PLC Counters Explained: CTU, CTD and CTUD (with Live Examples)

The CTU (count up) counter is the most common PLC counter instruction. It increments its current value (CV) by one on each rising edge of the CU input, and its QU done bit turns on when CV reaches or exceeds the preset PV.

How it works, step by step. At the start of each scan, the PLC evaluates the rung containing the CTU instruction. If the CU input transitions from FALSE to TRUE (a rising edge) since the last scan, CV is incremented by one. If CV is now greater than or equal to PV, QU (the done bit, called .DN in Allen-Bradley) is set. The counter holds its CV value between scans — it does not reset automatically. When a RES instruction is energised on a separate rung, CV returns to zero and QU clears.

Note that CV can accumulate beyond PV. If no reset logic is in place, CV continues counting as long as rising edges arrive. This is usually intentional in cycle-count applications but can cause unexpected behaviour if you forget the reset. Always pair a CTU with reset logic.

Real-world uses of the CTU counter

Part counting on a production line. A proximity sensor at the end of a conveyor pulses once per part. A CTU with PV = 100 counts parts into a box or bin. When CV reaches 100, QU fires: a diverter solenoid opens, directing the conveyor to an empty container, and a RES resets the counter for the next batch. This is the single most common CTU application in manufacturing.

Machine cycle counting for preventive maintenance. A sensor detects each closing of a punch press die. A CTU accumulates total press cycles. When CV reaches the maintenance interval (say, 50,000 cycles), QU triggers a maintenance-required alert on the HMI. The maintenance engineer resets the counter after servicing. Using a CTU here rather than a timer is correct because the wear mechanism is cycles, not hours.

Conveyor zone counting. In a multi-zone conveyor system, a CTU at each handoff point counts parts entering the zone. A second CTU or a CTUD tracks parts leaving. The difference between the two CV values gives you the number of parts currently in the zone — useful for anti-collision logic and zone fill detection.

Operator button press validation. Requiring an operator to press a confirm button exactly three times before a hazardous action is permitted. A CTU counts the presses; QU at PV = 3 enables the action. A timer resets the counter if 10 seconds pass without the third press, preventing accidental authorisation from stray presses.

The CTUD (count up / count down) counter is the most versatile of the three counter types. It combines both CU and CD inputs in a single instruction, allowing CV to increment and decrement from the same counter. It produces two done bits: QU fires when CV reaches PV (the full condition) and QD fires when CV reaches zero (the empty condition).

How it works. On each rising edge of CU, CV increments by one. On each rising edge of CD, CV decrements by one. If CU and CD both pulse within the same scan cycle, the net effect depends on the platform — most IEC 61131-3 implementations either cancel (net zero) or prioritise one direction. Check your hardware documentation. QU is set when CV is greater than or equal to PV; QD is set when CV is less than or equal to zero. Both bits can be simultaneously TRUE only in the edge case where PV is zero.

The LD input loads PV into CV (same as CTD). The R input resets CV to zero and clears QU and QD. Neither QU nor QD prevents further counting — the CTUD will count past PV upward and below zero downward unless external logic gates the count inputs.

Real-world uses of the CTUD counter

Buffer zone tracking. Parts enter an accumulation conveyor buffer through an entry photoelectric sensor (pulses CU) and exit through an exit sensor (pulses CD). The CTUD CV tracks the exact number of parts currently in the buffer. QU at PV = 50 signals "buffer full — stop upstream conveyor" and QD at zero signals "buffer empty — stop downstream conveyor". This is the canonical CTUD application.

Car park occupancy system. A CTUD with PV = 200 (total spaces) counts vehicles entering (CU) and exiting (CD). QU fires when all spaces are full — a sign at the entrance changes from OPEN to FULL. QD at zero (theoretically impossible in a real car park, but a useful underflow guard) can alert to a sensor fault.

Warehouse bay tracking. Forklifts loading and unloading a bay trigger RFID gates that pulse CU (loading) and CD (unloading). The CTUD CV gives the current pallet count in the bay. QU at PV signals the bay is at capacity; QD signals the bay is empty and available for a new load.

Multi-pump alternation counter. A system with two duty pumps alternates which pump runs first on each start cycle. A CTUD tracks the cumulative start-count difference between the two pumps: each time pump A starts, CU pulses; each time pump B starts, CD pulses. The QU/QD bits signal when one pump has run significantly more than the other, triggering a switchover recommendation on the HMI.

A PLC counter can exceed its preset in either direction if the count inputs continue to fire after the done condition is reached. Understanding overflow (CTU counting past PV) and underflow (CTD or CTUD counting below zero) is essential for writing robust counter logic that does not misbehave in real production conditions.

CTU overflow. The CTU QU done bit sets when CV reaches PV, but the counter does not stop counting. If rising edges continue to arrive on CU — because the reset logic has not been triggered yet, or because the upstream sensor is still active — CV continues to increase past PV. QU remains TRUE as long as CV is at or above PV, so there is no double-trigger issue with the done bit. However, if CV is a 16-bit integer (maximum 32,767 in most platforms), it will eventually roll over to 0 or to the minimum negative value. In a production environment where hundreds of thousands of parts are counted without a reset, this rollover can cause subtle bugs. Always verify that your reset logic fires reliably before CV can reach the integer maximum.

CTD and CTUD underflow. A CTD counter with CV at zero will decrement into negative values if CD continues to pulse. QD remains TRUE as long as CV is at or below zero. In most applications this means the dispense or depletion process should gate the CD input using the QD bit — an XIO (normally-closed) QD contact in series with the count trigger prevents further decrement. Without this gate, CV can go arbitrarily negative, and when LD next loads PV, the previous debt of negative counts does not carry over (LD is a hard-load, not an add). Still, allowing large negative values is a sign of a logic gap — the machine is counting dispense events when no inventory is present.

Best practice: gate the count inputs on the done bits. For a CTU, use an XIO Counter1.QU contact in series with the sensor that drives CU. This gates the count input closed the moment QU fires, preventing further accumulation until after the reset. For a CTD, use an XIO Counter1.QD contact in series with the CD input. For CTUD, gate CU with XIO QU and gate CD with XIO QD. This single pattern eliminates both overflow and underflow at the source.

Platform-specific overflow limits. Allen-Bradley SLC-500 and ControlLogix CTU instructions use a signed 16-bit (SLC, maximum 32,767) or 32-bit (Logix, maximum 2,147,483,647) integer for PRE and ACC. IEC 61131-3 CTU and CTUD use INT (−32,768 to +32,767) or DINT (−2,147,483,648 to +2,147,483,647) depending on the function block variant. For applications counting in the millions of cycles, always choose a DINT-typed counter (CTU_DINT in IEC) or verify that your Allen-Bradley Logix counter uses a 32-bit accumulator.

Combining a CTU counter with a TON timer is one of the most common ladder logic patterns in manufacturing. The counter defines the quantity condition; the timer defines the dwell duration after that quantity is reached. Together they implement the "count X, then wait Y seconds" cycle that drives packaging machines, batching systems, and conveyor sorting everywhere.

The application. A conveyor deposits parts into a box. After 10 parts are deposited, the conveyor must stop for 5 seconds to allow the box to be removed and an empty box positioned. The conveyor then restarts automatically for the next 10 parts.

Rung 1 — Count incoming parts:

XIC PartSensor — CTU Counter1 PRE=10 / PV=10

Each rising edge on PartSensor (a photoelectric sensor at the box fill point) increments Counter1.ACC / CV. When CV reaches 10, QU (.DN in Allen-Bradley) turns on.

Rung 2 — Start the conveyor hold timer on QU:

XIC Counter1.QU — TON ConveyorHold_Timer PRE=5000 ms

The moment Counter1.QU turns on, the TON's enable rung goes TRUE and the timer begins counting. ACC increments toward 5,000 ms.

Rung 3 — Stop the conveyor while QU is TRUE:

XIO Counter1.QU — OTE Conveyor_Motor

A normally-closed Counter1.QU contact in series with the conveyor motor output drops the motor output the moment QU fires. The conveyor stops immediately on the 10th part.

Rung 4 — Reset counter and timer when the hold is complete:

XIC ConveyorHold_Timer.DN — RES Counter1
XIC ConveyorHold_Timer.DN — RES ConveyorHold_Timer

When the TON DN bit fires at 5,000 ms, the two RES rungs execute: Counter1.ACC / CV resets to zero (clearing QU), and the timer ACC resets to zero. With QU now FALSE, rung 2's enable drops — the TON resets — and rung 3's XIO re-closes, restarting the conveyor for the next 10-part cycle.

This four-rung pattern is self-resetting and runs indefinitely without operator intervention. The scan-cycle order matters: place the RES rungs after both the CTU and TON rungs so the reset executes on the same scan that DN fires, not one scan earlier.