Motor start/stop seal-in circuit

Scenario

Every controls engineer learns the three-wire motor start/stop circuit before writing their first PLC program. It appears verbatim in IEC 61131-3 training materials, Allen-Bradley certification exams, and factory acceptance tests worldwide. The seal-in contact latches the motor ON after the momentary start pushbutton is released — the fundamental pattern for any maintained-output logic.

Annotation key

• XIC(tag, address, name=...) — Examine If Closed: contact passes power when the referenced bit is 1 (true)
• XIO(tag, address, name=...) — Examine If Open: contact passes power when the referenced bit is 0 (false); normal for stop buttons wired N.C.
• OTE(tag, address, name=...) — Output Energize: coil energizes the referenced bit when rung has power
• parallel: branch: — models a parallel contact branch (logical OR)
• The MOTOR_AUX contact in the parallel branch is the seal-in: once the motor output energizes, the aux contact closes and holds the rung true even after the START_PB releases

How to read

The rung reads left to right. Power flows if either the start button (XIC START_PB) or the aux contact (XIC MOTOR_AUX) is closed, and the stop button (XIO STOP_PB) is not pressed. When the output coil (OTE MOTOR_CMD) energizes the motor, it also drives the aux contact bit — latching the rung high. Pressing STOP breaks the series path and de-energizes the rung, dropping the motor and the seal-in simultaneously.