LADDER LOGIC VIDEO
Start/Stop Seal-In Circuit in Ladder Logic, Explained Rung by Rung (Video)
The most common rung in industrial control, in Siemens and Rockwell notation, with the NC stop-button wiring explained properly and a credited video lesson.
Start stop ladder logic uses one rung: a normally open start contact in parallel with a seal-in contact from the motor output, both in series with the stop, driving the motor coil. Press start and the output holds itself on; press stop and it drops. The stop button is wired normally closed, so a broken wire also stops the motor.
In this lesson by Tim Wilborne, a start/stop circuit with a seal-in is programmed in RSLogix 500 for an Allen-Bradley MicroLogix. It is one of the lessons in our Allen-Bradley RSLogix 500 and MicroLogix course, in the module on XIC, XIO, OTE and the seal-in. Follow along, then check your work against the walkthrough below, which also shows the same logic in Siemens TIA Portal.
The rung, piece by piece
Take a motor with a green start button, a red stop button and a contactor driven from a PLC output.
- Stop contact, in series. Every path to the coil passes through it, so when it goes false nothing can hold the coil on.
- Start contact. A momentary push button. On its own it would only run the motor while you hold it down.
- Seal-in contact, in parallel with start. A contact that looks at the motor output itself. Once the output is on, this contact is true and provides a second path around the start button.
- Motor coil. The output that drives the contactor.
The same logic in Siemens and Rockwell notation
Siemens draws a normally open contact and a coil. Rockwell uses instruction names: XIC (examine if closed), XIO (examine if open) and OTE (output energise). The logic is identical; only the symbols and addresses differ. In Studio 5000 you would use tag names such as Start_PB instead of I:0/0, but the instructions are the same.
Why the stop is wired NC but programmed as XIC
This is the part that confuses nearly everyone at first, so take it slowly.
A contact instruction in a PLC does not know how the button is wired. It looks at a bit. A normally open contact in Siemens, or XIC in Rockwell, is true when the bit is 1. A normally closed contact, or XIO, is true when the bit is 0.
Now the wiring. The stop button is wired normally closed: when nobody is pressing it, current flows and the input bit is 1. Press it and the circuit opens, so the bit goes to 0.
So in the program, the stop is an XIC (in Siemens, a normally open contact). It is true, passing power, while the stop button is not pressed. Press stop, the bit goes to 0, the contact goes false and the rung drops.
Why not wire it normally open and use XIO? Because of what happens when something breaks. With a normally closed stop button, a cut wire, a loose terminal or a lost supply sends the bit to 0, exactly as if someone had pressed stop. The motor stops and will not restart until the fault is fixed. With a normally open button and XIO, the same broken wire leaves the bit at 0 for ever, the XIO stays true, and pressing stop does nothing. You would only find out when you needed it.
Rung by rung: what the PLC sees
Here is the main rung through one full cycle, with the stop button wired normally closed.
| Moment | Stop bit | Start bit | Motor output | Why |
|---|---|---|---|---|
| Idle | 1 | 0 | 0 | No path through start or seal-in |
| Press start | 1 | 1 | 1 | Path through stop and start |
| Release start | 1 | 0 | 1 | Seal-in contact holds the path |
| Press stop | 0 | 0 | 0 | Stop contact false; rung breaks |
| Release stop | 1 | 0 | 0 | Seal-in already dropped; needs start |
| Stop wire cut | 0 | 0 | 0 | Same as pressing stop: fails safe |
Real circuits add a few more rungs and contacts:
- Overload, in the main rung. The motor overload relay's auxiliary contact, wired normally closed, goes in series with the stop as another XIC. When the overload trips, the motor stops and needs a fresh start.
- Rung 2: run lamp. A contact drives a run indicator. Using the contactor's own auxiliary contact as a feedback input is better than using the output bit, because it shows the contactor really pulled in.
- Rung 3: reversing interlock. With a forward and a reverse contactor, put an XIO of the forward output in the reverse rung and the reverse output in the forward rung, so both can never be on together. Keep the electrical interlock in the wiring as well; the software one does not replace it.
Seal-in or set/reset?
Siemens has set and reset coils (S and R), and Rockwell has OTL and OTU (latch and unlatch). Both hold an output on too. The difference shows after a power cut. A seal-in rung drops out when power is lost and needs a new start press. A latched bit can be kept through a power cycle, depending on how memory is configured, which can mean a machine starting by itself when power returns. For starting motors, that is why the seal-in is the usual choice.
Emergency stop is a different circuit
A stop button in PLC logic is a normal operational stop. An emergency stop is a safety function and should not depend on a standard PLC program alone. ISO 13850 sets the design principles for the emergency stop function on machinery, and points to IEC 60204-1 for doing it with electrical equipment. In practice that means a safety relay or a safety PLC; the standard PLC can still read the E-stop state for alarms and screens. Our Siemens Safety Integrated course covers the safety PLC side.
Practise it
The practice task from our RSLogix 500 course: program a start/stop for a fan with the stop button wired normally closed, use an XIC for the stop, and explain in one sentence why that is the fail-safe choice. Then build the same rung in TIA Portal. If you can explain it out loud, you understand one of the most common patterns in industrial control.
Learn it properly, free
The seal-in appears in the Allen-Bradley RSLogix 500 course, in Siemens TIA Portal in the module on latching, holding and interlocks, and in Industrial Motor Control as the hard-wired three-wire starter it came from. The PLC foundations path puts the Siemens course together with networks, HMI and panel design. All are free in full. If you pass a final assessment and want the certificate, it costs from US$2.99 for a beginner course. It is an EDWartens Certificate of Completion with a verification code, not a Siemens or Rockwell qualification.
Take the free course
Questions
What is a seal-in circuit in a PLC?
A rung where a contact from the output is placed in parallel with the start button, so the output keeps itself on after the button is released. The stop contact in series breaks it.
Why is the stop button wired normally closed?
So that a broken wire, loose terminal or failed supply has the same effect as pressing stop. If it were wired normally open, a broken wire would leave the stop button unable to stop the motor.
Should the stop be XIC or XIO in RSLogix?
XIC, when the button is wired normally closed. The input bit is 1 while nobody presses the button, so XIC passes power until stop is pressed or the circuit breaks.
What is the difference between a latching circuit and a seal-in?
A seal-in holds the output on through a parallel contact and drops out when power is lost. A latch uses set and reset coils (Siemens) or OTL and OTU (Rockwell) and can keep its state through a power cycle, depending on memory settings.
Can a PLC stop button be used as an emergency stop?
Not on its own. ISO 13850 covers the emergency stop function on machinery, and in practice it is built with a safety relay or a safety PLC rather than a standard PLC input.
Sources
Written by the EDWartens engineering team for general education. Product names are trademarks of their owners; mentioning them does not imply endorsement. Prices and terms of other providers were checked on the date shown and can change.
