CTU — Count Up

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Description

CTU (Count Up) adds one to its accumulator each time its rung goes from false to true. It counts the transition, not the scan: a rung held true counts once, however many scans it stays true. When the accumulator reaches the preset the DN bit sets, and the count keeps going past the preset until a RES instruction clears it. Use CTU to count events: parts past a sensor, cycles of a machine, presses of a button, pulses from a timer. Do not put an ONS in front of a CTU; the counter already detects the edge. Do not use CTU to measure time; that is a timer.

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Operands

Operand	Type	Format	Valid Range	Required	Description
Counter	COUNTER	Tag name or Array[n]	A COUNTER tag; a CTU and a CTD may share one	Yes	The structure that holds this counter’s preset, accumulator and status bits.
Preset	DINT	Whole number	−2,147,483,648 to 2,147,483,647	Yes	The count at which DN sets. Stored in the tag as .PRE; editable online.
Accum	DINT	Whole number	Read-only on the face	—	The current count, .ACC. Retained until RES.

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Scan Behavior

Prescan

CU is cleared. ACC, DN, OV and UN keep their values: a counter is retentive.

Rung-condition-in is false

CU clears. Nothing else changes; the count is held.

Rung-condition-in is true

If the rung was false on the previous scan, this is a rising edge: ACC increases by one. CU sets. DN is then set if ACC is greater than or equal to PRE, otherwise cleared. If ACC was already at the DINT maximum it wraps to the DINT minimum and OV sets. If the rung was already true last scan, nothing happens.

Postscan

Nothing special.

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Member Bits

Member	Type	Set when	Cleared when
.CU	BOOL	The rung is true	The rung is false
.CD	BOOL	A CTD on the same counter has a true rung	That CTD’s rung is false
.DN	BOOL	ACC is greater than or equal to PRE	ACC drops below PRE (a CTD, or RES)
.OV	BOOL	ACC wraps from +2,147,483,647 to −2,147,483,648	RES only
.UN	BOOL	A CTD on the same counter wraps the other way	RES only
.PRE	DINT	The preset	—
.ACC	DINT	The count; keeps counting past PRE	RES only

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Example

Scenario: A machine builds parts in batches of five. Every completed cycle counts one up. A part that fails inspection is pulled from the batch and must come back off the count, which a CTD on the same counter does (see the CTD page). A lamp lights when five good parts are done.

Tags:

• Cycle_Complete — Cycle complete, BOOL (true at the end of each machine cycle)
• Inspect_Fail — Inspection failed, BOOL (true while a rejected part is in the station)
• Counter — COUNTER, Preset 5
• BatchDone — Batch complete lamp, BOOL
• Ack — Operator acknowledge, BOOL

Rung 1 (parts made):

—|XIC Cycle_Complete|———[CTU Counter  Preset 5]———

Rung 2 (parts rejected, explained on the CTD page):

—|XIO Inspect_Fail|———[CTD Counter  Preset 5]———

Rung 3 (the lamp):

—|XIC Counter.DN|———(OTE BatchDone)———

Rung 4 (the reset):

—|XIC Ack|———(RES Counter)———

Each time Cycle_Complete goes true the counter’s rung sees a rising edge and the Accum climbs by one; holding Cycle_Complete true does not count again. CU lights while the rung is true. The screenshot below was taken after the first cycle, with the rung still held: Accum 1, CU lit, DN dark.

At five, DN lights and BatchDone turns on. The count does not stop at the preset: a sixth cycle takes the Accum to 6 with DN still lit.

Press Ack: RES clears the Accum to 0 and DN to 0, the lamp goes out, and the next batch counts up from zero.

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See Also

• CTD — Count Down (the same counter, counting the other way)
• RES — Reset (the only way to clear a counter)
• TP — Pulse Timer (a self-retriggering TP makes a handy pulse source for counting)
• ONS — One-Shot Rising (not needed in front of a CTU)
• Timer and Counter Instructions — Category index

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Notes

The count runs past the preset. Unlike a timer, a counter does not stop at PRE. DN stays on while ACC is at or above it. Reset the counter when the event it counted has been handled, or the count keeps growing: XIC CTU.DN → RES CTU on its own rung is the classic self-resetting batch counter.

Overflow does not fault. At the DINT maximum the accumulator wraps to the minimum and OV sets; counting continues and the scan is never stopped. OV stays set until RES. This is the Logix behaviour; on an SLC 500 the wrap happened at 32,767.

Negative presets. The compare is signed. A CTU with a preset below zero has DN on from the moment it is reset, since an Accum of 0 is already at or above the preset, and every count up keeps it there. That is rarely what you want from a CTU; it matters mostly when a CTU shares a counter with a CTD that is counting down to a negative target (see the CTD page).

Up and down on one counter. A CTU and a CTD may name the same COUNTER tag. They share ACC, PRE and DN; CU belongs to the CTU and CD to the CTD. Use the pair to track a quantity that both rises and falls, such as parts in a buffer.

RES clears everything but the edge memory. ACC, DN, OV and UN go to 0. The counter remembers that its rung was true, so a rung still held across the reset does not count again until it has gone false and true once more.

Counter arrays. The Counter operand may be an element of a COUNTER array, for example Counts[2].