IEC Function Blocks

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IEC Function Blocks

UniLogic supports key IEC 61131-3 function blocks directly in the ST editor, including bistables, edge detection, counters, timers, and RTC blocks.

These are stateful blocks; they maintain internal data across scan cycles, unlike simple one-shot functions. All blocks must be declared as instances.

Bistable (RS, SR)

A bistable latch remembers its state; once set, it stays set until explicitly reset, even if the trigger that set it is no longer active. This makes bistable blocks ideal for:

Motor run/stop control - press Start to latch ON, press Stop to latch OFF
Alarm latching - latch an alarm TRUE when a fault occurs, clear it only when acknowledged
Mode selection - hold a machine in a state until a condition explicitly changes it

Two types are available. The difference is only in what happens if both inputs are TRUE at the same time:

Block	Dominant Input	Wins when both S and R are TRUE
RS	R (Reset)	Q becomes FALSE - Stop always wins. Use when safety requires that reset takes priority
SR	S (Set)	Q becomes TRUE - Start always wins. Use when you need the set condition to take precedence

Edge Detection

These function blocks detect signal transitions and fire a single-cycle pulse on the output Q.

R_TRIG (Rising Edge) - Pulse when CLK transitions from 0 -> 1
F_TRIG (Falling Edge) - Pulse when CLK transitions from 1 -> 0

Declaration and Usage

Q is a Read-only output member. CLK is the Hidden input member.
Both blocks maintain internal state (M) across cycles - never reset Q manually.

Counters: CTU, CTD, and CTUD

Counters track events based on signal transitions.

CTU - Count Up

Counts up from 0 to a preset value.

Property	Type	Access	Description
CU	BIT	Input (Hidden)	Count-up trigger. A rising edge increments the counter value CV by 1
R	BIT	Input (Hidden)	Resets the output Q and the counter value CV
PV	INT	Input (Hidden)	The counter limit value
Q	BIT	Read-only output	Set to TRUE when the counter value CV reaches PV
CV	INT	Read-only output	The current count value

Declaration and Usage example:

VAR
    Counter : CTU;
END_VAR

Counter(
    CU := SensorPulse,
    R := ResetButton,
    PV := 10
);

ReachedLimit := Counter.Q;
CurrentCount := Counter.CV;

CTD - Count Down

Counts down from a preset value to 0.

Property	Type	Access	Description
CD	BIT	Input (Hidden)	Count-down trigger. A rising edge decrements the counter value CV by 1
LD	BIT	Input (Hidden)	Load preset value
PV	INT	Input (Hidden)	The counter limit value
Q	BIT	Read-only output	Set to TRUE when the counter value CV reaches PV
CV	INT	Read-only output	The current count value

Declaration and Usage example:

VAR
    DownCounter : CTD;
END_VAR

DownCounter(
    CD := Pulse,
    LD := LoadButton,
    PV := 20
);

ReachedLimit := Counter.Q;
CurrentCount := Counter.CV;

CTUD - Count Up/Down

Counts upward on rising edges of CU and downward on rising edges of CD.

Property	Type	Access	Description
CU	BIT	Input (Hidden)	Count-up trigger. Increments the counter on a rising edge
CD	BIT	Input (Hidden)	Count-down trigger. Decrements the counter on a rising edge
R	BIT	Input (Hidden)	Resets the counter to 0
LD	BIT	Input (Hidden)	Load preset value
PV	INT	Input (Hidden)	Preset value
QU	BIT	Read-only output	TRUE when CV >= PV
QD	BIT	Read-only output	TRUE when CV <= 0
CV	INT	Read-only output	The current count value

Declaration and Usage example:

VAR
    UpDownCounter : CTUD;
END_VAR

UpDownCounter(
    CU := AddItem,
    CD := RemoveItem,
    R := ResetCounter,
    LD := LoadPreset,
    PV := 100
);

ReachedLimit := Counter.Q;
CurrentCount := Counter.CV;

CU and CD are edge-sensitive: holding them TRUE does not continuously increment or decrement the counter.
R and LD are level-sensitive and do not require a rising edge.
The IEC 61131-3 standard defines PV and CV as INT values, but PV may also be provided as a constant.

Timers: TON, TOF, and TP

UniLogic supports three IEC-standard timer function blocks.

Function Block	Name	Description
TON	Timer ON-Delay	Delays turning an output ON
TOF	Timer OFF-Delay	Delays turning an output OFF
TP	Timer Pulse	Generates a fixed-duration output pulse

Legacy timer implementations (from previous UniLogic versions) have been renamed with the _UNI_suffix: TON_UNI_, TOF_UNI_, TP_UNI_, TA_UNI_, TE_UNI_, and TIMER_RESET.
Existing programs using these names continue to work without modification.

TIME Data Type

The PT and ET properties use the TIME data type. For full details on TIME literals, supported units, syntax rules, and valid ranges, see Time Literals.

TIME literal examples:

T#5s (* 5 seconds *)

T#2m30s (* 2 minutes and 30 seconds *)

T#1h15m20s (* 1 hour, 15 minutes, 20 seconds *)

TIME#500ms (* 500 milliseconds, alternative prefix *)

IEC Function Block Structure

All three timer types share a common structure:

Property	Type	Access	Description
IN	BIT	Input (Hidden)	Control input. Rising edge or level triggers the timer depending on type
PT	TIME	Input (Hidden)	Preset time. The target duration
Q	BIT	Read-only output	Timer output. Meaning depends on timer type
ET	TIME	Read-only output	Elapsed time. Counts up from 0 to PT while the timer is active

Declaring IEC Timers

VAR

T1 : TON;

T2 : TOF;

T3 : TP;

END_VAR

TON - On-Delay Timer

Use TON to delay turning an output ON. The output Q becomes TRUE only after the input IN has been continuously TRUE for the preset duration PT.

Behavior:

When IN becomes TRUE:

The timer starts counting (ET increments from 0)
When ET >= PT, Q becomes TRUE

When IN becomes FALSE before ET reaches PT:

The timer resets: ET = 0
Q remain FALSE

Usage example:

T1(IN := StartSignal, PT := T#5s);

Motor := T1.Q; (* Motor starts 5 s after StartSignal rises *)

TOF - Off-Delay Timer

Use TOF to delay turning an output OFF. The output Q becomes TRUE immediately when IN rises, then becomes FALSE after IN has been FALSE for the preset duration PT.

Behavior:

When IN = TRUE:

Q = TRUE immediately
ET remains 0

When IN becomes FALSE:

The timer starts counting (ET increments from 0)
Q remains TRUE while the timer runs
After PT has elapsed, Q becomes FALSE

If IN rises again while the timer is running:

The timer stops and resets (ET = 0)
Q remains TRUE

Usage example:

T2(IN := RunSignal, PT := T#3s);

Valve := T2.Q; (* Valve stays open 3 s after RunSignal falls *)

TP - Pulse Timer

Use TP to generate a fixed-duration output pulse. On a rising edge of IN, Q becomes TRUE and stays TRUE for exactly PT, regardless of the subsequent state of IN.

Behavior:

On a rising edge of IN:

Q becomes TRUE immediately
ET starts counting from 0

While the timer is running (ET < PT):

Q remains TRUE regardless of IN state
A new rising edge on IN has no effect

When ET reaches PT:

Q becomes FALSE
ET resets to 0

Usage example:

T3(IN := Trigger, PT := T#500ms);

Buzzer := T3.Q; (* Buzzer sounds for exactly 500 ms per trigger *)

RTC (Real-Time Clock)

The RTC function block maintains a continuously increasing DATE_AND_TIME value starting from a preset timestamp.

Unlike elapsed-time calculations, the CDT value represents an absolute timestamp that advances while the block is enabled.

RTC Structure

Property	Type	Access	Description
EN	BIT	Input (Hidden)	Enables the RTC block
PDT	DATE_AND_TIME	Input (Hidden)	Preset date and time
Q	BIT	Read-only output	TRUE while the RTC is counting
CDT	DATE_AND_TIME	Read-only output	Current date and time value

Declaration and Usage Example:

VAR
MyRTC : RTC;
InitTime : DATE_AND_TIME := DT#2025-01-01-00:00:00;
END_VAR

MyRTC(
EN := TRUE,
PDT := InitTime
);

CurrentTime := MyRTC.CDT;
Running := MyRTC.Q;