PLC Scan Cycle & Ladder Logic Basics

A PLC doesn’t run its program once — it runs it over and over, thousands of times per second, in a continuous loop called the scan cycle. Every single scan, the PLC does three things in order: it reads all its inputs, it executes the ladder logic program using those input values, and it updates all its outputs based on what the logic decided. Then it starts over. This loop never stops while the PLC is in Run mode.

1. Read Inputs — the PLC checks the current state of every input (every pushbutton, sensor, switch) and stores those values.
2. Execute Logic — the PLC runs through the ladder logic program, rung by rung, top to bottom, using the input values it just read.
3. Update Outputs — the PLC applies the results of that logic to the real-world outputs (energizing or de-energizing contactors, valves, lights).

Then it repeats — continuously, many times per second.

Ladder logic is drawn like a ladder: two vertical power rails on the left and right, with horizontal “rungs” in between. Each rung represents one line of logic. On a rung, contacts (which represent inputs) are drawn like railroad-tie symbols, and a coil (which represents an output) sits at the end of the rung. If the contacts on a rung create a complete path from the left rail to the right rail, the rung is “true,” and the coil energizes — turning that output ON.

One of the most common rungs you’ll ever see: a Start pushbutton (Normally Open) in series with a Stop pushbutton (Normally Closed), controlling a Motor coil. Pressing Start closes its contact, completing the rung, and energizes the Motor coil. But there’s a catch — release the Start button, and the contact opens again. So a second contact, wired in parallel with the Start contact and driven by the Motor coil itself, “seals in” the circuit, keeping the rung true even after Start is released. Press Stop, and its Normally Closed contact opens, breaking the rung and de-energizing the Motor — no matter what else is happening.

• Assuming the PLC checks inputs continuously in real time — it only sees the value from the start of that scan
• Forgetting that a very short input pulse can be missed entirely if it changes faster than the scan cycle
• Reading a rung right-to-left instead of left-to-right
• Forgetting the seal-in contact and wondering why a motor stops the instant a pushbutton is released
• Confusing a contact (input reference) with a coil (output reference) when reading a rung

“When you’re staring at a ladder diagram trying to figure out why an output won’t energize, don’t just look at the coil — trace the rung backward, contact by contact, and ask which one isn’t making a complete path. The scan cycle means the PLC is only ever reacting to what it saw at the start of that scan, so if something changed a moment too late, it’ll show up next scan, not this one.”