See your PLC ladder logic execute, rung by rung.

What is the PLC scan cycle?

Every PLC runs the same continuous loop: read all inputs into an image table, execute the control program from the first rung to the last, write the computed outputs back to the physical terminals, then perform housekeeping (communications, watchdog reset, self-diagnostics). This loop — typically 1–20 ms — is the scan cycle.

Understanding the scan cycle is not optional. It determines when a coil set on rung 5 becomes visible to a contact on rung 3 (answer: next scan), why a 200 µs input pulse can be invisible to the controller, and how periodic tasks differ from the main cyclic task. Engineers who miss this write programs that look correct on paper but misbehave in production.

How the highlight works in the simulator

The simulator's scan-cycle highlight overlays each rung with a colour-coded state indicator as the virtual processor evaluates it:

• Yellow — the rung is currently being evaluated.
• Green — rung condition was true; output coil energised.
• Grey — rung was false; output stayed de-energised.

In slow mode the processor pauses after each rung and waits for you to advance manually. This is the closest thing to a hardware step debugger you will find in a browser-based PLC environment.

Why scan-cycle visibility changes how you learn

Most PLC beginners learn by running a program and observing outputs — which tells them what happened but not why. Scan-cycle highlight closes that gap. When a motor refuses to start, you can step through the rungs one by one, watching each contact state, and identify the exact rung — and exact contact — where the logic breaks.

This mirrors the real-world workflow engineers use with ladder-logic monitoring on connected hardware. Learning it in the simulator — where you can slow everything down and repeat scenarios without downtime — builds the intuition that carries into production work.

Supported dialects

Scan-cycle highlight works across all 8 supported dialects. The display adapts to the syntax of each dialect: IEC 61131-3 Structured Text blocks, Allen-Bradley-style mnemonic rungs, Siemens STL networks, and Mitsubishi GX Works mnemonics are all supported. See the dialect comparison page for a full breakdown.

Use it in the free curriculum

Scan-cycle highlight is available from Lesson 1 in the 12-lesson free curriculum. Lesson 3 (Timers & Counters) and Lesson 4 (Seal-in Rungs) use it heavily to demonstrate rung-order timing effects and latching behaviour.

FAQ

Does the scan-cycle highlight require a paid plan?

No. The highlight is available on all tiers, including the Free tier. Full slow-mode access is included with a free account.

Can I use it on mobile?

Yes. The simulator is responsive and the highlight overlay scales to smaller screens. A tablet or desktop gives the most comfortable experience.

Is there a tutorial for first-time users?

Yes — the curriculum lessons include step-by-step instructions for enabling slow mode in each exercise.