PLC Output Coil: OTE, SET/RESET & Examples
Quick answer
A PLC output coil writes the rung result to a Boolean tag when the rung is evaluated. A standard coil is true while the rung is true and false while it is false; SET/RESET or latch/unlatch instructions retain state and are different instructions.
Key Takeaways
- A PLC output coil writes the rung result to a Boolean tag when the rung is evaluated. A standard coil is true while the ...
- Beginner-level topic in Ladder Logic Elements
- Commonly used in: Non-safety motor, valve and lamp commands, Internal sequence-state and permissive bits
- Related to: Latch Coil, Unlatch Coil, TON (Timer On-Delay)
Detailed Definition
An output coil is the Ladder Diagram element that receives the result of logic operation at the right side of a rung. It can address an internal Boolean or a mapped output tag. The drawing resembles a relay coil, but the PLC is assigning data in memory; a physical output changes only through the controller and module update path.
A standard, non-retentive coil must be evaluated repeatedly. When its rung is false, it writes false. A SET/RESET pair or Rockwell OTL/OTU pair has different state-retaining behavior. Do not call every coil “latched,” and do not assume a tag becomes safe merely because the routine containing its normal coil was skipped.
Evidence and scope
The assignment behavior and vendor names below were checked against current Rockwell Studio 5000 V38.01 and Siemens STEP 7 V21 documentation. Prescan, retentive memory and physical-output behavior still depend on the selected controller, task and I/O configuration.
Technical review:
Critical behavior
- A standard coil is an assignment, not stored history: true rung writes true and false rung writes false whenever the instruction is executed.
- A physical output and its command tag are not the same thing; module update timing, inhibit state, forcing and field wiring affect the real device.
- SET/RESET, latch/unlatch and retentive variables have different restart and de-energization behavior from a standard coil.
- If more than one instruction writes the same tag, the last executed write normally determines the value seen after that execution.
- Rockwell documents that an OTE data bit is cleared during prescan; do not generalize that startup behavior to every platform or every coil type.
Verification checklist
- 1Cross-reference the destination tag and remove unintended duplicate writes.
- 2Confirm whether the instruction is standard, set/reset, latch/unlatch or safety-rated logic.
- 3Test false-to-true, true-to-false, routine-disabled, prescan and power-cycle cases.
- 4Verify mapped I/O, module state and field feedback separately from the internal command.
- 5Prove the required safe state through the project safety lifecycle; a normal coil example is not a safety design.
IEC and vendor terminology
Similar-looking instructions do not always have identical execution, initialization or storage behavior.
| Platform | Common term | What to verify |
|---|---|---|
| IEC 61131-3 / generic LD | Coil or assignment | Writes the result of the rung to a Boolean variable. Separate set and reset forms provide stored state. |
| Rockwell Studio 5000 | OTE / OTL / OTU | OTE follows rung condition. OTL sets a retained bit and OTU clears it; they must not be treated as synonyms for OTE. |
| Siemens STEP 7 | Assignment / Set output / Reset output | The standard assignment writes the RLO; S and R coils implement stored state. |
| CODESYS and IEC-style IDEs | Coil / Set coil / Reset coil | Names and symbols vary, so confirm execution and retain settings in the target runtime help. |
Standard output-coil rung
StartRequest StopHealthy PermissivesOK MotorCommand
|-------| |-------------| |--------------| |---------------( )-------|
TRUE TRUE TRUE TRUEWhat each coil form does
Use the behavior column—not the symbol’s shape alone—to decide which instruction the design needs.
| State / phase | Rung condition | Standard coil | SET / latch | RESET / unlatch |
|---|---|---|---|---|
| False | Writes false | No state change | No state change | |
| True | Writes true | Writes/stores true | Writes/stores false | |
| Instruction skipped | No write occurs; prior tag value may remain | Stored state remains | Stored state remains |
Working LD and ST example
Write one command tag in one place
This pattern keeps permissives visible and prevents two unrelated routines from fighting over the physical command.
Ladder Diagram
|--[ StartRequest ]--[ StopHealthy ]--[ PermissivesOK ]--( MotorCommand )--|
|--[ MotorCommand ]----------------------------------------( Local:2:O.Data.0 )--|Structured Text
MotorCommand := StartRequest
AND StopHealthy
AND PermissivesOK;
Local_2_O_Data_0 := MotorCommand;Expected result: MotorCommand and the mapped output are true only while all three conditions are true. A safety function still requires its approved safety controller, devices and validation.
Failure modes and diagnostic checks
| Symptom | Likely cause | Check next |
|---|---|---|
| Output flickers or ends in the wrong state | Two or more coils or assignments write the same tag | Cross-reference every write and consolidate command ownership. |
| Output stays true when a routine is disabled | The standard coil is no longer being evaluated | Define an explicit disabled-state write in a routine that still executes. |
| Output changes after download or mode transition | Prescan, retain or first-scan behavior was assumed | Test the exact controller transition and document initialization. |
| Tag is true but the field device is off | I/O connection, forcing, module, wiring or power problem | Trace command tag, mapped output, module state and measured field voltage in order. |
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 Energize (OTE), Studio 5000 V38.01
Rockwell Automation — Rung-condition and prescan execution behavior
- Overview of LAD elements, STEP 7 V21
Siemens — Standard coils and coils with additional functions
- IEC 61131-3:2025 publication record
IEC — Current programming-language standard publication
Continue with the practical guide
Common Questions
What is Coil (Output)?
A PLC output coil writes the rung result to a Boolean tag when the rung is evaluated. A standard coil is true while the rung is true and false while it is false; SET/RESET or latch/unlatch instructions retain state and are different instructions.
When should I use Coil (Output)?
Coil (Output) is particularly useful in scenarios such as Non-safety motor, valve and lamp commands and Internal sequence-state and permissive bits. Consider implementing it when you need reliable, efficient solutions for these types of applications.
What should I verify before using Coil (Output)?
Cross-reference the destination tag and remove unintended duplicate writes. Confirm whether the instruction is standard, set/reset, latch/unlatch or safety-rated logic. Test false-to-true, true-to-false, routine-disabled, prescan and power-cycle cases. Verify mapped I/O, module state and field feedback separately from the internal command. Prove the required safe state through the project safety lifecycle; a normal coil example is not a safety design.
What are related concepts I should learn?
To fully understand Coil (Output), you should also familiarize yourself with Latch Coil, Unlatch Coil, and TON (Timer On-Delay). These concepts work together in industrial automation systems.
Continue Learning
Ready to deepen your understanding of Coil (Output)? Here are some recommended resources:
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