How to use interrupts in Allen - Bradley PLC programming?

Interrupts play a crucial role in Allen - Bradley PLC programming, offering a way to handle events that require immediate attention outside of the normal program scan cycle. As a trusted Allen - Bradley PLC supplier, I've witnessed firsthand how effectively using interrupts can enhance the performance and responsiveness of industrial automation systems. In this blog, I'll guide you through the process of using interrupts in Allen - Bradley PLC programming.

Understanding Interrupts in Allen - Bradley PLCs

Before delving into how to use interrupts, it's essential to understand what they are. In the context of Allen - Bradley PLCs, an interrupt is a signal that causes the PLC to temporarily halt its current program execution and jump to a specific sub - routine or interrupt service routine (ISR). Once the ISR is completed, the PLC resumes its normal program execution from where it left off.

Interrupts are particularly useful in scenarios where certain events need to be processed immediately. For example, in a manufacturing line, if a safety sensor is triggered, an interrupt can be used to stop all machinery instantly to prevent accidents. Another use case could be in a high - speed counting application, where an interrupt can be used to capture the exact moment a pulse is received, ensuring accurate counting.

Types of Interrupts in Allen - Bradley PLCs

Allen - Bradley PLCs support several types of interrupts, each designed for different purposes:

Input - Based Interrupts

These interrupts are triggered by changes in the state of an input module. For instance, when an input signal goes from low to high or vice versa, an interrupt can be generated. Input - based interrupts are commonly used in applications where the detection of a specific input event is critical, such as detecting the presence of a part on a conveyor belt.

Timer - Based Interrupts

Timer - based interrupts are triggered after a specified time interval has elapsed. They are useful for tasks that need to be performed at regular intervals, like sampling a temperature sensor every few seconds or performing a self - diagnostic check at fixed time intervals.

Communication - Based Interrupts

In applications where the PLC communicates with other devices, communication - based interrupts can be used. For example, when a new message is received from a remote device via a communication protocol like Ethernet/IP, an interrupt can be triggered to process the incoming data immediately.

Configuring Interrupts in Allen - Bradley PLCs

Now that we understand the types of interrupts, let's look at how to configure them in Allen - Bradley PLCs. The process may vary slightly depending on the specific PLC model, but the general steps are as follows:

Step 1: Select the Interrupt Type

First, you need to determine which type of interrupt is appropriate for your application. If you are dealing with a real - time event detection, an input - based interrupt might be the best choice. For periodic tasks, a timer - based interrupt would be more suitable.

Step 2: Assign an Interrupt Number

Each interrupt in the PLC is assigned a unique number. This number is used to identify the interrupt and to associate it with the corresponding interrupt service routine. You need to choose an available interrupt number and document it for future reference.

Step 3: Define the Interrupt Service Routine (ISR)

The ISR is the code that will be executed when the interrupt is triggered. In Allen - Bradley PLC programming, you can create a separate sub - routine for the ISR. The ISR should be designed to handle the specific event that triggered the interrupt quickly and efficiently. For example, if the interrupt is triggered by a safety sensor, the ISR might include code to stop all motors and set a fault flag.

Step 4: Enable the Interrupt

Once the interrupt type, number, and ISR are defined, you need to enable the interrupt in the PLC configuration. This step allows the PLC to recognize the interrupt conditions and jump to the ISR when they are met.

Example of Using Input - Based Interrupts

Let's take a closer look at an example of using input - based interrupts in an Allen - Bradley PLC. Suppose we have a conveyor belt system, and we want to detect when a part passes a certain point on the belt. We can use an input - based interrupt to capture this event.

We'll assume we are using an Allen Bradley 2080 - LC50 - 24QVB PLC for this application.

Hardware Setup: Connect a proximity sensor to one of the input channels of the PLC. The proximity sensor will detect the presence of the part on the conveyor belt.
PLC Programming:
- First, create a new sub - routine for the ISR. In this sub - routine, we can increment a counter to keep track of the number of parts that have passed the sensor.
- Next, configure the input - based interrupt. Select the input channel connected to the proximity sensor and assign an interrupt number.
- Enable the interrupt in the PLC configuration.

Here is a simplified example of the ladder logic code for the ISR:

Allen Bradley 2080-LC50-24QVB 1768-L43S

// ISR for part detection
LD I:0/0  // Check if the input from the proximity sensor is high
JSR SBR0  // Jump to the sub - routine if the input is high

// Sub - routine SBR0
SBR SBR0
CTU C5:0  // Increment the counter
RET

Example of Using Timer - Based Interrupts

Now, let's consider an example of using timer - based interrupts. Suppose we have a temperature monitoring system, and we want to sample the temperature every 5 seconds. We'll use an Allen Bradley 1756 - L75 Controller for this application.

Hardware Setup: Connect a temperature sensor to an analog input module of the PLC.
PLC Programming:
- Create a new sub - routine for the ISR. In this sub - routine, we'll read the temperature value from the analog input and store it in a memory location.
- Configure a timer to generate an interrupt every 5 seconds. Assign an interrupt number to this timer - based interrupt.
- Enable the interrupt in the PLC configuration.

Here is a simplified example of the ladder logic code for the ISR:

// ISR for temperature sampling
TON T4:0, 500  // Timer set for 5 seconds
LD T4:0.DN  // Check if the timer has timed out
JSR SBR1  // Jump to the sub - routine if the timer has timed out

// Sub - routine SBR1
SBR SBR1
MOV AI:0/0 N7:0  // Move the temperature value from the analog input to a memory location
RET

Best Practices for Using Interrupts in Allen - Bradley PLC Programming

When using interrupts in Allen - Bradley PLC programming, there are several best practices to keep in mind:

Keep ISRs Short: The ISR should be designed to perform its task as quickly as possible. Since the PLC halts its normal program execution when an interrupt is triggered, a long - running ISR can cause delays in the overall system operation.
Avoid Complex Logic in ISRs: Complex logic in an ISR can make the code difficult to debug and maintain. Try to keep the ISR focused on the specific task related to the interrupt event.
Use Global Variables Carefully: If you need to use global variables in the ISR, make sure to handle them properly to avoid race conditions. A race condition can occur when multiple parts of the program try to access and modify the same variable simultaneously.
Test Thoroughly: Before deploying the PLC program in a production environment, test the interrupt functionality thoroughly. This includes testing different interrupt scenarios and ensuring that the ISR behaves as expected.

Conclusion

Interrupts are a powerful feature in Allen - Bradley PLC programming, allowing for real - time event handling and improved system responsiveness. By understanding the different types of interrupts, how to configure them, and following best practices, you can effectively use interrupts in your industrial automation applications.

As an Allen - Bradley PLC supplier, we have a wide range of PLC models, including the Allen Bradley 2080 - LC50 - 24QVB, Allen Bradley 1756 - L75 Controller, and 1768 - L43S Allen - Bradley Compact, to meet your specific requirements. If you are interested in purchasing Allen - Bradley PLCs or need further assistance with PLC programming, including interrupt configuration, please feel free to contact us for a procurement discussion.