ladder logic

Developing Software With Ladder Logic

The latest market research predicts the industrial automation market to nearly double from $164. billion in 2020 to $306.2 billion by 2027. Logic would dictate that keeping up with developing automation technology and the ladder logic it is run on is a skill that will be in high demand in the coming years.

With an average of $72,731 a year salary in the United States, it is easy to see why so many are considering careers in automation engineering.

Are you an automation professional or hobbyist? Want to be sure you understand what ladder logic is and how it is being used to create ever more complex automation? Then come along as we go up and down the logic ladder and explore its importance.

What Is Ladder Logic Software Exactly?

In order to properly give justice to a ladder logic definition, we must go back before the time of using it in software. Ladder logic was originally a written method to design relay racks in the manufacturing industry. The process control values were given symbols and depicted visually on a ladder diagram.

We may even need to break down the definition further to include “logic” as its base. Logic, as it relates to programming, is a set of principles that arrange elements of a system to perform a certain task. In other words, it is logic that dictates the order in which something is created (manufactured).

For our purposes, ladder logic sets the rules in which we program our PLC (Programmable Logic Controller). All robotic automation devices follow a set of logic instructions. Using a logic ladder is essentially a roadmap to an objective, the final produced result of the instructions.

For those that don’t think like a computer, ladder logic software is the recipe for cooking a meal that your family will love. The family is your boss, partner, or customer, and the meal being the final product produced through the manufacturing process.

Basics of Ladder Logic Software

Much the same as all computer languages, ladder logic programming operates with binary signals. Each signal is set to zero or one. In code-speak this is called a boolean. Almost every basic PLC instruction comes down to true or false.

Each “rung” of the ladder can be seen as a “when then this” statement in programming. The process progresses up the ladder as each permission is met. The set of conditions that sets the value to true or false determines if the program continues up the ladder in the process.

As each rung of the ladder takes the climber further to his goal, so does every rung of the logic ladder. The PLC executes the instructions when the logic parameters are met. It may help to grasp this concept by explaining the two basic instructions of ladder logic.

Examine if Closed

The input instruction Examine if closed (XIC) evaluates the condition to be true when the bit is set to 1 (HIGH). The bit is set to 0 (LOW) when the condition is found to be false.

Output Energize

The output Energize (OTE) instruction will set the bit to 1 (HIGH) when true and 0 (LOW) when false. The entire PLT instruction set runs on evaluations of these two instructions, one input and one output.

In addition, programmers must familiarize themselves with ladder logic symbols. Fortunately, there aren’t many to learn. A complete list and explanation of ladder logic symbols can be found here.

Ladder Logic Examples

Perhaps the best example of PLC programming ladder logic for beginners is the basic motor control circuit. Please understand that it is possible to wire a mother starter without the use of a PLC. That being said, there are many benefits to programming a PLC for the motor starter, including:

  • Including 3 phase circuit overload
  • Using a motor contactor to bridge high and low voltage circuits
  • Including a latch circuit for continuous operation

This straightforward circuit has many applications including conveyors, starters, process initiation, and more.

Our first objective when building this circuit is to program the input and output values of the circuit when the start pushbutton is pressed. When the momentary start push button is pressed, digital inputs are assigned values of 1, and when released are assigned values of 0.

The motor is tied to an output under the condition the instruction is set to 1 (High). This will energize the coil in a contactor and allow current to flow. Now we are faced with a problem. The user must keep their finger down on the button to keep the motor running.

This is where ladder logic begins to work in practice. The next rung in the ladder keeps the motor running after the button is pressed. To complete the logic circuit, we need to program an XIO instruction into both branches (rungs of the ladder).

The upper branch and the lower branch. The upper branch stops the motor from being started when the Stop button is pressed. The lower XIO stops the motor when the stop button is pressed. For a visual representation, use a ladder logic simulator to show the complete circuit and animate it by pressing the buttons.

Ladder Logic and Basic PLC Automation

Ladder logic is a great tool for visualizing and programming the path of current through a circuit. The more advanced and complex the circuit(s) the more important it becomes to have a representation of the circuit to work with.

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