EP003 — Student Guide

By the end of EP003 you should be able to:

1. Explain what a scan cycle is and why it repeats continuously.
2. List the three steps of a scan, in order.
3. Read a simple ladder rung and identify its contacts and coil.
4. Explain how a Start/Stop/Seal-In circuit works.
5. Describe why scan timing matters when troubleshooting fast-changing signals.

Scan Cycle

The continuous loop a PLC runs while in Run mode: read inputs, execute logic, update outputs, repeat.

e.g., a PLC scanning 1000 times per second updates outputs almost instantly from a technician's perspective.

Rung

One horizontal line of ladder logic running between the left and right power rails.

e.g., a single line of ladder logic controlling one motor start/stop circuit.

Contact

A symbol on a rung that references an input or internal bit and either passes or blocks the logical path.

e.g., a Start pushbutton's NO contact referenced in a rung.

Coil

The symbol at the end of a rung representing the output that energizes when the rung is true.

e.g., the output coil that energizes a motor contactor.

Normally Open (NO)

A contact that is open — no current flows — until the device is actuated.

e.g., a Start pushbutton — open until pressed.

Normally Closed (NC)

A contact that is closed — current flows — until the device is actuated. Used for safety circuits so a broken wire fails safe.

e.g., an E-Stop button — closed until pressed, so a broken wire is caught as a fault, not ignored.

Seal-In Circuit

A parallel contact driven by the output itself that holds a rung true after the momentary Start contact opens.

e.g., a Start/Stop/Seal-In rung that keeps a motor running after the Start button is released.

Scan Time

How long one complete scan takes; it determines the fastest signal change the PLC can reliably see.

e.g., a program with a 5ms scan time updates roughly 200 times per second.

Ladder Logic

The graphical programming language modeled on relay control diagrams, built from rails, rungs, contacts, and coils.

e.g., the graphical rung-and-rail programming language most PLCs use.

Run Mode

The PLC operating state in which the scan cycle executes and outputs are controlled.

e.g., the PLC actively scanning and controlling the machine, as opposed to Program mode.

The five OCG-Q003 questions, listed for reading. Answers live in the interactive version.

1. What are the three steps of a PLC scan cycle, in order?
2. In ladder logic, what does a coil represent?
3. In a Start/Stop/Seal-In circuit, why is the Stop pushbutton wired as Normally Closed?
4. True or False: A PLC checks its inputs continuously, in real time, during logic execution.
5. True or False: A ladder rung is evaluated left to right.

• Ladder Troubleshooting: 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.