PLC EXERCISES · DRILL 2 OF 3
Seal in circuitBuild a start/stop latching circuit, then run it.
Make a light stay on after Start is released, then add a Stop button that turns it off. The same pattern runs almost every motor.
A push button only stays pressed while your finger is on it. To keep a light (or a motor) running after Start is released, the rung has to hold itself on. That’s a seal-in circuit, also called a holding or latching circuit: a contact for the output itself, in parallel with Start.
Then a Stop button in series breaks the rung. Because Stop is wired normally closed, its input bit is ON until it’s pressed, so it’s examined with an XIC. That surprises a lot of people.
Seal in circuit quick reference
The rung: [ Start_PB ] in parallel with [ Light1 ], then [ Stop_PB ] in series, then ( Light1 ).
| Contact | Instruction | Why |
|---|---|---|
| Start_PB (normally open) | XIC | Its bit is ON while pressed |
| Light1 (the seal) | XIC | Once the light is on, its own contact keeps the rung true |
| Stop_PB (wired normally closed) | XIC | Its bit is ON until pressed, so the XIC opens when Stop is pressed |
Why wire Stop normally closed?
If the Stop wire breaks or comes loose, the input bit goes OFF, exactly as if Stop were pressed, and the machine stops. A normally open Stop with a broken wire would never stop anything. That’s called failing safe.
Stop wins
Stop sits in series after the branch, so holding Start and Stop together leaves the output off.
Common questions
What is a seal in circuit?
A rung that keeps itself on after a momentary Start button is released. A contact for the output itself sits in parallel with Start, so once the output is on, it holds the rung true. It’s also called a holding or latching circuit.
Why is the Stop button XIC in a PLC program?
Stop is wired normally closed, so its input bit is ON until it’s pressed. XIC is true while the bit is ON, so pressing Stop opens the rung. A broken Stop wire also turns the bit OFF and stops the machine, which fails safe.
What happens if Start and Stop are pressed at the same time?
Stop wins. It’s in series after the branch, so with Stop pressed nothing reaches the output, not even a held Start.
What is the difference between a seal-in and OTL/OTU latch instructions?
OTL and OTU set and reset a bit that stays set, in most PLCs even through a power cycle. A seal-in drops out when power or the rung is lost, which is why motor starters use it: the motor won’t restart by itself when power returns.
YOUR NEXT STEP
See it run a motor
Level 1’s motor rung is this seal-in circuit with an overload, an E-Stop and a restart permission added. Wire the panel, then read the program. It’s free.
Training practice. Real machine control circuits also need the safety functions your risk assessment calls for (E-Stops, guards, overloads); a PLC rung alone isn’t a safety circuit.