PLC Programming Fundamentals – OTU Instruction

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:

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.