Elevator PLC Ladder Diagram & Ladder Logic (Run It Free)

Elevator PLC I/O table: inputs, sensors and outputs

A three-floor lift controller needs two classes of input: call and cabin buttons that create service requests, and position or safety sensors that feed back the physical state of the cab and door.

Inputs include exterior up-call buttons on floors 1 and 2, exterior down-call buttons on floors 2 and 3, and in-cabin floor buttons for each of the three floors. Sensor inputs are the three floor position sensors (FLOOR_SENSOR_1/2/3), a door-fully-open limit switch (DOOR_OPEN_LIMIT), a door-fully-closed limit switch (DOOR_CLOSE_LIMIT), a doorway obstruction photo-eye (OBSTRUCT_PE), and a cab overload weight switch (OVERLOAD_SW).

Outputs are the hoist motor up contactor (MOTOR_UP), the hoist motor down contactor (MOTOR_DN), a door-open motor (DOOR_OPEN_MOTOR), a door-close motor (DOOR_CLOSE_MOTOR), three floor indicator lamps (INDICATOR_LAMP_1/2/3) and two direction lamps (DIRECTION_UP_LAMP, DIRECTION_DN_LAMP).

Latching floor calls in ladder logic

The first ladder logic task is converting momentary button presses into sticky pending-call bits — one per floor. Any button that references floor N (an external call button or the in-cabin floor-N button) sets a CALL_FN latch. The latch is reset only when the cab arrives at floor N and FLOOR_SENSOR_N trips, so a call is never lost even if the passenger releases the button during travel.

Using SET and RESET coils for the latch means the pending state survives scan-to-scan without an external memory bit: the SET rung captures the button press in one scan, and the RESET rung clears it when FLOOR_SENSOR_N is true. This is the foundation of the entire elevator PLC program — all priority and motor-dispatch logic reads the CALL_F1/F2/F3 bits rather than the raw buttons.

Up/down motor interlock rung

The most safety-critical rule in any elevator PLC program is that MOTOR_UP and MOTOR_DN must never be energised at the same time — simultaneous contactors would short the motor supply and destroy the drive. The interlock is built as a pair of mirror-image rungs: the MOTOR_UP coil rung has a normally-closed MOTOR_DN contact in series, and the MOTOR_DN coil rung has a normally-closed MOTOR_UP contact in series.

Beyond the electrical interlock, both motor rungs are gated by DOOR_CLOSE_LIMIT (the cab must not move unless the door is fully shut) and by a normally-closed OVERLOAD_SW contact (an overloaded cab must not travel). This three-condition gate — motor intent, door closed, no overload — is the safest way to structure hoist motor control in ladder logic for a lift or elevator.

Elevator call-to-arrival timing diagram

The timing diagram traces a complete lift call cycle from floor 1 to floor 2: a momentary press of CALL_UP_2 sets the CALL_F2 latch, MOTOR_UP energises once DOOR_CLOSE_LIMIT is confirmed, the cab travels until FLOOR_SENSOR_2 trips, MOTOR_UP drops, DOOR_OPEN_MOTOR runs until DOOR_OPEN_LIMIT trips, and the 3-second dwell timer runs before DOOR_CLOSE_MOTOR takes over.

Note that MOTOR_UP is always OFF during the door-open and dwell phases — the DOOR_CLOSE_LIMIT gate ensures the hoist cannot re-start until the door has fully re-sealed. This is the sequence the live simulator graded test case validates automatically.

How to write an elevator PLC program step by step

Building the elevator ladder logic in the simulator follows a natural layered sequence. Start with the I/O wiring — assign the floor sensors, limit switches, photo-eye, overload switch, motor outputs, door motor outputs and lamps to the address slots provided in the wiring panel.

Then build the call latches (SET/RESET pairs for CALL_F1, CALL_F2, CALL_F3), followed by the floor-tracking latches (AT_F1/F2/F3 driven by FLOOR_SENSOR rising edges). Next add the one-hot destination selector that picks the nearest pending floor using the AT and CALL bits. Then wire the motor coils behind the three-condition gate (intent AND DOOR_CLOSE_LIMIT AND /OVERLOAD_SW AND /opposite-motor interlock). Finally build the door sequence: DOOR_OPEN_MOTOR on arrival, dwell TON, DOOR_CLOSE_MOTOR when dwell expires unless OBSTRUCT_PE is made.

The simulator's six graded test cases verify each layer independently, so you can build and pass them one at a time.

2, 3 and 4 floor elevator PLC ladder logic — scaling the call logic

The same ladder pattern scales cleanly from a 2 floor elevator PLC ladder logic exercise up to a 4 floor (and beyond) controller — only the number of repeated blocks changes, not the structure. For every floor N you add one CALL_FN latch (SET on any button referencing floor N, RESET when FLOOR_SENSOR_N trips) and one AT_FN floor-tracking bit. The nearest-floor destination selector then compares the current AT_FN position against every pending CALL bit and dispatches MOTOR_UP or MOTOR_DN accordingly — a 2 floor lift only ever travels in one direction at a time, while a 3 or 4 floor lift needs the up/down priority comparison so it serves the nearest call in its current direction first.

The motor interlock, door sequence, obstruction reversal and overload hold are written once and shared by all floors, so a 4 floor elevator PLC program is mostly copy-and-relabel of the per-floor call and tracking rungs. Everything on this page is runnable and auto-graded in your browser — and to keep a copy of this elevator PLC ladder diagram (I/O table plus every rung) beside you while you build, press Ctrl/Cmd+P on this page and choose Save as PDF.