PLC Scan Cycle Simulator

Input Scan → Program Execution → Output Update → Housekeeping · Live I/O & ladder

When to use: Use this to understand how a PLC executes a program. A PLC does not react instantly — it repeats a fixed scan cycle: it reads all physical inputs into an image table, executes the ladder logic top-to-bottom using that frozen snapshot, then writes results to physical outputs. Toggle the field inputs, then Run or single-Step to watch each phase and the live I/O states. The example program is a motor seal-in latch with a 3-second TON start delay and an E-Stop interlock.

STEP 1

Input Scan

STEP 2

Program Execution

STEP 3

Output Update

STEP 4

Housekeeping

Completed Scans: 0

🎚️ Field Inputs (click to toggle)

I0 — Start PB

Momentary start

I1 — Stop PB

Momentary stop

I2 — E-Stop (NC)

Healthy = closed

📥 Input Image Table

Snapshot of inputs taken at Step 1. Logic uses this, not live inputs.

I00

I10

I21

🪜 Ladder Logic

Rung 0 — Run latch (seal-in)
(I0 + M0) · /I1 · I2 → M0○ FALSE
Rung 1 — TON T0 (3.0 s) acc=0.0s
M0 → TON T0 (done → Q0)○ FALSE
Rung 2 — Run lamp
M0 → Q1○ FALSE
Rung 3 — Delay lamp
M0 · /T0.DN → Q2○ FALSE

🧠 Internal Memory

M0 — Run Latch (seal-in)0

📤 Output Image Table

Written to physical outputs at Step 3 (Output Update).

Q0 — Motor

On after 3 s delay

Q1 — Run Lamp

On while running

Q2 — Delay Lamp

On during start delay

IEC 61131-3 Reference: The PLC scan = read inputs → solve logic → write outputs → housekeeping, repeated continuously. Total scan time (here 500 ms/phase) determines I/O responsiveness. Inputs changing mid-scan are not seen until the next Input Scan — this is why scan time matters for fast processes.