A momentary START button can't hold a motor running by itself — the motor's own contact has to seal itself in. Press START and RESET below to see exactly how that self-latching rung works.
Ladder logic is the dominant programming language for programmable logic controllers (PLCs), designed to visually resemble a relay control schematic so electricians and technicians could read and troubleshoot PLC programs using the same mental model as the physical relay panels PLCs replaced. Each horizontal 'rung' represents one logical statement, evaluated left to right, with contacts (inputs/conditions) and coils (outputs) forming the logic.
A momentary pushbutton like START only closes its contact while physically held — release it and the contact opens again. Without additional logic, the motor would only run while someone kept their finger on the button. Seal-in logic solves this: once the motor coil M energizes, M's own auxiliary contact (wired in parallel with START) closes and keeps the rung energized on its own, even after START is released — the circuit is now 'sealed in' by its own output.
The STOP contact is normally-closed, meaning current flows through it by default and it opens (breaking the circuit) when pressed. Placing it in series after the START/seal-in parallel combination means pressing STOP breaks the entire rung regardless of whether START or the seal-in contact is currently closed — de-energizing M, which in turn opens M's own seal-in contact, so the circuit stays off even after STOP is released.
Ladder diagrams are read left-to-right per rung, top-to-bottom across rungs, evaluated continuously in a fast scan cycle. This left-to-right, relay-schematic-like structure is precisely why ladder logic remains the standard for industrial control decades after PLCs replaced physical relay panels — it lets an electrician troubleshoot a live program the same way they'd trace a relay circuit with a meter.
That would require an operator to physically hold the button the entire time the motor runs, which is impractical and unsafe for any real industrial process. Seal-in logic lets a brief momentary press start a process that then continues running on its own until a deliberate stop condition occurs.
It means the contact conducts (is 'closed', allowing current/logic flow) in its default, unpressed state, and opens (breaks the circuit) when actuated. This is intentional for stop/safety functions — it means a broken wire or a de-energized stop button circuit fails safe by stopping the process, rather than failing to a running state.
Yes — despite structured text, function block diagrams, and other IEC 61131-3 languages being available on modern PLCs, ladder logic remains extremely common in industrial practice specifically because of its direct visual correspondence to the physical relay logic that industrial electricians have historically been trained to read and troubleshoot.
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