Understanding the One Shot Rising (OSR) Instruction

The One Shot Rising (OSR) instruction, also known as a positive edge trigger, is a fundamental tool in PLC programming. It's designed to detect the rising edge of a signal – the precise moment when a digital input or a bit transitions from a logic low (0) to a logic high (1). Upon detecting this rising edge, the OSR instruction generates a pulse that lasts for only one scan cycle of the PLC. This single-scan pulse allows for the execution of specific actions or operations only once during the transition, preventing repeated or unintended executions.

How the OSR Instruction Works

The OSR instruction operates based on a memory bit associated with it. This memory bit stores the previous state of the input signal. During each scan cycle, the PLC compares the current state of the input signal with the value stored in the memory bit. If the current state is high (1) and the previous state (stored in the memory bit) was low (0), the OSR instruction detects a rising edge. It then sets its output bit to high (1) for a single scan cycle. After that scan, the output bit returns to low (0), and the memory bit is updated with the current state of the input, ready for the next scan cycle.

The following diagram illustrates the basic principle of the One Shot Rising instruction:

Practical Examples of OSR in PLC Programming

The OSR instruction finds its application in a wide range of industrial automation scenarios. Here are a few practical examples:

1. Starting a Process with a Single Push Button Press

In many applications, a process should only start once when a start button is pressed. Using a standard contact would cause the process to restart every scan cycle as long as the button is held down. The OSR instruction ensures that the process initiation logic is executed only once, even if the button is held for an extended period.

2. Counting Events

When counting events, such as the number of items passing through a sensor, the OSR instruction is vital. Without it, a single item could trigger the sensor multiple times due to bouncing or prolonged presence, leading to an inaccurate count. The OSR ensures that each item is counted only once.

3. Triggering an Alarm

In alarm systems, it's often necessary to trigger an alarm only once when a fault condition arises. The OSR instruction prevents the alarm from repeatedly triggering as long as the fault condition persists, avoiding unnecessary alerts and potential confusion.

4. Recipe Selection in Batch Processing

In batch processing applications, selecting a specific recipe should only happen once when a selection button is pressed. The OSR instruction prevents accidental recipe changes if the button is held down or experiences contact bounce.

Example: Controlling a Filling Station

Consider a filling station where a button press initiates a filling sequence. The sequence should only start once per button press. Here's a simplified ladder logic example (conceptual):

// Ladder Logic for Filling Station

// Rung 1: Detect button press rising edge
// Input: Start_Button (Digital Input)
// Output: Start_Filling_OSR (OSR Instruction)

// Rung 2: Trigger Filling Sequence (Executed only once per button press)
// Input: Start_Filling_OSR.Q (OSR Output)
// Output: Filling_Sequence_Active (Internal Bit)

// Rung 3: Filling Sequence Logic (Simplified)
// Input: Filling_Sequence_Active
// Output: Valve_Open, Pump_On, etc. (Physical Outputs)

OSR vs. Other Similar Instructions

While the OSR instruction is powerful, it's essential to understand its differences from other similar instructions, such as the One Shot Falling (OSF) instruction and standard contacts.

This table highlights the key differences and helps you choose the appropriate instruction for your specific needs.