Discrete Input and Output Wiring
Objective. Students wire sinking and sourcing discrete input devices (pushbuttons, limit switches, proximity sensors) and output devices (pilot lights, solenoids, relay coils) to PLC I/O modules within allowable current ratings.
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 Input and Output Wiring
What "Discrete" Means
A discrete signal is simply ON or OFF — a single bit of information. Every pushbutton, limit switch, proximity sensor, pilot light, and solenoid coil connected to a PLC in this lab is a discrete device. The PLC reads discrete inputs through its input module and drives discrete outputs through its output module. Getting the wiring right is the foundation of every PLC-controlled machine.
PLC I/O Module Basics
A PLC input module converts a field voltage — typically 24 VDC in modern industrial systems — into a logic-level signal the CPU can read. The module has individual input terminals (often labeled X0, X1, X2 … or I:0/0, I:0/1 … depending on the manufacturer), a common terminal (COM or 0 V), and internal optocouplers that isolate field wiring from the CPU.
A PLC output module does the reverse: it switches a field voltage to power a field device. Transistor (DC solid-state) output modules are common for 24 VDC loads. Each output channel has a maximum current rating — typically 0.5 A to 2 A per point — and a module total. Never exceed either rating. Overloading an output damages the transistor or blows the module fuse.
Sinking and Sourcing — Current Flow Direction
These terms describe which direction current flows through the input or output terminal.
- Sinking (NPN): The device or module provides a path to 0 V (negative). Current flows FROM the positive supply, THROUGH the field device, INTO the module terminal, and the module sinks it to common (0 V). An NPN sensor has an open-collector output that pulls the signal line toward 0 V when active.
- Sourcing (PNP): The device or module provides positive voltage. Current flows FROM the module terminal, THROUGH the field device, TO common (0 V). A PNP sensor sources current out of its output wire when active.
Key rule: A sinking input card pairs with a sourcing (PNP) sensor; a sourcing input card pairs with a sinking (NPN) sensor. Some cards are selectable — check the module datasheet and wiring diagram before connecting anything.
In this lab the trainer uses a sinking input card. You will wire the PNP proximity sensor to it. The NPN sensor demonstration by the instructor shows why polarity reversal causes the input to never activate.
Three-Wire Sensor Wiring
A three-wire proximity sensor has:
- Brown wire: +24 VDC supply
- Blue wire: 0 V (common)
- Black wire: Signal output (NPN sinks to 0 V; PNP sources +24 V)
For a PNP sensor on a sinking card, connect brown to +24 V, blue to 0 V, and black to the input terminal. When the sensor detects a target, the black wire rises to near +24 V, current flows into the sinking module, and the CPU reads a logic 1.
Pushbutton and Limit Switch Wiring
Two-wire contact devices (pushbuttons, limit switches) are simpler: one wire to +24 V and the other to the input terminal. The module sees voltage when the contact closes. Wire a normally open (NO) contact so the input is OFF at rest and turns ON when pressed or actuated. Wire a normally closed (NC) contact so the input is ON at rest and drops OFF when actuated — useful for safety interlocks.
Output Wiring — Pilot Lights and Solenoids
A 24 VDC pilot light connects with one wire to the output terminal and the other to 0 V common. When the output transistor turns ON, +24 V appears at the terminal and current flows through the lamp to common.
A solenoid valve coil is an inductive load. Inductive loads generate a voltage spike (back-EMF) when switched OFF. Always use a suppression device — a flyback diode across a DC coil — to protect the output transistor. In this lab the relay module provides that protection and also allows a 24 VDC output transistor to switch higher-voltage or higher-current loads safely. Wire the output terminal to the relay coil positive; relay coil negative to 0 V. The relay's dry contacts then control the solenoid.
Verifying Wiring Before Energizing
Before applying power:
1. Remove the module or confirm output power is OFF.
2. Set your multimeter to DC voltage.
3. With input power ON but outputs de-energized, probe each input terminal to COM. You should read near 0 V on an open contact and near +24 V when the device is actuated.
4. Confirm no terminal-to-terminal shorts with the continuity function.
After energizing, force outputs ON in the programming software (with the PLC in a safe, controlled state and all personnel clear of actuated devices) to confirm each field device responds to the correct output point.
Safety Requirements
- Lockout/Tagout (LOTO): De-energize and lock out the trainer power source before adding or moving any wires. Verify zero energy with your multimeter before touching terminals. OSHA 29 CFR 1910.147 applies.
- PPE: Safety glasses are required at the trainer at all times. Use insulated tools.
- Current ratings: Check the module datasheet. Never connect a load that exceeds the per-point or module total current rating.
- Polarity: Reversing DC polarity can destroy transistor output modules and sensor outputs instantly.
- Inductive loads: Always confirm suppression is in place before switching a solenoid or relay coil through a solid-state output.
- Forced outputs: Forcing an output bypasses ladder logic. Announce the force, ensure no one is in the machine envelope, and remove all forces before handing the trainer back.
Key terms10
Worked examples4
- Identify the per-point current: 7 mA = 0.007 A.
- Multiply by the total number of points: 0.007 A × 16 = 0.112 A.
- Convert to milliamps for clarity: 0.112 A = 112 mA.
- Check per-point rating: each solenoid draws 0.6 A; the per-point limit is 1.0 A. 0.6 A < 1.0 A — each individual point is within rating.
- Calculate total load: 4 solenoids × 0.6 A = 2.4 A.
- Compare to module total rating: 2.4 A < 4.0 A — the module total is also within rating.
- Conclusion: the wiring plan is acceptable.
- Logic 0 check (target absent): measured 0.8 VDC. The logic-0 threshold requires less than 5 VDC. 0.8 V < 5 V — reads as a valid logic 0.
- Logic 1 check (target present): measured 23.6 VDC. The logic-1 threshold requires greater than 15 VDC. 23.6 V > 15 V — reads as a valid logic 1.
- Both measurements fall within the module's stated thresholds.
- Pilot light: (25 mA ÷ 500 mA) × 100 = 5%.
- Solenoid coil: (150 mA ÷ 500 mA) × 100 = 30%.
- Both are well below 100%, so neither point is overloaded.
The math in this lesson1
Materials: PLC trainer, NPN and PNP proximity sensors, pushbuttons, limit switches, pilot lights, relay module, solenoid valve coil, multimeter, wire strippers, ferrule crimper, terminal-block screwdrivers, I/O wiring diagram worksheet
Check: Instructor observes live test: each input illuminates the correct I/O indicator on the CPU and each forced output activates the correct field device — 100% of wired points must pass; any miswire is corrected and retested before sign-off.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
