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newSchoolWorks.com › VoTechWorks › Building & Construction › Electrical › Programmable Logic Controller (PLC) Introduction for Electricians › Lesson 8
BLD-ELEC05-07 · Lesson 8 · 22.5 clock hours · lab

Timers and Counters in Ladder Logic

Objective. Students will configure and program TON timer and CTU counter instructions to control the sequence and count of output events in a PLC application.

Timers and Counters in Ladder Logic

Why Timers and Counters Matter

Most industrial and commercial electrical systems need more than simple on/off control. A ventilation fan may need to run for a set period after a room is vacated. A conveyor may need to count packages before triggering a diverter. Timers and counters are the PLC instructions that make time-based and quantity-based control possible without relays, clock motors, or mechanical counters. Learning to configure them correctly is a core electrician skill whenever you work on automated systems.

The TON (Timer On-Delay) Instruction

The Timer On-Delay (TON) instruction is the most common timer in ladder logic. It begins accumulating time when its rung goes true (the enable condition is met) and activates its done bit after the accumulated value reaches the preset value.

Every TON instruction block contains four key fields:

When the rung goes false before the timer finishes, the accumulated value resets to zero on most TON implementations — the timer is non-retentive by default. If you need the timer to hold its accumulated value across power interruptions or rung-false conditions, you would use a retentive timer (RTO), but the TON is the standard starting point.

Practical sequence for a five-second on-delay:

1. Examine the start button contact (XIC) in rung 0 — this is the enable condition.

2. Place the TON instruction at the end of rung 0. Set Preset = 5000 (5 seconds × 1000 ms).

3. In rung 1, use an XIC contact referencing the timer's `.DN` bit to energize the output coil.

4. Download, go online, press the start button, and watch the ACC climb to 5000. The output energizes at that point.

The CTU (Count Up) Instruction

The Count Up (CTU) instruction increments its accumulated count by one each time its rung transitions from false to true — that is, on each rising edge of the enable condition. It does not count continuously while the rung stays true; it counts events.

Key fields in a CTU block:

Resetting a Counter

Because the CTU is retentive — it holds its count even when the rung goes false — you must use a RES (Reset) instruction to clear the accumulated value back to zero. Place the RES instruction in a separate rung, referencing the same counter address. Wire the rung to a dedicated reset pushbutton input. When that rung goes true, ACC returns to zero and the done bit de-energizes.

The same RES instruction is used to reset retentive timers (RTO). For a standard TON, a RES instruction is not needed because the timer resets automatically when the rung goes false.

Connecting Timers and Counters in a Program

You often chain these instructions together. For example, a CTU can count cycles and, when its done bit activates, energize a TON to hold an alarm on for ten seconds before automatically resetting. The done bit of one instruction becomes the enable contact of another — this is standard ladder logic composition.

Safety Reminders for PLC Lab Work

Lockout/Tagout (LOTO): Before making any physical wiring changes to your trainer station, de-energize the unit and follow your facility's lockout/tagout procedure. LOTO applies even on low-voltage trainers whenever you access terminal blocks.

PPE: Safety glasses are required at PLC trainer stations. If your trainer is connected to 120 V AC or higher for output loads, treat exposed terminals as energized until proven otherwise.

Software vs. Hardware: Downloading a program to a running PLC can immediately change output states. Confirm the trainer is in a safe condition — outputs unloaded or loads understood — before you download. Always announce to your lab partner that you are downloading.

Never force outputs in a live system without authorization. Forcing an output bypasses the program logic and can energize a device unexpectedly.

What to Record in Your Lab Notebook

For every lab exercise, your notebook must include: the rung diagram (drawn or printed), the preset and accumulated values you used, the measured result (stopwatch time or count), and a brief explanation of why the output behaved as it did. Complete records are a professional habit and are required to pass today's check.

Key terms10

TON (Timer On-Delay)A PLC timer instruction that begins accumulating time when its rung goes true and activates its done bit after the accumulated value reaches the preset; resets automatically when the rung goes false.
Preset (PRE)The target value programmed into a timer or counter instruction; the accumulated value must reach this value before the done bit activates.
Accumulated Value (ACC)The current running total within a timer (elapsed time in milliseconds) or counter (number of events counted); updated every PLC scan.
Done Bit (.DN)A status bit associated with a timer or counter that turns true when the accumulated value equals or exceeds the preset; used as a contact in other rungs to trigger outputs.
CTU (Count Up)A PLC counter instruction that increments its accumulated value by one on each rising-edge transition of its enable rung; retentive — holds its count when the rung goes false.
RES (Reset)A PLC instruction placed in its own rung and referencing a specific counter or retentive timer address; when the rung goes true it clears the accumulated value to zero and de-energizes the done bit.
Rising EdgeThe moment a signal transitions from false (0) to true (1); the CTU increments its count on this transition, not while the signal stays continuously true.
Retentive Timer/CounterA timer or counter that holds its accumulated value when its enable rung goes false; requires a RES instruction to clear; the CTU is retentive by design, while the standard TON is non-retentive.
Enable Bit (.EN)A status bit that goes true as soon as a timer or counter rung goes true; indicates the instruction is currently active and accumulating.
Lockout/Tagout (LOTO)An OSHA-required safety procedure that ensures hazardous energy is isolated and cannot be re-energized while workers are accessing equipment; required before any physical wiring changes, even on training stations.

Worked examples4

1. You need a TON timer to delay an output by 7.5 seconds. Your PLC platform stores timer values in milliseconds. What value do you enter in the Preset field?
  1. Identify the desired delay: 7.5 seconds.
  2. Convert seconds to milliseconds: 7.5 s × 1000 ms/s = 7500 ms.
  3. Enter 7500 in the Preset field of the TON instruction.
Answer: Preset = 7500 (milliseconds), which equals a 7.5-second on-delay.
2. During your lab test of the five-second TON, you press the start button and measure the actual delay with a stopwatch three times, recording 5.1 s, 4.9 s, and 5.0 s. Does the timer pass the ±0.5-second acceptance criterion for all three trials?
  1. Programmed preset = 5 seconds. Acceptable range = 5.0 s − 0.5 s to 5.0 s + 0.5 s = 4.5 s to 5.5 s.
  2. Trial 1: 5.1 s — within 4.5–5.5 s? Yes.
  3. Trial 2: 4.9 s — within 4.5–5.5 s? Yes.
  4. Trial 3: 5.0 s — within 4.5–5.5 s? Yes.
  5. All three trials fall within the acceptable range.
Answer: All three measured delays pass the ±0.5-second criterion. The timer is functioning correctly.
3. Your CTU counter has a Preset of 5 and the accumulated value is currently 3. The operator presses the start button twice more in succession. What is the new accumulated value, and is the done bit energized?
  1. Current ACC = 3. Each press of the start button produces one rising-edge event on the CTU rung.
  2. First press: ACC = 3 + 1 = 4. Is ACC ≥ PRE (5)? No — done bit remains off.
  3. Second press: ACC = 4 + 1 = 5. Is ACC ≥ PRE (5)? Yes — done bit turns on.
Answer: After two more button presses, ACC = 5 and the done bit (.DN) energizes, activating the indicator output.
4. You have a TON with Preset = 10000 ms. You want to verify the actual delay. You run five stopwatch trials and record: 10.0 s, 10.1 s, 9.8 s, 10.2 s, and 10.0 s. Calculate the average measured delay and determine whether the average is within ±0.5 s of the 10-second preset.
  1. Sum the five measurements: 10.0 + 10.1 + 9.8 + 10.2 + 10.0 = 50.1 s.
  2. Divide by the number of trials: 50.1 ÷ 5 = 10.02 s.
  3. Acceptable range: 10.0 − 0.5 = 9.5 s to 10.0 + 0.5 = 10.5 s.
  4. Is 10.02 s within 9.5–10.5 s? Yes.
Answer: Average measured delay = 10.02 seconds, which is within the ±0.5-second acceptance window. The timer passes.

The math in this lesson2

Learners convert seconds to millisecond presets and average stopwatch trials to verify timer accuracy.
N-Q.1S-ID.2
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates a TON (timer on-delay) instruction on the projector, explaining preset, accumulated value, enable bit, and done bit; students annotate a printed timer instruction reference sheet and answer four prediction questions before the instructor reveals the program output. · Lab exercise: students add a TON timer to their existing start/stop program so that pressing the start button causes the output to energize after a five-second delay; they set the preset, download, and test by timing the actual delay with a stopwatch, recording results. · Counter exercise: students program a CTU counter to count the number of times the start button is pressed and illuminate a second output (indicator light) when the count reaches five; they test, reset the counter using a reset input, and document the rung logic and results in their lab notebooks.
Materials: Computer workstations with PLC programming software, PLC trainer stations, stopwatch or phone timer, printed timer/counter instruction reference sheets, lab notebooks
Check: Both the timer and counter programs are tested live by the instructor; timer delay must be within ±0.5 seconds of the programmed preset and the counter output must activate at the correct count; lab notebook entries must be complete; pass requires both functions operating correctly.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

PLC Programming — Timers
1. In a TON instruction, what event causes the done bit (.DN) to energize?
PLC Programming — Timers
2. A TON timer on a PLC platform that stores time in milliseconds must produce a 12-second delay. What preset value should the programmer enter?
PLC Programming — Timers
3. What happens to the accumulated value of a standard (non-retentive) TON timer when its enable rung goes false before the preset is reached?
PLC Programming — Counters
4. A CTU counter has a Preset of 5 and an accumulated value of 5. The operator presses the count input button one more time. Which statement best describes the counter behavior?
PLC Programming — Counters
5. Which instruction is used to clear a CTU counter's accumulated value back to zero?
OSHA 10 — Electrical Safety
6. Before making wiring changes at a PLC trainer station, which safety procedure must the student follow first?
PLC Programming — Counters
7. A student programs a CTU with a Preset of 5 to count start-button presses. After exactly 5 presses, what must the student observe to confirm the counter is operating correctly?
OSHA 10 — Electrical Safety
8. A technician downloads a new program to a running PLC trainer. The trainer's output is wired to a motor contactor. What is the greatest immediate hazard of this action?
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