Block Reference

This page lists every function block you can use in your PLC programs — both in the visual FBD editor and in Structured Text code.

Logic Blocks

Use these to combine TRUE/FALSE signals.

[IMAGE: FBD editor showing AND block with two sensor inputs connected to a motor output]

Example in ST:

MotorRun := Sensor1 AND Sensor2;    // Both must be active
AlarmActive := Fault1 OR Fault2;     // Either fault triggers alarm
DoorClosed := NOT DoorOpen;          // Invert signal

Timers

Add delays to your logic. Essential for real automation — almost every program uses at least one timer.

TON — On-Delay Timer

Waits before turning on. Use when you need a delay after a condition becomes true.

Common uses: Debounce a sensor, delay a conveyor start, wait before extending a cylinder.

StartDelay(IN := StartButton, PT := T#2s);
IF StartDelay.Q THEN
    ConveyorSpeed := 500.0;  // Starts 2 seconds after button press
END_IF

TOF — Off-Delay Timer

Stays on for a while after the input turns off. Use when something should keep running briefly.

Common uses: Keep a conveyor running after the last box passes, hold a valve open.

RunOnTimer(IN := BoxDetected, PT := T#5s);
ConveyorRun := RunOnTimer.Q;  // Keeps running 5s after last box

TP — Pulse Timer

Turns on for an exact duration, then turns off. Use for timed actions.

Common uses: Extend a cylinder for a fixed time, flash a warning light, trigger a sort pulse.

PushPulse(IN := SortSignal, PT := T#500ms);
PusherExtend := PushPulse.Q;  // Pushes for exactly 500ms

[IMAGE: Trace view showing TON, TOF, and TP timing diagrams side by side]

Counters

Count events — how many boxes passed, how many cycles completed.

CTU — Count Up

Counts rising edges. Outputs TRUE when the count reaches the target.

BoxCounter(CU := SensorPulse, PV := 50, RESET := ResetButton);
IF BoxCounter.Q THEN
    // 50 boxes counted — trigger action
    BatchComplete := TRUE;
END_IF

CTD — Count Down

Starts at a value and counts down to zero.

CTUD — Up/Down Counter

Counts both up and down. Useful for tracking inventory (items in minus items out).

[IMAGE: FBD editor showing a CTU counter connected to a sensor input and a comparison block]

Edge Detection

Detect the exact moment a signal changes — not that it IS true, but that it just BECAME true.

R_TRIG — Rising Edge

Pulses TRUE for one scan when the input goes from FALSE to TRUE.

Use when: You want to count button presses (not hold), trigger an action once per detection.

ButtonEdge(CLK := StartButton);
IF ButtonEdge.Q THEN
    // Runs exactly once per button press
    ToggleMotor := NOT ToggleMotor;
END_IF

F_TRIG — Falling Edge

Pulses TRUE for one scan when the input goes from TRUE to FALSE.

Use when: You want to detect when a sensor clears (box left the zone).

Latches

Remember a state until explicitly changed. Like a light switch — stays on until you turn it off.

MotorLatch(S := StartButton, R := StopButton OR Fault);
MotorRun := MotorLatch.Q;  // Stays on until stop or fault

Math Blocks

Do calculations with numbers.

// Clamp conveyor speed between 0 and 1000 mm/s
SafeSpeed := LIMIT(0.0, RequestedSpeed, 1000.0);

Comparison Blocks

Compare two values — result is TRUE or FALSE.

Selection Blocks

Choose between values based on a condition.

SEL — Binary Select

Pick one of two values based on a TRUE/FALSE condition:

// Run at 500 if in fast mode, 200 if in slow mode
Speed := SEL(FastMode, 200.0, 500.0);

MUX — Multi Select

Pick from multiple values using an index number:

// Select speed based on gear (0, 1, 2, 3)
Speed := MUX(GearNumber, 0.0, 100.0, 300.0, 500.0);