Wiring Field Devices to PLC Inputs and Outputs

Wiring Field Devices to PLC Inputs and Outputs

Why This Skill Matters

A programmable logic controller (PLC) is only as reliable as the wiring that connects it to the real world. Field devices—push buttons, limit switches, pilot lights, sensors, and more—communicate with the PLC through its input and output (I/O) modules. A single reversed wire or missed connection can prevent a machine from starting, damage output modules, or create an unsafe condition. Electricians who can read an I/O wiring diagram and wire field devices correctly are valuable on any commercial or industrial job.

Safety First: Lockout/Tagout Before You Touch a Terminal

Before wiring or inspecting any PLC trainer station, you must follow the written lockout/tagout (LOTO) procedure. OSHA's Control of Hazardous Energy standard (29 CFR 1910.147) requires that all energy sources be isolated and de-energized before work begins on equipment. On a PLC trainer this means:

1. Notify anyone affected by the shutdown.

2. Identify every energy source (electrical supply to the trainer).

3. Shut down the trainer using the normal stopping procedure.

4. Isolate the energy source—flip the disconnect or circuit breaker to the OFF position.

5. Apply your personal lockout device and tag.

6. Verify absence of voltage using a calibrated voltage tester on every terminal that could be energized before you touch any wire.

Never assume the trainer is de-energized because someone else says so. Test it yourself. Wear safety glasses whenever working at a terminal strip; a loose ferrule launched by a screwdriver can cause eye injury.

Understanding the I/O Wiring Diagram

An I/O wiring diagram shows every field device, the wire number or color assigned to each conductor, the terminal block designation (e.g., TB1), and the specific PLC input or output channel address. Before picking up a wire, read the whole diagram and answer these questions:

• What is the power supply voltage? (This lab uses 24 VDC.)
• Which terminals are the common (0 V) return, and which carry the switched signal?
• Which channels are discrete inputs (DI) and which are discrete outputs (DO)?
• What wire color or label does the diagram specify for each conductor?

Following the diagram exactly—not from memory—is a professional habit that prevents errors and is required for NCCER competency.

Discrete Input Wiring: Push Button and Limit Switch

A discrete input channel reads one of two states: ON (current flowing) or OFF (no current). The PLC's input module supplies a small sensing voltage—24 VDC in this lab—and monitors whether current returns through the field device.

Normally Open (NO) push button: When the button is not pressed, the circuit is open and the input reads OFF. When pressed, current flows and the input reads ON. Wire one terminal of the button to the 24 VDC source terminal at the module and the other terminal to the assigned input channel terminal.

Normally Closed (NC) limit switch: When the actuator is not tripped, the circuit is closed and the input reads ON. When the actuator is tripped (e.g., by a machine part), the circuit opens and the input reads OFF. The wiring connections are the same as for the NO button, but understand that the default (un-tripped) state is ON—the opposite of the push button.

Terminal labeling: every wire end must be labeled with the wire number shown on the diagram. Use pre-printed wire labels or a label maker. Unlabeled wires cannot be traced quickly during troubleshooting and are a quality deficiency in professional work.

Discrete Output Wiring: 24 VDC Pilot Light

A discrete output channel energizes a load when the PLC program turns it ON. The output module switches voltage to the load. For a sourcing (PNP) output module the module supplies 24 VDC to the load when active; the load's other lead returns to the common (0 V) terminal.

Polarity matters. The 24 VDC pilot light used in this lab is a DC device. Connect the positive (+) lead of the pilot light to the output channel terminal and the negative (−) or common lead to the 0 V common terminal. Reversing polarity will prevent the light from illuminating and may damage it. Check the diagram for the polarity markings before terminating the output wiring.

Wire Preparation and Ferrules

Control wiring in a PLC panel uses fine-stranded wire, typically 22–18 AWG, which can loosen or break strands if inserted bare into a cage-clamp or screw terminal. Ferrules (hollow metal sleeves crimped onto the stripped end) prevent this. To install a ferrule:

1. Strip the wire to the length marked on the ferrule barrel (usually 8–10 mm).

2. Insert the stripped end fully into the ferrule.

3. Crimp with the correct ferrule crimping tool—pliers will not create a reliable crimp.

4. The crimped end should be secure; no bare strands should be visible outside the insulation sleeve.

Use the wire gauge range printed on the ferrule and match it to the terminal manufacturer's specification.

Point-to-Point Continuity Check

After wiring and before power-up, perform a point-to-point continuity check against the diagram. With the trainer locked out, set your digital multimeter (DMM) or continuity tester to the continuity function. Touch one probe to the source end of each wire and the other probe to the terminal where the diagram says it should land. A tone or low-resistance reading confirms correct connection. A silent or high-resistance reading means a missed connection, wrong terminal, or broken wire—write it on the punch list and correct it before proceeding.

What a Passing Result Looks Like

On first power-up after instructor sign-off: pressing the NO push button turns the input channel ON; tripping and releasing the NC limit switch causes its input channel to toggle; and the pilot light illuminates when the instructor activates the output from the PLC programming terminal. No smoke, no blown fuses, no reversed indicators. That is the standard.