Lesson 17 · Automation

PLC Inputs and Outputs

A PLC senses the world through inputs and acts on it through outputs. Get I/O and you understand how a PLC actually connects to a machine.

The PLC’s connection to the real world

A PLC only knows about the world through its inputs, and it only affects the world through its outputs. Everything the program does is: look at the inputs, decide, drive the outputs. Understanding I/O is understanding how a PLC actually touches your machine.

Inputs — what the PLC senses

An input is anything that tells the PLC about a condition. Wire a device to an input terminal, and when that device closes the circuit, the PLC sees that input turn ON (a 1); when it opens, OFF (a 0).

Discrete (digital) inputs: on/off devices — push buttons, limit switches, float switches, proximity switches, selector switches, relay contacts. The bread and butter.
Analog inputs: a varying signal — a 4-20mA or 0-10V reading from a temperature, pressure, or level transmitter. The PLC reads a number, not just on/off.

Outputs — what the PLC drives

An output is how the PLC makes something happen. When the program turns an output ON, the PLC closes that output’s circuit, energizing whatever’s wired to it.

Discrete (digital) outputs: switch things on and off — contactor coils, pilot lights, solenoid valves, small relays. Often the output drives a coil that drives a contactor that drives the motor.
Analog outputs: send a varying signal — a 4-20mA command to a VFD telling it what speed to run, or to a valve telling it how far to open.

Key idea: the PLC doesn’t switch motor power directly. A small output energizes a contactor coil; the contactor’s heavy poles switch the motor. The PLC handles the logic; contactors and drives handle the power — exactly the decision-vs-action split from the relay lessons.

Sinking, sourcing, and voltages

I/O comes in different electrical flavors — 24V DC is most common for inputs, with outputs as relay, transistor (sinking/sourcing), or triac types depending on what they switch. The practical point for now: inputs and outputs are rated for specific voltages and types, and you match the field device to the right module.

What to take away

Inputs let the PLC sense the world (discrete on/off or analog values); outputs let it act on the world (switching coils and lights, or commanding drives and valves). The PLC reads inputs, runs the logic, and sets outputs — and it leans on contactors and drives to handle the actual power.

Concept lesson: this one is about understanding equipment and ideas rather than wiring a circuit, so there’s no Sandbox build for it. Next: why all of this beats a panel full of relays.