Read inputs
Copy field terminals into the input image. A later field change remains pending.
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See what a PLC does between a button press and a motor output
Step through read inputs → solve program → write outputs. Change a field device between phases and watch why the ladder does not see it until the next scan.
Interactive three-wire starter
Change a physical input, then advance one PLC phase at a time. The input image only changes during Read inputs.
Scan0
- 1. Read inputs
The PLC copies the physical input terminals into its input image table.
- 2. Solve program
The ladder is solved from the frozen input image and the previous output image.
- 3. Write outputs
The solved output image is copied to the physical output terminals.
Physical field devices
These can change at any moment.
Input image snapshot
START
Input image
STOP_OK
Input image
OL_OK
Input image
Program solve
Left power railRight power rail
—| |—STOP_OKXIC
—| |—OL_OKXIC
—| |—STARTXIC
—| |—MOTORseal
( MOTOR )OTE
Series path TRUE × Start/seal branch FALSE = solved MOTOR FALSE
Physical output
M
MOTOR OFF
Changes only at Write outputs
Ready
Press Start, then read the physical inputs to begin.
The key mental model
The program usually reads a snapshot, not the terminals live
In the basic cyclic model, the input phase freezes field states into memory. The program solves every rung against that image, and the physical outputs update later. This makes scan behavior deterministic—but creates a one-scan delay for changes that arrive after the snapshot.
Solve ladder
Evaluate contacts and coils top-to-bottom using the frozen images.
Write outputs
Apply the solved output image to field terminals, then begin again.
Engineering boundary
This visualizer teaches the common input-image/program/output-image model. Real PLCs may use immediate I/O, remote racks updated asynchronously, periodic or event tasks, interrupts, and vendor-specific scheduling. Use the controller manual and measured task timing for real safety or high-speed design decisions.
Teach with real learner data
Our study of 10,272 grading runs found seal-in and Stop logic among the hardest beginner concepts.
Read the data studyDownload the free lab manual
Twelve browser-runnable PLC labs with objectives, instructor notes, grading, and no special hardware.
Open the lab manualEstimate scan time
Turn instruction count, I/O points, CPU class, and communications overhead into a useful estimate.
Use the calculatorQuestions
PLC scan cycle questions
A cyclic PLC first reads physical inputs into an input image, then executes the user program against that snapshot, then writes the solved output image to physical outputs. Communications and housekeeping usually happen around those core phases.
Most cyclic PLC programs solve against a snapshot captured at the input-read phase. If a physical input changes after that snapshot, ordinary program logic will not see the new value until the next input read.
The Start input and previous motor output form a parallel branch. A healthy Stop circuit and overload are in series with that branch. Once energized, the previous motor output holds the rung true until Stop or overload opens the series path.
No. The input-image and output-image model is a useful foundation, but real controllers can also use immediate I/O instructions, asynchronous remote I/O, event tasks, interrupts, and different scheduling models. Check the manual for the controller and task type you are using.
