder logic diagram

Control Behavior: The light comes on only when switch A is off, and switch B is off. Otherwise, it should be off

Truth table

Ladder diagram

Conclusion

In conclusion, ladder logic is a programming language that is widely used in the field of industrial automation. It is a graphical language that is used to create logic diagrams that control various processes in manufacturing and other industries. Ladder logic is easy to read and understand, flexible, and cost-effective, making it an ideal solution for many control systems. It is having some limitations. It is not recommended for complicated control systems, or systems that need high-speed processing.

PLC basic knowledge is required then use PLC BASIC Knowledge

FAQs on PLC Programming Ladder Logic

1. What are the six basic rules of ladder logic?

Rule 1: Use Rungs and Rails.
Rule 2: Follow the Scan Order. 
Rule 3: Use Proper Addressing and Naming. 
Rule 4: Use Comments and Documentation. 
Rule 5: Use Subroutines and Functions. 
Rule 6: Test and Debug Your Program.

2. What is ladder logic based on?

Ladder logic has evolved into a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. Ladder logic is used to develop software for programmable logic controllers (PLCs) used in industrial control applications.

3. What are the principles of ladder logic?

The basic principles of ladder logic are straightforward. Each rung on the ladder defines one operation in the control process. A rung has read rails on the left, write rails on the right, and the logic operations in between.

4. Why relays may be used instead of PLC?

While PLCs offer many advantages, relays do still have a place within a control system. They provide a simple and efficient tool that requires no advanced programming for proper commissioning. Relays can be particularly useful: where there are only a few IO points per control system.

For more knowledge read below articles
PLC Interview Questions and Answers
PLC Programming - Basic Knowledge