Ladder Logic Basic Example

Ladder Logic Interlocking I

Ladder Logic Basic Example

In this post, you are going to see a Ladder Logic Interlocking example. In this way, you can get to know the applications of this programming language. This example will consist of making an interlocking (self-retention) of a single-phase motor. To make it easier, we will go over everything step by step.

1. Physical connection

The first step of this Ladder Logic Basic Example would be to carry out the physical connection diagram between the PLC and the motor, but it is not necessary. You can appreciate all the required physical elements to be able to perform a real test.

Physical connection diagram between the PLC and the motor

Although, keep in mind that not all the actuators can be coupled directly to the PLC. In some cases, it is necessary that between these there is a security key, such as thermomagnetic key contactors. Moreover, not all the PLCs are fed with the same voltage level. Usually, they are powered with 24V, 110V or 220V. This is why it is important to check the user manual before executing the respective connections.

2. Identify the physical inputs and outputs

Secondly, you have to identify the physical inputs and outputs, which have been previously connected to the PLC. In this situation, there are:

Physical inputs and outputs between the PLC and the motor
  • 2 inputs
  • Start button: assigned to the I0.0 INPUT.
  • Stop button: assigned to the I0.1 INPUT.
  • Motor: assigned to the Q0.0 OUTPUT.

3. Ladder Diagram for this Ladder Logic Interlocking

Thirdly, it is the moment to make the Ladder diagram. In this example, we are going to do an interlocking, which means that at the moment that when someone presses the Start button, the motor (actuator) would have to be ON. Then, the first rung would be composed with the I.0.0 input, which is also assigned to the Start button and to the Q0.0 output that is assigned to the motor (actuator). So at the moment that someone presses the Start button, the Q0.0 coil is energized. Consequently, if we do not press the button, it de-energizes the coil and our goal is not that. For this reason, we need the interlocking.

Ladder Diagram for this Ladder Logic Interlocking
Ladder Diagram

In order to guarantee the interlocking, in the first rung, we use a Normally Open Contact for the I0.0 Input and, at the same time, a Coil for the Q0.0 Output. Secondly, we add a Normally Close Contact for the I0.1 Input, which is assigned to the Stop button. 

Ladder Diagram process for this Ladder Logic Interlocking

Summary of this Ladder Logic Interlocking

To summarizing, firstly the Start button is pressed. Immediately, it energizes the Q0.0 coil, so the motor activates. In case that you want to stop the process and return to the beginning, press the Stop button because it will de-energize the coil.

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