EN, TT, DN — a timer's three faces

You know T1.DN — the done bit, true when the timer finishes. But a Logix timer publishes three status bits, and the other two are what turn a timer from a delay into a sequencer.

The trio

• EN — enable. Mirrors the rung feeding the timer. Rung true, EN true. Nothing more — but nothing less, and that "nothing less" is the trick below.
• TT — timer timing. True only while the timer is accumulating: the rung is true and the accumulator hasn't reached the preset yet. This is the bit that means "we are in the middle of the delay."
• DN — done. True from the moment the accumulator reaches the preset until the rung drops.

Read the diagram top to bottom at any instant and you can tell exactly where the timer is in its life. EN and TT rise together; at the preset TT hands off to DN; when the rung drops, all three reset at once.

TT is a free output window

"Run the purge fan for the first 20 seconds after start" — that's one XIC of TT. No comparison, no second timer, no relay:

──────[start conditions]──────( TON T1, 20 s )
──────[T1.TT]─────────────────( Purge fan )

The fan runs while T1 times and stops by itself at the preset. Anything that should happen during a delay rides TT; anything that happens after it rides DN.

EN is a free seal-in

Here's the move the batch print makes, and it's worth staring at:

    Stop        Start
──────[/]──┬─────[ ]─────┬──────[/T2.DN]──────( TON T1 )
           └───[T1.EN]───┘

The seal branch doesn't use the motor's contact or an internal relay — it uses the timer's own EN bit. Press Start: the rung goes true, EN goes true. Release Start: the rung stays true through EN itself. The timer holds its own rung up while it works.

It's the same self-feeding loop as every seal-in — the coil just happens to be a timer. One instruction gives you the delay AND the memory. When the rung finally breaks (stop, or the /T2.DN at the end of the sequence), EN drops, the seal dies, and the timer resets to zero — ready for the next cycle.

Try the bits live

Open the builder with the batch print loaded: the contact dropdown now offers .DN, .TT and .EN for every timer you place. Pulse Start and watch rung 0 stay lit through the T1.EN branch — the next lesson runs that whole sequence.