Coils (output instruction)
OTU: Output Unlatch
Clears the bit to 0 and leaves it there, even after the rung goes false.
Also called: reset coil, unlatch, RST, -(U)-.
OTU(coil)What it does
OTU is the other half of OTL. It only ever writes a 0, and only when its rung is true. Together the two make a latch that any number of conditions anywhere in the program can set and clear.
The pair is more flexible than a seal-in, because the set and the reset are separate rungs that can live wherever they read best. It is also easier to get subtly wrong, because nothing forces the two to be consistent.
When both the OTL rung and the OTU rung are true on the same scan, the one later in the scan wins. That is not ambiguity in the controller, it is a definite outcome, but it is one you have to choose on purpose rather than discover.
A rung you can run
Set and reset from two different places
XIC(Cycle_Start)XIO(Estop_Active)OTL(Cycle_Active)
XIC(Cycle_Complete)OTU(Cycle_Active)
XIC(Estop_Active)OTU(Cycle_Active)Two independent reasons to clear the state, both later in the scan than the set rung, so an emergency stop always wins over a start.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | OTU |
| Siemens (TIA Portal) | R, the reset coil |
| Mitsubishi (GX Works) | RST |
| CODESYS and IEC 61131-3 | R, the reset coil |
Common mistakes
- Assuming set wins over reset, or the reverse. Scan order decides, so put the one that must win last and write a comment saying that is deliberate.
- Clearing a fault latch on the acknowledge alone, without also requiring the fault condition to have cleared. That lets an operator acknowledge their way past a live fault.
Practise it
Interactive exercises that use OTU. Each one checks your ladder against a real scan.
- Set / ResetLatch and unlatch instructions (OTL / OTU) instead of a seal-in branch, and the crucial detail of which one wins when both fire, decided purely by rung order.
- State Coil / Fault CoilA fault lamp that latches ON when any alarm condition occurs, even briefly, and only clears on Reset. And if the condition is still present when Reset is released, the fault comes right back.