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Binary Circuits in Ladder Diagram

Sample selection of POUs in the ladder diagram

Example 13. Example: Push button switches a lamp

LD block: FB_AssignToLamp.fb.ld

_c4ld_img_tutorial_assign_to_lamp

Equivalent in ST:

FUNCTION_BLOCK FB_AssignToLamp
VAR_INPUT    
    bPushButton : BOOL := FALSE;
END_VAR
VAR_OUTPUT
    bLamp : BOOL;
END_VAR
VAR
END_VAR
bLamp := bPushButton;


Example 14. Sample counter block: FB_StorageCounter

The counter block FB_StorageCounter encapsulates a CTUD counter (preselection: PV= 100).

_c4ld_img_tutorial_storage_counter.png


Example 15. Sample start/stop block: FB_Latch

xMotor := (xStart OR xMotor) AND NOT xStop

The second branch line of the ParallelClosed branch reads back its own output xMotor (self-holding). xStop is in series as an opener.

  • xStart briefly TRUE→ xMotor stays on (holds itself)

  • xStop TRUE → xMotor shuts down.

_c4ld_img_tutorial_latch.png


Example 16. Example of an operator

  • Network 1: iSum := iValue1 + iValue2 about a ADD-Operator block.

  • Network 2: xGreater := (iValue1 > iValue2) about a GT-Operator Block.

Both operator blocks have the EN enable input and the ENO enable output. Inputs are iValue1 and iValue2. Outputs are iSum and xGreater

PROGRAM PRG_CompareArithmetic
VAR
    iValue1  : INT := 10; // first operand
    iValue2 : INT := 5;  // second operand
    iSum     : INT;       // result iValue1 + iValue2
    xGreater : BOOL;      // TRUE if iValue1 > iValue2
END_VAR
_c4ld_img_tutorial_compare_arithmetic.png


Example 17. Example: Set/Reset coils, SR latch PRG_SetReset

  • Network 1: xSet → Set coil (S) on xLatch (Modifier.Storage = 1).

  • Network 2: xReset → Reset coil (R) to xLatch (Modifier.Storage = 2).

xLatch remains after xSet saves TRUE and is reset by xReset.

FUNCTION_BLOCK FB_Latch
VAR_INPUT
    xStart : BOOL; // start button (NO)    
    xStop  : BOOL; // stop button (logic: TRUE = pressed)
END_VAR
VAR_OUTPUT
    xMotor : BOOL; // latched motor output
END_VAR
VAR
END_VAR
_c4ld_img_tutorial_prg_setreset.png


Example 18. Example of main program: PLC_PRG

  • Network 1:

    Instantiates Storage1 : FB_StorageCounter and wires the signal inputs (xLightBarrierIn, xLightBarrierOut, xAck, xPreset) to the outputs for signal lamps (xLampFull, xLampEmpty) and for the display (iDisplay).

  • Network 2:

    xLamp50 := (iDisplay >= 50) via a GE comparator.

PROGRAM PLC_PRG
VAR
    Storage1   : FB_StorageCounter; // counter instance
    xLightBarrierIn  : BOOL; // infeed light barrier  -> xInfeed
    xLightBarrierOut : BOOL; // outfeed light barrier -> xOutfeed
    xAck       : BOOL; // acknowledge / reset   -> xReset
    xPreset    : BOOL; // preset to 100         -> xLoad
    xLampFull  : BOOL; // "storage full" lamp
    xLampEmpty : BOOL; // "storage empty" lamp
    xLamp50    : BOOL; // ">= 50% full" lamp
    iDisplay   : INT;  // stock display
END_VAR
_c4ld_img_tutorial_plc_prg.png


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