Understanding the One-Shot Instruction

The one-shot instruction, often abbreviated as ONS (One-Shot) or sometimes called a rising-edge trigger, is a logic element in PLC programming that allows a rung of ladder logic to execute only once when a specific input condition transitions from FALSE to TRUE. Think of it as a "momentary" switch in the digital realm. It ensures that a particular sequence of actions is initiated only once, preventing unintended repetitions or continuous operations.

The Basic Principle

The core principle behind the one-shot instruction is edge detection. It monitors the state of an input signal and triggers an action only when it detects a rising edge (a transition from OFF to ON). Once triggered, the one-shot instruction remains active for a single PLC scan cycle, allowing the subsequent logic to execute. Even if the input signal remains TRUE, the one-shot will not trigger again until the input transitions back to FALSE and then to TRUE again. This is crucial for preventing unintended multiple executions.

How the One-Shot Instruction Works

To fully grasp the utility of the one-shot instruction, it's essential to understand its inner workings within the PLC scan cycle.

PLC Scan Cycle and One-Shot

A PLC operates in a cyclical manner, repeatedly scanning its inputs, executing the program logic, and updating its outputs. This cycle is known as the PLC scan cycle. The one-shot instruction is evaluated during each scan cycle. When the input condition transitions from FALSE to TRUE, the one-shot sets an internal bit to TRUE for a single scan. This TRUE state enables the execution of the subsequent logic on that rung. After that single scan, the internal bit is reset to FALSE, even if the input remains TRUE. The one-shot will only trigger again when the input transitions back to FALSE and then TRUE again.

Ladder Logic Example

Consider a simple ladder logic example:

--[Input Contact]----[ONS Instruction]----(Output Coil)
                | Input_Signal       | OneShot_Bit         | Output_Device

In this example, when Input_Signal transitions from FALSE to TRUE, the OneShot_Bit is set to TRUE for one scan cycle. This activates the Output_Device for that single scan. Even if Input_Signal remains TRUE, Output_Device will not be energized again until Input_Signal goes FALSE and then TRUE again.

Detailed Diagram

Practical Applications of One-Shot Instructions

The one-shot instruction finds its utility in a wide array of PLC programming scenarios.

Data Logging and Initialization

When initializing a process or system, it's often necessary to record initial parameters or settings. The one-shot instruction can be used to ensure that this data logging occurs only once at the beginning of the process, preventing continuous overwriting of the initial values.

Recipe Selection

In batch processing applications, a one-shot instruction can be used to trigger the selection of a specific recipe based on an operator input. This ensures that the recipe selection logic is executed only once, preventing accidental changes during the process.

Alarm Triggering

One-shot instructions can be used to trigger alarms or notifications when a specific event occurs. For example, when a tank level exceeds a certain threshold, a one-shot instruction can trigger an alarm message. This ensures that the alarm is triggered only once, even if the tank level remains above the threshold.

Pulse Generation

The one-shot instruction can be used to generate short pulses for triggering specific actions. This is useful in applications where a brief signal is required to activate a device or initiate a process.

Advantages of Using One-Shot Instructions

Employing one-shot instructions in PLC programming offers several key advantages:

Precise Control

One-shot instructions provide precise control over the execution of logic, ensuring that actions occur only once when intended.

Prevention of Repetition

They prevent unintended repetitions of operations, which can be crucial in processes where multiple executions could lead to errors or damage.

Simplified Logic

One-shot instructions can simplify complex logic by eliminating the need for additional code to prevent multiple executions.

Improved Efficiency

By preventing unnecessary operations, one-shot instructions can improve the efficiency of PLC programs and reduce scan cycle times.

One-Shot vs. Latching Circuits

While both one-shot instructions and latching circuits can control the execution of logic, they serve different purposes. Here's a comparison:

"The one-shot instruction is best suited for scenarios where a single, momentary action is required, while latching circuits are ideal for maintaining a state or condition until explicitly reset."

Advanced One-Shot Techniques

Beyond the basic implementation, there are several advanced techniques that can enhance the utility of one-shot instructions.

Cascading One-Shots

Multiple one-shot instructions can be cascaded to create sequential operations. This allows you to trigger a series of actions in a specific order, with each action occurring only once.

Using One-Shots with Timers

Combining one-shot instructions with timers can create time-delayed actions. For example, you can trigger a timer with a one-shot instruction to initiate a process after a specific delay.

Resetting One-Shots Programmatically

In some cases, you may need to reset a one-shot instruction programmatically. This can be achieved by toggling the input signal from TRUE to FALSE and back to TRUE within the PLC program.