EP002 — Student Guide

By the end of EP002 you should be able to:

1. Explain the difference between a digital input and a digital output.
2. Identify common digital input devices found in the field.
3. Identify common digital output devices found in the field.
4. Describe how a PLC processes an incoming input signal.
5. Explain how outputs control equipment through the control circuit.
6. Use PLC LED indicators to gather evidence while troubleshooting.
7. Follow a logical, evidence-based troubleshooting process from input to load.

PLC

Programmable Logic Controller — the industrial computer that scans inputs, solves program logic, and commands outputs.

e.g., a small conveyor line uses a PLC to sequence motors and sensors.

Digital Input

A two-state (ON/OFF) signal sent to the PLC by a field device such as a pushbutton, photoeye, or switch.

e.g., a pushbutton or photoeye sending a signal to the PLC.

Digital Output

A two-state (ON/OFF) command sent from the PLC to a field device such as a contactor, solenoid, or lamp.

e.g., the PLC turning on a motor contactor or a stack light.

NO (Normally Open)

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

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

NC (Normally Closed)

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.

Sourcing

A circuit arrangement where the device supplies (sources) positive current out to the load or input point.

e.g., a PNP proximity sensor sourcing +24V to the PLC input.

Sinking

A circuit arrangement where the device provides a path to common (sinks current) rather than supplying it.

e.g., an NPN sensor sinking to 0V (common).

PNP

A sourcing solid-state sensor output: it switches the positive supply to the PLC input.

e.g., a PNP inductive sensor, common in North American panels.

NPN

A sinking solid-state sensor output: it switches the input point to common/negative.

e.g., an NPN sensor, common on European-built machines.

Contactor

An electrically operated switch with a coil and heavy main contacts, used to start and stop motors and other large loads.

e.g., the device that closes to send 3-phase power to a conveyor motor.

Overload Relay

A protective device that trips the control circuit when a motor draws excessive current for too long.

e.g., trips and opens the circuit if a jammed conveyor draws too much current.

Solenoid Valve

An electrically actuated valve that directs air or fluid to move a cylinder or actuator.

e.g., shifts to extend a pneumatic cylinder on a packaging line.

DOL Starter

Direct-On-Line starter — applies full line voltage to the motor immediately on start.

e.g., a small pump motor started directly across the line.

Star-Delta Starter

A reduced-voltage starting method that begins in star (wye) then transitions to delta to limit inrush current.

e.g., used on a large compressor motor to limit inrush current at startup.

I/O

Input/Output — the collective term for the PLC's field connection points.

e.g., a PLC rack with 16 inputs and 8 outputs.

The five OCG-Q002 questions, listed for reading. Answers live in the interactive version.

1. A photoeye is a: (A) Digital Output (B) Digital Input (C) Relay (D) Contactor
2. Which device is normally controlled by a PLC output? (A) Pressure Switch (B) Proximity Sensor (C) Solenoid Valve (D) Selector Switch
3. If the Output LED is ON but the motor is OFF, what should you check next?
4. True or False: A PLC input receives information from the machine.
5. True or False: An illuminated output LED always guarantees the load is energized.

• Pushbutton: Never troubleshoot from assumptions. Verify the signal entering the PLC before replacing the device.
• Photoeye: A photoeye’s "ON" state depends entirely on how it’s configured. Always confirm light-operate vs. dark-operate before assuming a fault.
• Limit Switch: A limit switch is mechanical — wear and misalignment cause more failures here than electrical faults do.
• Proximity Sensor: These only detect metal — always confirm the target material and gap distance before condemning the sensor.
• Contactor: An energized coil doesn’t guarantee closed main contacts. Listen and look at the mechanism, not just the output LED.
• Solenoid Valve: If an actuator won’t move, check the valve’s energized state before replacing the actuator itself.
• Stack Light: A stack light reports the PLC’s decision. It does not verify the machine actually completed the action.
• Motor Starter: A PLC command and the actual machine state are not always the same. Always verify the commanded output and the physical result independently.