OR Logic in Allen Bradley PLCs: A Beginner’s Guide

In the world of programmable logic controllers (PLCs), understanding Boolean logic is paramount. One of the fundamental logic gates is the OR gate. This article will delve into the concept of OR logic and demonstrate its application using Allen Bradley PLCs, a popular choice in industrial automation.

What is OR Logic?

OR logic is a fundamental concept in Boolean algebra. It operates on the principle that if at least one input is true, the output will also be true. Think of it like a light switch: if either switch is flipped on, the light turns on. In the context of PLCs, OR logic allows you to create conditions where if any one of several inputs is satisfied, a specific action is triggered.

Representing OR Logic

OR logic can be represented in several ways:

• Symbolically: A + B (where A and B are inputs)
• Truth Table:

A	B	A OR B
0	0	0
0	1	1
1	0	1
1	1	1

• Ladder Logic:

In ladder logic, OR logic is represented by a parallel connection of contacts. If any one of the contacts is closed, the output is energized.

Practical Example: Using OR Logic in a PLC

Let’s imagine a simple scenario: you have a conveyor belt with two limit switches (A and B) positioned at different points. You want to activate a motor (M) to stop the conveyor belt if either limit switch is activated. This is where OR logic comes in.

Here’s how you would implement this using an Allen Bradley PLC:

1. Define Inputs: You would define two input variables, A and B, corresponding to the limit switches. These inputs would be configured to read the state of the switches (open or closed).
2. Define Output: You would define an output variable, M, corresponding to the motor. This output would be configured to control the motor’s state (on or off).
3. Create Ladder Logic: In your ladder logic program, you would create a parallel connection of contacts. One contact would be connected to input A, and the other to input B. The common output of these contacts would be connected to the motor output (M).

Now, if either limit switch A or limit switch B is activated (closed), the corresponding contact will close, energizing the motor output and stopping the conveyor belt.

Conclusion

OR logic is a fundamental building block in PLC programming. By understanding its principles and applying it in practice, you can create complex and efficient automation solutions. This article provided a basic introduction to OR logic and its application using Allen Bradley PLCs. As you progress in your PLC journey, you will encounter more sophisticated logic gates and combinations, but a firm grasp of OR logic is essential for building upon your knowledge.