Moving, comparing and calculating

Boxes (MOV, ADD, MUL…) run when power reaches their EN input and do nothing otherwise. They work on bytes, words (INT), double words (DINT) and REALs; the suffix of the mnemonic tells you which: MOVB, MOVW, MOVD, MOVR.

Move

MOVW +500, VW10 copies 500 into VW10. MOVW C0, VW12 copies a counter's current value; MOVW T32, VW100 a timer's. Moving a timer or counter value is how you read it into your data.

Compare

A comparison is a contact: it passes power when the comparison is true. LDW>= VW10, +300 starts a network with "VW10 ≥ 300"; AW<, OW= combine it. Byte comparisons (LDB) are unsigned, word and double-word comparisons signed, and LDR compares reals.

Start againAdd a networkDownload .awlDownload symbol table

ContactsNormally open contactNormally closed contactRising edge (P)Falling edge (N)NOTCompareCoils and boxesCoilSet (S)Reset (R)TON on-delayTOF off-delayTONR retentiveCTU count upCTD count downMOV_W move word

Drag an instruction onto the rung (or click it, then click a place). Drop below a contact to put it in parallel. Click an element to edit its address; Delete removes it.

Network 1✕

SMB28 > 200CMP B>

LEVEL_HIGHQ0.0

Network 2✕

SMB28 < 50CMP B<

LEVEL_LOWQ0.1

PLC-200 panelTiming diagramSTL viewSymbol table

SIMATIC S7-200 · CPU 224SFRUNSTOPRUNSTOP

Q0.0LEVEL_HIGH0

Q0.1LEVEL_LOW0

SMB28

0

SMB29

0

TimePause0.1×0.25×1×4×One scanCold restartt = 0.00 s · 0 scans

No errors: the program compiles.

Turn the potentiometer SMB28 on the panel: LEVEL_HIGH lights above 200, LEVEL_LOW below 50.

Integer arithmetic

In SIMATIC STL, arithmetic is two-operand: +I IN1, OUT means OUT := OUT + IN1. In LAD, the ADD_I box has IN1, IN2 and OUT; when OUT is not IN1, Micro/WIN inserts a MOV first. Watch for:

• Overflow: an INT stops at +32767. An +I that goes beyond it gives a wrong result and sets the overflow bit SM1.1. Use DINT (+D) for large values or clamp them.
• Division: /I keeps the integer quotient; DIV gives quotient and remainder in a double word. Dividing by zero sets SM1.3 and leaves OUT unchanged.
• Order: compute in a scratch word, then copy the result, so that an intermediate value never reaches an output.

ExampleCounter and integer mathsBlinker: timers and SM0.5Logic stack: ALD, LPS, LPPStart againOpen .awl fileOpen symbol table

Program (STL, or a whole Micro/WIN export)Network 1 // Count parts on I0.0; I0.1 resets LD I0.0 EU LD I0.1 CTU C0, +10 Network 2 // Copy the count, compute boxes of 4 and the remainder LD SM0.0 MOVW C0, VW10 MOVW VW10, VW12 /I +4, VW12 LDW>= C0, +10 = Q0.0Symbol table (Symbol, Address, Comment)PART I0.0 part sensor RESET_CNT I0.1 reset FULL Q0.0 10 parts

No errors: the program compiles.

PLC-200 panelTiming diagramProgram status

SIMATIC S7-200 · CPU 224SFRUNSTOPRUNSTOP

I0.0PART(ON)

I0.1RESET_CNT(ON)

Q0.0FULL0

TimePause0.1×0.25×1×4×One scanCold restartt = 0.00 s · 0 scans

Real numbers

For physics (speeds, T/2 of a pendulum) convert to REAL: ITD (INT → DINT) then DTR (DINT → REAL), compute with +R -R *R /R SQRT, and come back with ROUND (to the nearest) or TRUNC (towards zero), then DTI if it must fit a word. TP 4 computes $T/2 = \pi\sqrt{L/g}$ this way from the potentiometer SMB29.

ExplorerGo deeper: derivations and open questions

Scaling a potentiometer. SMB28 gives 0–255. To express it as 0–125 % in integer arithmetic, multiply first and divide last: MUL 125 then DIV 255 keeps the precision; dividing first would give 0 or 1. In REAL arithmetic the order does not matter, but each conversion costs scan time.

BCD and the 7-segment display

A 7-segment display decodes BCD: each decimal digit on 4 bits. IBCD VW0 turns 1234 into 16#1234, one digit per nibble, ready for the display; BCDI does the reverse for a thumbwheel. SEG makes the 7-segment pattern of one hexadecimal digit when the display is driven segment by segment. The PLC-200 display (TP 3) takes BCD data on Q0.2–Q0.5 with digit latches on Q0.6–Q1.1.