Beginner lesson 1 of 17 · 20 min Free
The PLC scan cycle, explained
Explain what a controller reads, what it commands, and the three steps of one scan, then wire a pushbutton to a lamp in the Workspace.
Before you start
- None. This module assumes you have never used a PLC or written a program.
Inputs and outputs
An input is a signal the controller reads, like Pushbutton. An output is a command it sends, like Lamp.
A contact reads a tag
A Normally Open Contact passes power while its tag is on. It tests the Pushbutton tag, not the button itself.
One scan: read, decide, write
Each scan reads the inputs, runs the rung, then writes the outputs. With Pushbutton on, power flows and Lamp turns on.
Step through one scan
This is the simulator from the Workspace running a two-rung program. Set the pushbutton, run one scan, and see what the controller read, which rung passed power, and what it wrote. Nothing changes between scans.
Build these rungs yourself in the Workspace, starting from the lesson's own project.