What a PLC actually does

The gray box in the panel replaced a wall of relays, and it did it with one trick: a loop that never stops.

Three jobs, one loop

A PLC — programmable logic controller — does exactly three things:

1. Read every input: pushbuttons, photo-eyes, limit switches, sensor signals. A snapshot of the plant, frozen for this instant.
2. Solve the logic: run your ladder program top to bottom against that snapshot.
3. Write every output: motor starters, valves, lights — whatever the logic decided.

Then it does it again. And again — a full lap (a scan) takes a few milliseconds, so to a human the response looks instant.

Why the loop matters to you

Two consequences show up in real troubleshooting:

• The PLC acts on snapshots. A pulse faster than one scan can be missed; that's why counting fast events uses high-speed counter hardware, not a plain rung.
• Order is real. Rungs solve top to bottom, and a rung near the bottom sees what the rungs above it already decided this same scan. Ladder is not a wiring diagram frozen in time — it's a program with an order.

Inputs and outputs are just bits

Everything the PLC knows about the plant lives in memory as bits and numbers: a pressed button is a 1, a released one a 0, an analog pressure is a number (that's what the signal scaling tool converts). The ladder program is nothing more than rules about those bits.

Why this matters

Every mystery you'll ever chase in a PLC — an output that won't turn on, a seal-in that won't drop — is answered by walking this loop: what did it read, what did the logic say, what did it write. The next lesson teaches you to read the logic itself.