OTL Output Latch: Rockwell PLC Behavior & Examples
Quick answer
Rockwell OTL (Output Latch) sets a BOOL to TRUE when its rung is true. When the rung later becomes false, OTL makes no change, so the bit remains TRUE until an OTU, another write or initialization clears it.
Key Takeaways
- Rockwell OTL (Output Latch) sets a BOOL to TRUE when its rung is true. When the rung later becomes false, OTL makes no c...
- Beginner-level topic in Ladder Logic Elements
- Commonly used in: Stored sequence and mode requests, Non-safety alarm memory
Detailed Definition
OTL is a state-setting output instruction in Allen-Bradley and Rockwell ladder logic. It differs from OTE: OTE writes both the true and false rung result, while OTL only writes TRUE when enabled. OTU is the paired instruction that writes FALSE.
“Retentive” here describes how the bit behaves when the OTL rung goes false; it is not a complete promise about power loss, downloads, controller mode changes or nonvolatile storage. Use OTL/OTU for an internal request or sequence state with explicit ownership. Mapping a latched bit directly to a physical actuator can create an uncontrolled restart path.
Evidence and scope
OTL and OTU behavior was checked against Rockwell Logix Designer V38 documentation. Other platforms have SET/RESET or RS/SR instructions with similar intent but not guaranteed identical startup, priority or storage behavior.
Technical review:
Critical behavior
- OTL writes TRUE only when its rung is true; a false OTL rung does not clear the bit.
- OTU writes FALSE only when its rung is true. The final value depends on every write that executes.
- Rockwell warns that overwritten output tags or shared structure members can cause unexpected operation.
- OTL does not have a prescan action in current Logix documentation; project initialization and produced/consumed or I/O behavior still need separate checks.
- A latched internal request should be separated from the final actuator command and its permissives.
Verification checklist
- 1Cross-reference the target BOOL and eliminate unintended multiple writers.
- 2Test OTL only, OTU only, both conditions and neither condition.
- 3Move the controller through relevant operating modes and power states.
- 4Verify that permissives gate the final command continuously.
- 5Confirm that the tag is not being used as a substitute for certified safety logic.
IEC and vendor terminology
Similar-looking instructions do not always have identical execution, initialization or storage behavior.
| Platform | Common term | What to verify |
|---|---|---|
| Rockwell Studio 5000 | OTL / OTU | OTL sets the BOOL when enabled; OTU clears it when enabled. False rungs make no change to the target bit. |
| Rockwell standard output | OTE | Writes the current rung result and therefore clears its bit when the rung is false. It is not an OTL substitute. |
| Siemens STEP 7 | S / R coil; RS / SR | Similar stored-state intent, but simultaneous-input priority and restart semantics must be selected and tested explicitly. |
| CODESYS / IEC style | RS / SR | Bistable function blocks expose defined reset- or set-dominance rather than relying on two separate writes. |
OTL/OTU ownership pattern
|----[ Start_Pulse ]----------------------(OTL) Cycle_Active----|
|----+----[ Stop_Pulse ]------------------(OTU) Cycle_Active----|
| +----[ Process_Fault ]------------------------------------|
|----[ Cycle_Active ]----[ Permissives_OK ]----(OTE) Run_Cmd---|OTL and OTU scan-order table
Assume both instructions address Cycle_Active. When both execute true in one task scan, the later write determines the final value.
| State / phase | OTL rung | OTU rung | Final BOOL |
|---|---|---|---|
| Neither executes | FALSE | FALSE | Previous value |
| Latch only | TRUE | FALSE | TRUE |
| Unlatch only | FALSE | TRUE | FALSE |
| OTL then OTU | TRUE first | TRUE later | FALSE |
| OTU then OTL | TRUE later | TRUE first | TRUE |
Working LD and ST example
Translate an OTL/OTU pair into explicit reset-dominant ST
Make priority visible and keep the stored request separate from the continuously evaluated output.
Ladder Diagram
|----[ Start_Pulse ]--------------------(OTL) Cycle_Request----|
|----[ Stop_Pulse ]---------------------(OTU) Cycle_Request----|
|----[ Cycle_Request ]----[ Healthy ]----(OTE) Conveyor_Run----|Structured Text
IF StopPulse OR NOT Healthy THEN
CycleRequest := FALSE;
ELSIF StartPulse THEN
CycleRequest := TRUE;
END_IF;
ConveyorRun := CycleRequest AND Healthy;Expected result: Stop or unhealthy state wins over Start. ConveyorRun drops immediately with Healthy even if another code path has not yet cleared CycleRequest.
Failure modes and diagnostic checks
| Symptom | Likely cause | Check next |
|---|---|---|
| A latched bit changes with no visible OTL or OTU rung true | The tag is written by an alias, HMI, produced/consumed value, initialization routine or another instruction. | Use the controller cross-reference and trend every write source, not only OTL/OTU instructions. |
| The output restarts after a mode transition | A stored request remained true and the final command lacks a restart inhibit or first-scan policy. | Test Program-to-Run, power-cycle and communications recovery with the real I/O path. |
| Stop and Start together produce inconsistent results | Two separate writes make priority depend on routine and task execution order. | Encode a single reset-dominant state equation or document and verify the intended order. |
| OTU clears an internal tag but not the machine | The physical output is driven by a different tag, force, safety controller or external holding circuit. | Trace the command through output mapping, module data, safety path and field feedback. |
Current primary and technical sources
These sources support the behavior summarized on this page. The project's controller, firmware and IDE help remain authoritative for implementation.
- Output Latch (OTL), Logix Designer V38
Rockwell Automation — Set behavior, false-rung behavior and overwrite warning
- Output Unlatch (OTU), Logix Designer V38
Rockwell Automation — Clear behavior and execution table
- RS bistable function block
CODESYS — Reset-dominant alternative with an explicit equation
Continue with the practical guide
Common Questions
What is OTL (Output Latch)?
Rockwell OTL (Output Latch) sets a BOOL to TRUE when its rung is true. When the rung later becomes false, OTL makes no change, so the bit remains TRUE until an OTU, another write or initialization clears it.
When should I use OTL (Output Latch)?
OTL (Output Latch) is particularly useful in scenarios such as Stored sequence and mode requests and Non-safety alarm memory. Consider implementing it when you need reliable, efficient solutions for these types of applications.
What should I verify before using OTL (Output Latch)?
Cross-reference the target BOOL and eliminate unintended multiple writers. Test OTL only, OTU only, both conditions and neither condition. Move the controller through relevant operating modes and power states. Verify that permissives gate the final command continuously. Confirm that the tag is not being used as a substitute for certified safety logic.
What are related concepts I should learn?
To fully understand OTL (Output Latch), you should also familiarize yourself with TON (Timer On-Delay), TOF (Timer Off-Delay), and CTU (Count Up). These concepts work together in industrial automation systems.
Continue Learning
Ready to deepen your understanding of OTL (Output Latch)? Here are some recommended resources:
Was this helpful?
Let us know if this glossary term helped you understand OTL (Output Latch) better.
Your feedback helps us improve our glossary and create better content for the PLC programming community.
Quick Info
- Category
- Ladder Logic Elements
- Difficulty
- Beginner
- Tier
- Important
About Ladder Logic Elements
Contacts, coils, timers, counters, and ladder diagram components