PLC-Course

Sprint 4 Cheat Sheet — IL, ST, SFC

📜 The Five IEC 61131-3 Languages

Language	Style	Best for	Siemens name
LD (Ladder Diagram)	Graphical (rungs)	Discrete interlocks, safety	KOP
FBD (Function Block Diagram)	Graphical (blocks)	Process control, reusable blocks	FUP
IL (Instruction List)	Textual (assembly-like)	Legacy, low-level — being deprecated	AWL
ST (Structured Text)	Textual (Pascal-like)	Math, loops, complex conditions	SCL
SFC (Sequential Function Chart)	Graphical (steps)	Sequences, batch, recipes	GRAPH

📝 Instruction List (IL) — At a Glance

LD   START       (* Load START   *)
ANDN STOP        (* AND with NOT STOP *)
ST   MOTOR       (* Store to MOTOR *)

Common operators: LD, ST, AND, OR, ANDN, ORN, NOT, S, R, JMP, CAL.

⚠️ IEC 61131-3 Edition 3 (2013) deprecates IL. Don’t write new code in it. Read it for legacy maintenance.

🧮 Structured Text (ST) — Pascal for PLCs

(* Variables *)
VAR
    Start    : BOOL;
    Stop     : BOOL;
    Motor    : BOOL;
    Speed    : REAL;
END_VAR

(* Logic *)
IF Start AND NOT Stop THEN
    Motor := TRUE;
END_IF;

IF Stop THEN
    Motor := FALSE;
END_IF;

(* Math + scaling: 4–20 mA → 0–100% *)
Speed := (Current_mA - 4.0) / 16.0 * 100.0;

Operators:

• Boolean: AND, OR, XOR, NOT
• Comparison: =, <>, <, >, <=, >=
• Assignment: :=
• Math: +, -, *, /, MOD, ** (power)

Control flow: IF…THEN…ELSIF…ELSE…END_IF, CASE…OF, FOR…TO…DO, WHILE…DO, REPEAT…UNTIL, EXIT, RETURN.

Data types: BOOL, INT, DINT, REAL, LREAL, TIME, STRING, ARRAY, STRUCT.

🧭 Sequential Function Chart (SFC) — Visual State Machines

An SFC is steps (boxes) connected by transitions (horizontal bars with conditions). Exactly one step is active at a time per chain.

    ┌──────────┐
    │ Step 1   │  ← Initial step (double border)
    │ Idle     │
    └────┬─────┘
         ─── (Start_Button)         ← Transition condition
    ┌────▼─────┐
    │ Step 2   │
    │ Filling  │
    │ N: Pump  │   ← Action (N = Non-stored)
    └────┬─────┘
         ─── (Level >= 80)
    ┌────▼─────┐
    │ Step 3   │
    │ Mixing   │
    │ N: Mixer │
    └────┬─────┘
         ─── (T#30s elapsed)
         ▼
    ... and so on

Action qualifiers:

• N — Non-stored (active while step is active)
• S — Set (latches on, must be reset)
• R — Reset
• L — Limited duration
• D — Delayed
• P — Pulse (one scan)

🌳 Language Decision Tree

flowchart TD
    Q1{Step-by-step<br/>sequence?}
    Q1 -->|Yes| SFC[SFC]
    Q1 -->|No| Q2{Heavy math,<br/>loops, strings?}
    Q2 -->|Yes| ST[ST]
    Q2 -->|No| Q3{Reusable<br/>signal-flow block?}
    Q3 -->|Yes| FBD[FBD]
    Q3 -->|No| Q4{Discrete<br/>interlocks?}
    Q4 -->|Yes| LD[LD]
    Q4 -->|No| Q5{Legacy code?}
    Q5 -->|Yes| IL[IL — deprecate]
    Q5 -->|No| Reconsider[Re-scope]

🔁 Mixing Languages — The Right Move

Real programs use multiple languages in different POUs (Program Organization Units):

• Main program in SFC for overall flow
• Per-step actions in LD or FBD for interlocks
• Math-heavy function blocks in ST
• Safety logic in LD (auditable)

This is normal. IEC 61131-3 was designed for it.

🏷️ Siemens Translation Crib

IEC 61131-3	Siemens TIA Portal
LD	KOP (Kontaktplan)
FBD	FUP (Funktionsplan)
IL	AWL (Anweisungsliste) — only S7-300/400 classic
ST	SCL (Structured Control Language)
SFC	GRAPH

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