Back to tutorials

PLC Programming Fundamentals – OTL Instruction

Vladimir Romanov
June 15, 2020
Table of Contents

Introduction of Output Latch

The OTL, also known as the Output Latch, instruction will force a single bit of logic into a high state if all the conditions leading to it are true. It’s a basic instruction which is powerful but can cause programmers a lot of grief if used improperly or too frequently. This instruction is found on the right side of a ladder logic rung and will switch a bit to a HIGH state once it executes. Unlike the OTE Instruction, the OTL will never turn the bit LOW. In order to make that happen, you can leverage other instructions which accomplish exactly that. That being said, the most common pairing with an OTL is an OTU (Output Unlatch).

What is the purpose of the Output Latch?

In PLC programming, an output is tied to an electrical device that most often performs a mechanical action. A few examples of such devices include motor starters, relays, valves, solenoids and lights. The purpose of energizing an output is to toggle between the two states of the device tied to this output. In other words, energizing a motor coil may start the motor. Similarly, energizing a valve will toggle the state and open or close the valve depending on the initial state.

The latch portion comes into play when the output must be energized for an extended duration. In other words, once the conditions are met for the output to be energized, an output latch instruction will set the output to HIGH until it is unlatched. The rung in which this instruction is used will not be monitored until a separate condition unlatches the output through the use of an OTU instruction.

Example & Usage of OTL

Here’s a real-world scenario of an OTL 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 Start”) is connected to Input 0 (I:0/0 of the PLC).
  4. An operator presses the start button.
  5. The XIC instruction is tied to I:0/0.
  6. The OTL instruction is tied to O:0/o.
  7. The OTL instruction energizes the output (O:0/0) while the XIC is TRUE.
  8. The output (O:0/0) remains HIGH when the XIC is released.

Programming example in RSLogix 500:

OTL Output Latch Instruction RSLogix 500

The OTL instruction will energize the output of the PLC which will allow the motor tied to the starter to run. The bit which was energized by the OTL instruction will remain energized after the release of the start button or any other condition tied to the input of the OTL.

Data Types Allowed for OTL

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

  • Boolean – The OTL may only set TRUE or 1 or HIGH.

Important Notes

  • Note 1 – The OTL instruction was initially used to set 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 energize any boolean within the program. Through this property, programmers can utilize the OTL 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

Got a question or comment?

Related Tutorials


Opto 22 groov RIO Getting Started

The groov RIO module from Opto 22 has been released only a few months ago. The groov RIO is a feature packed piece of hardware that is easy to install, configure and deploy for many different field applications. The module comes pre-loaded with an array of software tools used across the industry and is ready to hit the floor running out of the box. Furthermore, the module is equipped with user configurable input and output nodes that have the capability unlike anything else on the market.

August 2, 2020
Vladimir Romanov

Micro800 PLC Programming Getting Started

The Micro800 Control Systems by Rockwell Automation provide a cost effective solution for small to medium automation systems. The controllers within this family are typically used on stand alone machines or small process applications. The family offers five distinct controller types that are further customizable through add-on modules: Micro810, Micro820, Micro830, Micro850 and Micro870.Micro800 Programmable Logic Controllers Getting StartedUnlike the other Rockwell Automation controllers, this line of PLCs is programmed through a dedicated software: Connected Components Workbench. From within this tool, the programmer can flash the firmware, load the program and monitor the state of the controller. Furthermore, Connected Components Workbench (CCW) is often used to program other non-PLC Rockwell Automation devices such as Variable Frequency Drives (VFDs) and certain Safety Relays.

July 26, 2020
Vladimir Romanov

An Introduction to DeviceNet Industrial Networks

In this tutorial you will learn about the DeviceNet industrial network, go over a basic offline configuration, and explore some advanced features.

July 24, 2020
Jacques Venter