Timers and counters

"Wait three seconds, then start the conveyor." "Count twelve bottles, then index the case." Plants run on delays and counts — and in ladder logic both are boxes with a number inside.

TON: the on-delay timer

A TON starts accumulating time while its rung is true, and sets its done bit (DN) when the accumulator reaches the preset:

• Rung true → the accumulator climbs: 0.0, 0.1, 0.2 … up to the preset.
• Accumulator = preset → T1.DN turns ON, and stays on while the rung holds.
• Rung false → the accumulator resets to zero. A TON has no memory of half-finished timing — interrupt it at 4.9 of 5.0 seconds and it starts over from nothing.

The done bit is just a bit: other rungs examine it with an ordinary -| |- contact. "Warn horn for 5 s, then run" is two rungs: switch → TON, and T1.DN → conveyor.

CTU: the count-up counter

A CTU adds one every time its rung goes from false to true — it counts edges, not time spent on:

• Photo-eye blocked → one count. Held blocked for a minute → still one count. Edges, not duration.
• Accumulator reaches preset → C1.DN turns ON.
• Unlike a timer, a counter keeps its count when the rung drops — that is its job. It only goes back to zero when a RES (reset) instruction fires.

See both run

The ladder logic simulator has both circuits live: watch the TON accumulator climb in real time, and pulse the photo-eye to walk the counter to its preset — then reset it for the next batch.

And then stop pulsing it by hand. The box counter in the builder comes with a real belt: boxes ride past the eye at a fixed spacing, the beam makes and breaks once per box, and the CTU counts five and stops the belt. The edge you were faking with your finger is the edge the machine gives you for free — which is also how you find out what happens to a counter wired to the belt motor instead of the eye. Try it: it counts to one and sits there forever.

Why this matters

Contacts, coils, a seal-in, a timer, a counter: that's the working vocabulary of most ladder programs you'll ever open. The rest is the same five ideas, repeated with more wires.