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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Robotics / Mechatronics › Programmable Logic Controllers for Mechatronics › Lesson 3
MFG-MECH15-02 · Lesson 3 · 16 clock hours · lab

Discrete I/O Wiring and Module Configuration

Objective. Students will wire a discrete input module to pushbuttons and limit switches and wire a discrete output module to pilot lights and a relay coil, following NEC/NFPA 79 wiring practices and using LOTO before any wiring changes.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Discrete I/O Wiring and Module Configuration

What Is Discrete I/O?

A discrete (also called digital) I/O module deals with signals that have only two states: ON or OFF. A pushbutton that is pressed sends a 24 VDC signal to a discrete input channel; a PLC output turns a pilot light fully on or fully off. This is different from analog I/O, which handles continuously varying signals such as 4–20 mA. In manufacturing and marine automation, discrete I/O is the backbone of machine control: limit switches detect part position, relay outputs energize motor starters, and indicator lights tell operators the system state.

Input Module Fundamentals: Sinking vs. Sourcing

The terms sinking and sourcing describe the direction of current flow at the I/O point.

Many industrial modules are sinking/sourcing (bi-directional) and accept either type. Always check the module datasheet to confirm current direction, voltage range (typically 12–24 VDC), and input impedance before wiring. Reversing polarity will not necessarily destroy the module immediately, but it will prevent correct operation and may damage components over time.

Limit switches and pushbuttons are wired in series with the supply rail and the input terminal per the provided wiring diagram. Normally open (NO) contacts pass current only when actuated; normally closed (NC) contacts pass current at rest. Confirm contact type on the device label before connecting.

Output Module Fundamentals: DC Sourcing and Relay Outputs

Discrete output modules come in three common types for industrial 24 VDC systems:

1. Transistor sourcing (PNP) output – The module provides positive voltage to the load when the output is ON. Current flows from the module's output terminal through the load to the common (0 V) rail. Used for pilot lights, solenoid valves, and relay coils.

2. Transistor sinking (NPN) output – The module sinks current; the positive supply connects to the load, and the output terminal provides the return path.

3. Relay output – An internal electromechanical relay isolates the PLC from the load circuit. Relay outputs can switch AC or DC loads and provide galvanic isolation. Use relay outputs for contactor coils or other inductive loads; always observe the module's maximum current rating and install a suppression diode or snubber across inductive coils to prevent voltage spikes.

For this lab, you will wire two sourcing DC output channels to pilot lights and one relay output channel to a small contactor coil. Follow the wiring diagram exactly — confirm the common terminal connection for each output group before energizing.

NFPA 79 Wiring Practices

NFPA 79, Electrical Standard for Industrial Machinery, governs the electrical installation of industrial equipment. Key practices that apply directly to this lab:

Lockout/Tagout Before Any Wiring Change

Before touching any terminal, you must execute the LOTO procedure under OSHA 29 CFR 1910.147 (Control of Hazardous Energy). On the trainer panel:

1. Notify affected persons and identify all energy sources (24 VDC supply, any AC feeds).

2. Shut down the equipment using the normal stopping procedure.

3. Isolate the energy source at the disconnect or power switch.

4. Apply your personal lock and tag to the isolation point.

5. Release or restrain stored energy (discharge capacitors, verify no residual voltage).

6. Verify absence of voltage with a calibrated DMM before touching any terminal. Test the meter on a known live source first, then test the circuit, then confirm the meter still functions — the test-before-touch sequence required by NFPA 70E.

Your instructor will verify your LOTO before you begin wiring. No exceptions: this is a safety-critical step that mirrors real shop requirements.

Module Configuration in PLC Software

After wiring is complete and verified, you will configure the I/O module in the PLC project software:

1. In the project tree, navigate to the I/O rack or chassis.

2. Add the correct module (match the catalog number on the physical module).

3. Assign the correct slot number — this must match the physical slot in the chassis.

4. Set the connection parameters (RPI — requested packet interval — and connection type as required by your platform).

5. Download the project, go online, and open the I/O status display.

6. Toggle each physical input switch and confirm the corresponding bit changes state in the software. Force each output ON from the software and confirm the field device activates.

Document all readings — supply voltage, measured voltages at input and output terminals with the load active — in your lab log. A complete, accurate log entry is required for credit.

Safety Reminders for This Lab

Key terms10

Discrete I/OInput or output signals that have only two states, ON or OFF (24 VDC present or absent), as opposed to analog signals that vary continuously.
Sinking input (NPN)An input module configuration in which current flows into the input terminal from the field device; the field device is connected between the positive supply and the input terminal.
Sourcing input (PNP)An input module configuration in which the module provides current out of the input terminal; the field device completes the circuit to the 0 V common.
Relay output moduleA PLC output module that uses an internal electromechanical relay to switch the load circuit, providing galvanic isolation and the ability to switch AC or DC loads.
Terminal torque specificationThe manufacturer-specified tightening torque for screw or spring-cage terminals, typically in inch-pounds; correct torque prevents loose connections and conductor damage.
Verify absence of voltageThe NFPA 70E required step of using a calibrated DMM to confirm that no voltage is present at a circuit before touching it, performed as test-meter-on-known-live, test-circuit, confirm-meter-still-works.
Slot number (PLC configuration)The physical position number of a module in the PLC chassis or rack; the software configuration must match this number for the controller to communicate with the correct module.
RPI (Requested Packet Interval)A configuration parameter on some PLC platforms (such as EtherNet/IP systems) that sets how frequently the controller updates I/O data from a module, expressed in milliseconds.
NFPA 79The National Fire Protection Association's Electrical Standard for Industrial Machinery; governs wiring methods, wire sizing, color coding, termination, and protection for machine control panels.
Inductive load suppressionA diode, varistor, or RC snubber wired across a relay coil or solenoid to absorb the voltage spike generated when current is interrupted, protecting the output transistor or relay contacts.

Worked examples4

1. A 24 VDC sourcing output module drives a pilot light through a circuit with 0.5 Ω of total wire resistance. The pilot light is rated 24 VDC, 25 mA. What voltage drop occurs across the wiring, and what voltage reaches the pilot light?
  1. Identify current through circuit: I = 25 mA = 0.025 A.
  2. Apply Ohm's law to find voltage drop across wire resistance: V_drop = I × R = 0.025 A × 0.5 Ω = 0.0125 V.
  3. Subtract voltage drop from supply to find voltage at the load: V_load = 24 V − 0.0125 V = 23.9875 V.
Answer: The wire drops approximately 0.013 V (13 mV), so the pilot light receives approximately 23.99 V — well within its rating. This confirms that 18–22 AWG wire has negligible resistance for low-current 24 VDC control circuits.
2. A relay output module coil circuit measures 22 Ω across the contactor coil terminals with the coil de-energized. When the output is forced ON and 24 VDC is applied, what current flows through the coil?
  1. Identify the known values: V = 24 V, R = 22 Ω.
  2. Apply Ohm's law: I = V / R = 24 V / 22 Ω.
  3. Calculate: I = 1.09 A (rounded to two decimal places).
  4. Compare to the output module's relay contact current rating (check datasheet — commonly 2 A for small relay outputs); 1.09 A is within a 2 A rating.
Answer: Approximately 1.09 A flows through the coil. Always verify this is below the module's maximum relay contact current rating before energizing.
3. You measure 12.6 VDC at an input terminal when a pushbutton is pressed, but the PLC status display shows the input bit as OFF. The module's minimum ON-state voltage threshold is listed as 15 VDC in the datasheet. What is the likely cause, and how would you diagnose it?
  1. Recognize that 12.6 V is below the module's 15 VDC minimum ON-state threshold, so the module correctly sees the signal as OFF.
  2. Measure the voltage at the 24 VDC power supply output terminals — if the supply reads significantly below 24 V (e.g., 12–13 V), the supply may be undersized, faulted, or wired to the wrong tap.
  3. Check for excessive voltage drop across connections: measure from supply positive to the input terminal with the button pressed; a large drop indicates a bad connection or undersized wire.
  4. Confirm the common (0 V) connection is secure on both the power supply and the input module common terminal.
Answer: The input signal (12.6 V) is below the module's minimum ON threshold (15 V). The most likely causes are a low supply voltage or significant voltage drop in the wiring or connections. Measure the supply at its terminals and at the module to isolate the fault.
4. You are scaling a 4–20 mA analog input on a separate trainer module (for reference practice). The sensor outputs 12 mA. The engineering range is 0–250 PSI. What pressure does 12 mA represent?
  1. Identify the live zero: 4 mA = 0 PSI; 20 mA = 250 PSI. The span in mA = 20 − 4 = 16 mA. The span in PSI = 250 − 0 = 250 PSI.
  2. Find how far the signal is above the live zero: 12 mA − 4 mA = 8 mA.
  3. Calculate the fraction of span: 8 mA / 16 mA = 0.5 (50%).
  4. Apply the fraction to the engineering range: 0.5 × 250 PSI = 125 PSI.
Answer: 12 mA corresponds to 125 PSI — exactly midscale, which makes sense because 12 mA is the midpoint of the 4–20 mA range.

The math in this lesson2

Learners scale a 12 mA analog signal to a 0–250 PSI engineering range.
F-LE.1A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Pre-lab LOTO practice: each student executes the approved LOTO procedure on the trainer panel, has it verified by the instructor, then wires four sinking/sourcing input circuits from a provided wiring diagram; instructor spot-checks polarity and wire termination torque. · Output wiring lab: students wire two sourcing DC output channels to pilot lights and one relay output channel to a small contactor coil; use DMM to verify continuity before energizing; power up and confirm outputs activate when forced in the PLC software. · Module configuration in software: students add the correct I/O module to the PLC project tree, assign slot number and connection parameters, go online, and confirm that live I/O status matches physical switch positions; document results in a lab log.
Materials: PLC trainer with discrete I/O modules, 24 VDC power supply, pushbuttons, limit switches, pilot lights, relay with coil, 18–22 AWG stranded wire, wire stripper, screwdrivers, DMM, wiring diagram set, NFPA 79 reference, LOTO kit, lab log notebook
Check: All wiring passes instructor continuity and polarity check before energizing; after power-up, all four inputs and three outputs respond correctly (100% pass required for safety-critical wiring); lab log entry complete with measured voltages recorded.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and LOTO
1. Before making any wiring change to the PLC trainer panel, what step must be completed first according to OSHA 29 CFR 1910.147?
Discrete I/O Wiring
2. A sourcing (PNP) discrete input module provides current out of its input terminal. Which wiring configuration correctly connects a normally open pushbutton to this module?
Measurement and Troubleshooting
3. After wiring, a student uses a DMM to verify continuity of an output circuit before energizing. The meter reads OL (overload/open) across the pilot light terminals with the common wire connected. What is the most likely cause?
Wiring Practices and Standards
4. According to NFPA 79, why must control wiring terminals be tightened to the manufacturer's specified torque?
Output Module Selection
5. A relay output module is used instead of a transistor output module to drive a contactor coil. What is the primary reason for this choice?
PLC Module Configuration
6. When configuring a discrete I/O module in PLC software, the slot number assigned in the project tree must match which physical attribute?
Electrical Safety / NFPA 70E
7. A student measures 24.0 VDC at the power supply output but only 11.2 VDC at an input module terminal when a limit switch is closed. Which NFPA 70E-required action should have been taken before this measurement to ensure safety?
Electrical Calculations
8. A transistor sourcing output module has a relay output channel wired to a 24 VDC contactor coil rated at 18 Ω. The supply is 24 VDC. Approximately how much current flows through the coil when the output is ON?
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