Back to tutorials
Beginner

PLC Programming Fundamentals – OTU Instruction

By
Vladimir Romanov
Table of Contents

Introduction

The OTU, also known as Output Unlatch, instruction will set a bit to LOW if all the conditions leading to it evaluate to true. This instruction is one of the fundamental instructions for Programmable Logic Controllers (PLCs), but it should be used with caution. This instruction will be found on the right side within a ladder logic structure and turn a bit to a LOW state if the preceding instructions evaluate to true.

Example & Usage of OTU

Here’s a real-world scenario of an OTU instruction:

  1. A Micrologix 1100 Allen Bradley PLC is used to control a process.
  2. A motor contactor is connected to Output 0 (O:o/o of the PLC).
  3. A normally open push button (“System Stop”) is connected to Input 0 (I:0/1 of the PLC).
  4. An operator presses the start button.
  5. The XIC instruction is tied to I:0/0.
  6. The OTU instruction is tied to O:0/o.
  7. The OTU instruction unlatches the O:0/0 bit after the “System Stop” button is pressed which pulls the XIC to HIGH.

Programming example in RSLogix 500:

OTU Output Unlatch Instruction RSLogix 500

Outcome:
The OTU instruction will set the O:0/0 bit to a logic LOW.

Data Types Allowed for OTU

The OTU instruction will work with the following data types within the RSLogix 500 environment:

  • Boolean – The OTU may only evaluate FALSE or 0 or LOW.

Important Notes

  • Note 1 – The OTU instruction was initially used to reset the status of miscellaneous outputs landed on the PLC. However, it may be used on any boolean within the program. In other words, it can de-energize any boolean within the program. Through this property, programmers can utilize the OTU instruction for a wide range of applications.
  • Note 2 – Although it may seem like a good idea to introduce OTL and OTU instructions for every bit, there’s a catch. Depending on how the code is executed, you may run into scenarios where the logic will depend on the position of your rungs within the program. Avoid using OTL & OTU instructions unless absolutely necessary for the application.

Video Tutorial

Back to tutorials

Related Tutorials

Intermediate

MicroLogix 1100 RSLogix 500 External IO PLC Programming Tutorial

The MicroLogix 1100 PLC is still heavily used in the industry and is one of the most recommended PLCs for learning PLC programming. The MicroLogix 1100 PLC is compact, offers a variety of I/O configurations and is inexpensive relative to other Rockwell options. Furthermore, the MicroLogix 1100 PLC is programmable using the free version of RSLogix 500 that can be downloaded from the official Rockwell website.

By
Vladimir Romanov
Beginner

PLC Simulator - PLC Programming & Testing within a Simulated Environment

A PLC Simulator is an excellent avenue to practice PLC ladder logic programming. Although the software environment is not the perfect substitute for hands on experience on an actual PLC, the simulation delivers a very accurate representation of ladder logic execution.

By
Vladimir Romanov
Intermediate

HMI Software - Different Tools, Applications and Solutions to Meet Your Needs

HMI Software is commonly referred to as the development application that will be used to create the runtime application for an HMI terminal. It’s important to understand this distinction as we will be going over the HMI development environments used by PLC and HMI programmers to create the screens rather than the end-user applications you will see at the plant level.Numerous HMI programming software solutions are found on the market. Although they aim to achieve the same result, different packages have a distinct advantage over others in terms of price, reliability, ease-of-use and technical capability. It is therefore possible to find the right solution that will fit your specific application if you spend the time to research all options.

By
Vladimir Romanov