Statement list (STL) and the .awl export

STL (statement list; IEC name: instruction list) is the text form of an S7-200 program. Every LAD network has an STL equivalent, and File → Export writes a whole program as STL in an .awl file. Reading STL tells you exactly what the CPU executes.

Lines, networks, comments

Network 1 // Start/stop station
// RUN is set by START, held by its own contact, released by STOP_NC
LD     START      // START pressed
O      RUN
A      STOP_NC
=      RUN

• A program is a list of networks; Network n // title starts one.
• // starts a comment: on its own line after the network line (network comment) or after an instruction.
• One instruction per line: a mnemonic (LD, A, =, TON…) then its operands separated by commas.
• Operands are addresses (I0.0, VW100, T37), constants (+50, 16#DB, 2#1010, 9.81) or symbols from the symbol table (START). The export may also quote symbols ("START").

The logic stack

Bit instructions work on a stack of bits. The top of the stack (TOS) is the current "power flow".

Instruction	Effect on the stack	LAD equivalent
LD bit / LDN bit	push bit / NOT bit	first contact of a rung or branch
A bit / AN bit	TOS := TOS AND bit / AND NOT bit	contact in series
O bit / ON bit	TOS := TOS OR bit / OR NOT bit	contact in parallel
ALD	pop two, push their AND	two blocks in series
OLD	pop two, push their OR	two blocks in parallel
LPS / LRD / LPP	push a copy / read the copy / pop it	a branch point feeding several outputs
NOT, EU, ED	invert TOS / rising / falling edge of TOS	NOT, P, N contacts
= bit, S bit, n, R bit, n	write TOS / set / reset n bits if TOS	coils

Boxes (TON, MOVW, +I, CTU…) execute when TOS is 1. A counter takes two or three stack levels (CU, then R; or CU, CD, R). The stack is 9 levels deep on the S7-200.

ExampleLogic stack: ALD, LPS, LPPStart/stop stationBlinker: timers and SM0.5Counter and integer mathsSCR stepsStart againOpen .awl fileOpen symbol table

Program (STL, or a whole Micro/WIN export)Network 1 // (A OR B) AND (C OR D): two blocks joined by ALD LD I0.0 O I0.1 LD I0.2 O I0.3 ALD = Q0.0 Network 2 // One condition, two outputs: LPS saves the stack, LPP restores it LD I0.4 LPS A I0.5 = Q0.1 LPP AN I0.5 = Q0.2Symbol table (Symbol, Address, Comment)

No errors: the program compiles.

PLC-200 panelTiming diagramProgram status

SIMATIC S7-200 · CPU 224SFRUNSTOPRUNSTOP

I0.0 (ON)

I0.1 (ON)

I0.2 (ON)

I0.3 (ON)

I0.4 (ON)

I0.5 (ON)

Q0.0 0

Q0.1 0

Q0.2 0

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

Open Program status: the first column is the TOS after each instruction, live. Change the inputs on the panel and watch ALD and LPS/LPP do their work.

Predict first

How do you write (I0.0 OR I0.1) AND (I0.2 OR I0.3) → Q0.0 in STL?

LD I0.0 / O I0.1 / A I0.2 / O I0.3 / = Q0.0LD I0.0 / O I0.1 / LD I0.2 / O I0.3 / ALD / = Q0.0LD I0.0 / A I0.2 / O I0.1 / A I0.3 / = Q0.0

The layout of an .awl export

ORGANIZATION_BLOCK MAIN:OB1
TITLE=Program comment
BEGIN
Network 1 // Network title
// Network comment
LD     START
…
END_ORGANIZATION_BLOCK
SUBROUTINE_BLOCK SBR_0:SBR0
TITLE=Subroutine comment
VAR_INPUT
Speed:INT;   // local variables of the subroutine
END_VAR
BEGIN
Network 1
…
END_SUBROUTINE_BLOCK

The main program is ORGANIZATION_BLOCK MAIN:OB1; subroutines and interrupt routines follow. The symbol table is not in the .awl: copy it from Micro/WIN (select the table, Ctrl+C) into the symbol box of the runner, or save it as a text file.

SIMATIC and International mnemonics

Micro/WIN can show STL with two mnemonic sets (Tools → Options → General). The booklet, the manual and this site use the SIMATIC set (LD, A, O, =). If an export looks unfamiliar, check that option before anything else.