Can someone help me to understand the [attached] STL code?

Hi,

Why not ask the AI? It should be an easy task for it. I asked Gemini 2.5 Flash, and it responded:

Let's break down this STL code section by section. STL can be a bit dense, but we'll go through it logically.

General Observations before Diving In:

• S7-300/400 PLC: This syntax is typical for older Siemens S7-300 or S7-400 PLCs.
• Data Blocks (DBs): The code heavily uses Data Blocks (DB 5, DB 6, DB 4) to store and retrieve data. This is good practice for organizing variables.
• Memory Areas: It uses various memory areas:
  • DBW: Data Block Word (16 bits)
  • DBB: Data Block Byte (8 bits)
  • MW: Memory Word (16 bits)
  • M: Memory Bit (Boolean)
  • L: Load (into Accumulator 1)
  • T: Transfer (from Accumulator 1)
  • A: AND logic
  • O: OR logic
  • AN: AND NOT logic
  • ON: OR NOT logic
  • S: Set bit
  • R: Reset bit
  • JC: Jump if condition is true (usually previous RLO is 1)
  • JU: Unconditional Jump
  • SLW: Shift Left Word
  • LAR1: Load Address Register 1
  • STW: Status Word
  • TAK: Transfer Accumulator 1 to Accumulator 2
  • XOW: Exclusive OR Word
  • SE: Start Edge (Timer)
  • ==I: Compare Integer Equal
• Comments: The comments are in German, which is common for Siemens PLCs, especially in Europe. I'll translate them as we go.
• Indirect Addressing: There's a lot of indirect addressing using AR1 (Address Register 1) and DBW [AR1,P#0.0]. This means the code is dynamically calculating memory addresses, often to access elements in an array or a list within a DB based on an index.

Section 1: Loading Status Word 1 (ZUSTANDSWORT 1)

 OPN   "DB 5"                      // ZUSTANDSWORT 1 (Status Word 1)
     L     #_NO                        // MW227 := DB5:DW[NR]
     T     DBW    0

     T     #conv_akku1
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T     STW
     L     #conv_akku1
     L     DBW [AR1,P#0.0]
     T     "MW 227"

Explanation:

1. OPN "DB 5": Opens Data Block 5. All subsequent DB operations will refer to DB 5 until another OPN instruction.
2. L #_NO: Loads the value of a temporary variable _NO into Accumulator 1.
3. T DBW 0: Transfers the value from Accumulator 1 to DBW 0 (the first word in DB 5). This DBW 0 likely holds an index or "number" (NR) that will be used for indirect addressing.
4. The next block (T #conv_akku1 to T "MW 227") is a standard pattern for indirect addressing in STL. It's loading a word from DB 5 at an offset determined by DBW 0 into MW 227.
   • T #conv_akku1: Saves the current content of Accumulator 1 (the _NO value) to a temporary variable.
   • L STW, T #conv_stw: Saves the current Status Word. This is crucial because indirect addressing instructions modify the Status Word, and you often need to restore it.
   • L DBB 1, SLW 4: Loads the byte at DBB 1 (which is the higher byte of DBW 0) and shifts it left by 4 bits. This is a common way to prepare an address for LAR1.
   • LAR1: Loads the content of Accumulator 1 (the calculated address) into Address Register 1.
   • L #conv_stw, T STW: Restores the Status Word.
   • L #conv_akku1: Restores the original content of Accumulator 1 (the _NO value).
   • L DBW [AR1,P#0.0]: This is the core of the indirect addressing. It loads the Word from DB 5 whose address is calculated by AR1 plus an offset of P#0.0 (zero offset). So, it's effectively loading DBW[NR] where NR is the value in DBW 0.
   • T "MW 227": Transfers this loaded word into Memory Word 227.
   • In short: MW 227 is being assigned a value from DB 5 at an index determined by _NO.

Section 2: Loading Status Word 2 (ZUSTANDSWORT 2)

L     DBW    0                    // =NR
     OPN   "DB 6"                      // ZUSTANDSWORT 2
     T     DBW    0
// MW229 := DW6:DW[NR]
     T     #conv_akku1
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T     STW
     L     #conv_akku1
     L     DBW [AR1,P#0.0]
     T     "MW 229"

Explanation:

This section is almost identical to the first, but it operates on DB 6 and loads the value into MW 229.

1. L DBW 0: Loads the value from DBW 0 (from the currently open DB, which is still DB 5 at this point, but the comment // =NR suggests it's the index NR).
2. OPN "DB 6": Opens Data Block 6.
3. T DBW 0: Transfers the NR value into DBW 0 of DB 6.
4. The indirect addressing block then loads DBW[NR] from DB 6 into MW 229.
   • In short: MW 229 is assigned a value from DB 6 at the same index NR.

Section 3: Storing Formal Parameters (FORMALPARAMETER SPEICHERN)

A     #V24A                       // FORMALPARAMETER SPEICHERN
     =     "M 211.0"                   // -EESB
     A     #REM
     =     "m 211.2"                   // -EEVO
     A     #HAON
     =     "m 211.5"                   // -EEVT
     A     #FEBA
     =     "M 211.6"                   // -EERM

Explanation:

This section takes input parameters (likely from a function call or a global area) and transfers their boolean states to specific memory bits (M 211.x). These bits probably represent internal flags or states derived from the formal parameters.

• A #V24A: Loads the boolean value of V24A into the RLO (Result of Logic Operation).
• = "M 211.0": Assigns the RLO to M 211.0.
• This pattern repeats for #REM (Remote?), #HAON (Hand/Auto ON?), and #FEBA (Feedback?). The comments indicate what these bits might represent (e.g., -EESB, -EEVO, -EEVT, -EERM are likely abbreviations for specific functions or states).

Section 4: Impulse Marker and Nahtstellenwort (Interface Word)

A     "m 211.4"                   // EST
     =     "M 230.4"                   // IMPULSEMERKER EST
     A     "M 227.3"
     A     "M 228.7"
     R     "M 228.7"                   // NAHTSTELLENWORT AKTUALISIEREN

Explanation:

1. A "m 211.4": Loads the state of M 211.4 (likely an "EST" status).
2. = "M 230.4": Assigns this state to M 230.4, which is commented as an "Impulse Marker EST". This suggests M 230.4 might be used for a one-shot or to detect changes in M 211.4.
3. A "M 227.3": ANDs the RLO with M 227.3.
4. A "M 228.7": ANDs the RLO with M 228.7.
5. R "M 228.7": Resets M 228.7. This sequence (A M227.3, A M228.7, R M228.7) is a common way to implement a reset condition. M 228.7 is reset only if both M 227.3 and M 228.7 were true. This is often used to "acknowledge" or clear a status bit. The comment "NAHTSTELLENWORT AKTUALISIEREN" means "Update Interface Word".

Section 5: Loading and Storing into DB 4

L     DBW    0
     OPN   "DB 4"
     T     DBW    0
// NW =? DB4:DW[=NR]
     T     #conv_akku1
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T     STW
     L     #conv_akku1
     L     DBW [AR1,P#0.0]             // DB4:DW[=NR] := NW
     L     "MW 211"

Explanation:

Similar to the previous DB operations, but now with DB 4.

1. L DBW 0: Loads the NR value (from DB 6, as it was the last OPN DB).
2. OPN "DB 4": Opens Data Block 4.
3. T DBW 0: Transfers NR to DBW 0 of DB 4.
4. The indirect addressing block loads a word from DB 4 at index NR.
5. L "MW 211": Loads the content of MW 211 into Accumulator 1. The comment // DB4:DW[=NR] := NW is a bit confusing here, as the loaded value from DB4 is not immediately used, and then MW 211 is loaded. It seems like the line L DBW [AR1,P#0.0] might be an error or part of a larger operation, and MW 211 is the actual value being prepared for the next step.

Section 6: Detecting Changes (AENDERUNG FESTSTELLEN)

// AENDERUNG FESTSTELLEN
     T     #conv_akku1
     TAK   
     T     #conv_akku2
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T     STW
     L     #conv_akku2
     L     #conv_akku1
     T     DBW [AR1,P#0.0]             // LAMPEN PRUEFEN AUSBLENDEN
     XOW   
     L     W#16#7FFF                   // KEINE AENDERUNG AN EINGAENGEN
     L     W#16#0
     ==I                               // -EUS  (ERM-UEBERWACHUNG)

Explanation:

This section appears to be comparing two values to detect a change, possibly for "Lamp Test" or input changes.

1. T #conv_akku1, TAK, T #conv_akku2: This sequence stores the current value of Accumulator 1 (which holds MW 211 from the previous step) into conv_akku1 and then into conv_akku2. This is a way to duplicate the value.
2. The indirect addressing block then loads a value from DB 4 (at index NR) into Accumulator 1 and then transfers conv_akku1 (which holds MW 211) into DBW [AR1,P#0.0]. This looks like it's writing MW 211 to DB4:DW[NR].
3. XOW: Exclusive OR Word. This is a key instruction for change detection. It performs a bitwise XOR operation between the content of Accumulator 1 and Accumulator 2. If the two words are identical, the result will be 0. If there are any differences (changes), the corresponding bits will be 1.
   • So, it's comparing the current MW 211 (which was in conv_akku1) with the value previously read from DB4:DW[NR] (which was in conv_akku2 after TAK). The result of the XOR is in Accumulator 1.
4. L W#16#7FFF: Loads the hexadecimal value 7FFF (binary 0111 1111 1111 1111) into Accumulator 1. This is a bitmask.
5. L W#16#0: Loads 0 into Accumulator 1.
6. ==I: Compares the two integers in Accumulator 1 and Accumulator 2 for equality. This instruction is usually preceded by a L instruction to load the second operand. The previous L W#16#0 loads 0 into Accu1, and the previous L W#16#7FFF loaded 7FFF into Accu1 (and then shifted to Accu2 by L again). This comparison seems to be checking if the result of the XOW (which is in Accu1) is equal to 0. If XOW resulted in 0 (meaning no change), then 0 == 0 would be true.
   • The comment "KEINE AENDERUNG AN EINGAENGEN" (No change at inputs) confirms this is for change detection. If the XOW result is 0, it means no change.

Section 7: Error Monitoring and Jump Conditions

This is where the logic gets more complex, with multiple conditional jumps (JC) based on various flags.

A(                                // -EERM (BEI EKS = 1
     A     "M 227.0"
     A     "M 211.6"                   //   KEINE DYN ERM-UEBERWACHUNG
     O     
     ON    "M 230.1"                   // SPRUNG AN ENDE
     )                                 // FEHLER ?
     JC    M001                        // FEHLERBEARBEITUNG

Explanation:

This block checks for an error condition and jumps to M001 (error handling) if true.

1. A( ... ): This indicates a nested RLO operation.
2. A "M 227.0": ANDs with M 227.0.
3. A "M 211.6": ANDs with M 211.6 (which was set by #FEBA). The comment "KEINE DYN ERM-UEBERWACHUNG" means "No dynamic ERM monitoring".
4. O: ORs the current RLO with the result of the previous ANDs.
5. ON "M 230.1": OR NOTs with M 230.1. The comment "SPRUNG AN ENDE" (Jump to end) suggests M 230.1 might be a flag to skip error handling.
6. JC M001: If the final RLO of this complex A(...) block is true, jump to M001 (Error Processing).

A     "M 227.3"                   // EKS
     JC    M002                        // EBFE
     A     "M 212.2"                   //pulse off
     A     #REM
     R     "M 230.1"                   //Reset on

Explanation:

1. A "M 227.3": Checks the state of M 227.3 (likely an "EKS" status).
2. JC M002: If M 227.3 is true, jump to M002.
3. A "M 212.2": ANDs with M 212.2 ("pulse off").
4. A #REM: ANDs with #REM.
5. R "M 230.1": If both M 212.2 and #REM are true, reset M 230.1 ("Reset on").

AN    "M 230.1"                   // EKV
     AN    "M 212.4"                   //    VO-BETRIEB
     O     "M 230.2"                   // ESB **** EKS-BEARBEITUNG
     JC    M003                        // EVO

Explanation:

Another conditional jump based on several flags.

1. AN "M 230.1": AND NOT with M 230.1.
2. AN "M 212.4": AND NOT with M 212.4 ("VO-BETRIEB" - "VO-Operation").
3. O "M 230.2": ORs with M 230.2 ("ESB"). The comment "EKS-BEARBEITUNG" means "EKS processing".
4. JC M003: If the RLO is true, jump to M003.

ON    "M 211.0"                   // ESF
     ON    "m 211.2"                   // EST
     ON    "M 212.6"                   // EKS
     S     "M 227.3"                   // KEIN RUECKSETZEN,FELERBEARB.
     S     "M 230.1"                   // EBVG STAFFEL GESCHALTET ?
     JC    M002                        // EKS

Explanation:

This block sets M 227.3 and M 230.1 based on several "OR NOT" conditions.

1. ON "M 211.0": OR NOT with M 211.0 ("ESF").
2. ON "m 211.2": OR NOT with M 211.2 ("EST").
3. ON "M 212.6": OR NOT with M 212.6 ("EKS").
4. S "M 227.3": If the RLO is true, set M 227.3. The comment "KEIN RUECKSETZEN,FELERBEARB." means "No reset, error processing".
5. S "M 230.1": If the RLO is true, set M 230.1. The comment "EBVG STAFFEL GESCHALTET ?" means "EBVG stage switched?".
6. JC M002: If the RLO is true, jump to M002.

AN    "M 212.1"                   // WEITER BEI VO-BETRIEB
     AN    "M 230.1"                   // EBVG
     JC    M003                        // EKS
     A     "M 212.1"                   // EVS
     A     "M 230.1"                   //    KEINE AENDERUNG
     =     "M 230.5"                   // EBVG
     JC    M004                        // EKS
     A     "M 212.1"                   // EVS
     =     "M 230.1"                   // EBVG
     =     "M 230.5"                   // EUS
     AN    "M 212.1"                   // EUMS ***STAFFEL-FAHREN AUF DAUER
     R     "M 227.0"                   //    ANFAHREN UEBER ZEIT

Explanation:

More conditional logic and jumps.

1. AN "M 212.1": AND NOT with M 212.1 ("WEITER BEI VO-BETRIEB" - "Continue with VO-Operation").
2. AN "M 230.1": AND NOT with M 230.1 ("EBVG").
3. JC M003: If RLO is true, jump to M003.
4. A "M 212.1": Loads M 212.1 ("EVS").
5. A "M 230.1": ANDs with M 230.1 ("KEINE AENDERUNG" - "No change").
6. = "M 230.5": Assigns the RLO to M 230.5 ("EBVG").
7. JC M004: If RLO is true, jump to M004.
8. A "M 212.1": Loads M 212.1 ("EVS").
9. = "M 230.1": Assigns RLO to M 230.1 ("EBVG").
10. = "M 230.5": Assigns RLO to M 230.5 ("EUS"). This looks like M 230.1 and M 230.5 are being set to the same value as M 212.1.
11. AN "M 212.1": AND NOT with M 212.1 ("EUMS ***STAFFEL-FAHREN AUF DAUER" - "Continuous staging drive").
12. R "M 227.0": If RLO is true, reset M 227.0 ("ANFAHREN UEBER ZEIT" - "Start over time").

Section 8: Timer Logic and Error Handling (M004, M005)

M004: A     "M 211.1"                   // EBVG
     JC    M005                        // IMP1
     A     "M 212.1"                   // EUS
     A     "M     201.0"               // -EUS
     S     "M 227.0"                   // -IMP2
     A     "M 227.0"                   // -EERM
     A     "M     201.7"               // -EST
     AN    "M 211.6"                   // -EST
     AN    "M 227.3"
     S     "M 227.3"                   // WEITER BEI STOERUNGSBEARB.
     A     "M 227.3"                   // WEITER BEI VO-BETRIEB
     JC    M002                        // EUS ***STAFFELFAHREN
     JU    M003

Explanation at M004:

1. M004:: Label for a jump target.
2. A "M 211.1": Checks M 211.1 ("EBVG").
3. JC M005: If true, jump to M005.
4. A "M 212.1": Loads M 212.1 ("EUS").
5. A "M 201.0": ANDs with M 201.0 ("-EUS").
6. S "M 227.0": If RLO is true, set M 227.0 ("-IMP2").
7. A "M 227.0": Loads M 227.0 ("-EERM").
8. A "M 201.7": ANDs with M 201.7 ("-EST").
9. AN "M 211.6": AND NOT with M 211.6 ("-EST").
10. AN "M 227.3": AND NOT with M 227.3.
11. S "M 227.3": If RLO is true, set M 227.3 ("WEITER BEI STOERUNGSBEARB." - "Continue with fault processing").
12. A "M 227.3": Checks M 227.3 ("WEITER BEI VO-BETRIEB" - "Continue with VO-operation").
13. JC M002: If true, jump to M002 ("EUS ***STAFFELFAHREN" - "EUS staged drive").
14. JU M003: Unconditionally jump to M003.

M005: A     "M 227.0"
     L     S5T#10MS                    // EKS
     SE    #_TIME
     A     "M 230.1"
     L     #KT1                        // EUS
     SE    #_TIME                      // EUS
     =     "M 227.0"                   // ERM
     A     "M 227.0"
     A     "M 211.6"                   //    KEINE AENDERUNG
     O     #_TIME                      // ERM
     JC    M003                        // EUS
     AN    "M 211.6"                   // EST
     A     "M 227.0"
     S     "M 227.3"                   // EQIT ***STOERUNGSBEARBEITUNG
     R     #_TIME                      // EQS

Explanation at M005:

This section involves timer logic (SE) and further error/status handling.

1. M005:: Label.
2. A "M 227.0": Loads M 227.0.
3. L S5T#10MS: Loads a time value of 10 milliseconds.
4. SE #_TIME: Starts a pulse timer (S5T) named _TIME. If M 227.0 is true, the timer starts for 10ms.
5. A "M 230.1": Loads M 230.1.
6. L #KT1: Loads the value of #KT1 (another time constant).
7. SE #_TIME: Starts the same timer _TIME again, but with #KT1 as the duration, if M 230.1 is true. This looks like a conditional re-triggering or setting of the timer.
8. = "M 227.0": Assigns the RLO (from the previous SE or the A M230.1 if the SE didn't activate) to M 227.0. This is a bit ambiguous without knowing the exact behavior of SE and = in this context. It might be setting M 227.0 based on the timer's output or the condition that triggered it.
9. A "M 227.0": Loads M 227.0.
10. A "M 211.6": ANDs with M 211.6 ("KEINE AENDERUNG" - "No change").
11. O #_TIME: ORs with the output of the timer _TIME. This means if _TIME is running or has finished, this condition becomes true.
12. JC M003: If RLO is true, jump to M003.
13. AN "M 211.6": AND NOT with M 211.6 ("EST").
14. A "M 227.0": ANDs with M 227.0.
15. S "M 227.3": If RLO is true, set M 227.3 ("EQIT ***STOERUNGSBEARBEITUNG" - "EQIT ***Fault Processing").
16. R #_TIME: Resets the timer _TIME.

Section 9: Error Handling (M002)

M002: A     "M     199.0"               // EKS
     S     "M 230.3"                   // EUS
     R     "M 230.1"                   // EQS
     R     "M 227.0"                   // ESB
     A     "M 230.3"                   // EVO
     A     "M 211.0"                   // ESF
     A     "m 211.2"                   // EST
     A     "M 212.6"                   // FM EST
     R     "M 227.3"                   // EQS
     R     "M 227.2"                   // EST
     R     "M 230.3"                   // FM EST
     A     "M 227.3"                   // WISCHMERKER EST
     AN    "M 227.2"                   // MAE-A
     S     "M 228.7"                   // FM EST
     S     "M     199.5"               // EKV ***VO-BETRIEB MOEGLICH
     S     "M 227.2"                   // EVT

Explanation at M002:

This section is a fault handling routine, resetting various flags and setting others.

1. M002:: Label.
2. A "M 199.0": Loads M 199.0 ("EKS").
3. S "M 230.3": If true, set M 230.3 ("EUS").
4. R "M 230.1": Reset M 230.1 ("EQS").
5. R "M 227.0": Reset M 227.0 ("ESB").
6. A "M 230.3": Loads M 230.3 ("EVO").
7. A "M 211.0": ANDs with M 211.0 ("ESF").
8. A "m 211.2": ANDs with M 211.2 ("EST").
9. A "M 212.6": ANDs with M 212.6 ("FM EST").
10. R "M 227.3": If RLO is true, reset M 227.3 ("EQS").
11. R "M 227.2": Reset M 227.2 ("EST").
12. R "M 230.3": Reset M 230.3 ("FM EST").
13. A "M 227.3": Loads M 227.3 ("WISCHMERKER EST" - "Wipe Marker EST").
14. AN "M 227.2": AND NOT with M 227.2 ("MAE-A").
15. S "M 228.7": If RLO is true, set M 228.7 ("FM EST").
16. S "M 199.5": If RLO is true, set M 199.5 ("EKV ***VO-BETRIEB MOEGLICH" - "EKV ***VO-Operation possible").
17. S "M 227.2": If RLO is true, set M 227.2 ("EVT").

Section 10: VO-Operation Logic (M003)

M003: AN    "M 230.2"                   // EKV
     AN    "m 211.5"                   // EKV
     O     "M 230.1"                   //    NICHT MOEGLICH
     R     "M 230.2"                   // EVT *** VO-BETRIEB
     JC    M006                        // EKV
     A     "m 211.5"                   // -EEVT
     S     "M 230.2"
     AN    "m 211.5"
     AN    "M 212.6"                   // ESB
     O                                 // EVO
     ON    "M 211.0"                   // EVB
     ON    "m 211.2"                   // EKV
     ON    "M 212.5"                   // EKS *** EVS SETZEN
     R     "M 230.2"                   // EST

Explanation at M003:

This section seems to control the "VO-BETRIEB" (VO-Operation) state.

1. M003:: Label.
2. AN "M 230.2": AND NOT with M 230.2 ("EKV").
3. AN "m 211.5": AND NOT with M 211.5 ("EKV").
4. O "M 230.1": ORs with M 230.1 ("NICHT MOEGLICH" - "Not possible").
5. R "M 230.2": If RLO is true, reset M 230.2 ("EVT *** VO-BETRIEB" - "EVT *** VO-Operation").
6. JC M006: If RLO is true, jump to M006.
7. A "m 211.5": Loads M 211.5 ("-EEVT").
8. S "M 230.2": If true, set M 230.2.
9. AN "m 211.5": AND NOT with M 211.5.
10. AN "M 212.6": AND NOT with M 212.6 ("ESB").
11. O: ORs with the previous RLO.
12. ON "M 211.0": OR NOT with M 211.0 ("EVB").
13. ON "m 211.2": OR NOT with M 211.2 ("EKV").
14. ON "M 212.5": OR NOT with M 212.5 ("EKS *** EVS SETZEN" - "EKS *** Set EVS").
15. R "M 230.2": If RLO is true, reset M 230.2 ("EST").

Section 11: Saving Status Words (ZW1, ZW2)

M006: A     "M 230.1"                   // ERM
     AN    "M 227.3"
     A     "M 211.6"                   // ZW1 SPEICHERN
     =     "M 230.5"
     OPN   "DB 5"
     L     "MW 227"
// ZW2 SPEICHERN
     T     #conv_akku1
     TAK   
     T     #conv_akku2
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T     STW
     L     #conv_akku2
     L     #conv_akku1
     T     DBW [AR1,P#0.0]
     OPN   "DB 6"
     L     "MW 229"
// EKS **** EBE ANSTEUERN
     T     #conv_akku1
     TAK   
     T     #conv_akku2
     L     STW
     T     #conv_stw
     L     DBB    1
     SLW   4
     LAR1  
     L     #conv_stw
     T   STW
     L     #conv_akku2
     L     #conv_akku1
     T     DBW [AR1,P#0.0]             // EST

Explanation at M006:

This section saves the values of MW 227 (Status Word 1) and MW 229 (Status Word 2) back into their respective Data Blocks (DB 5 and DB 6) using indirect addressing.

1. M006:: Label.
2. A "M 230.1": Loads M 230.1 ("ERM").
3. AN "M 227.3": AND NOT with M 227.3.
4. A "M 211.6": ANDs with M 211.6 ("ZW1 SPEICHERN" - "Save Status Word 1").
5. = "M 230.5": Assigns the RLO to M 230.5.
6. OPN "DB 5": Opens DB 5.
7. L "MW 227": Loads the content of MW 227 into Accumulator 1.
8. The indirect addressing block then writes the value of MW 227 into DB 5 at the address determined by DBW 0 (which still holds NR).
9. OPN "DB 6": Opens DB 6.
10. L "MW 229": Loads the content of MW 229 into Accumulator 1.
11. The next indirect addressing block writes the value of MW 229 into DB 6 at the address determined by DBW 0 (which still holds NR).

Section 12: Output Logic (M001)

M001: A     "M 230.1"                   // EKV
     AN    "M 227.3"
     O     "M 230.2"                   // ELPZ **** ELS BILDEN
     =     #DRON
     A     "M     199.2"               // EKS
     =     "M 227.4"                   // EST
     A     "M     199.1"               // SBL
     A     "m 211.4"
     AN    "M 227.3"                   //    LBL
     S     "M 227.4"
     A     "M 230.1"                   // ESF
     A     "M 227.3"                   // ESB
     A     "M       1.0"               // EVO
     S     "M 227.4"                   // EKV
     A(    
     A(                                // EGA
     ON    "M 212.6"                   // EQS
     ON    "M 211.0"
     ON    "m 211.2"                   // LBL
     O     "M 230.2"
     )                                 //    BETRIEB
     A     "M 212.2"                   // EKS
     O     "M 230.3"
     )                                 // EKV
     A     "M       1.0"               // EGA
     S     "M 227.4"
     A(                                // EST
     O     "M 230.1"                   // ERM
     O     
     A     "M 230.2"                   // HM1
     AN    "M 212.2"
     )     
     AN    "M 227.3"
     A     "M 211.6"
     S     "M 227.4"
     A     "M 227.4"
     =     #RULA

Explanation at M001:

This section, labeled M001 (which was the error handling jump target), now seems to be generating various output signals and status flags, including #DRON and #RULA.

1. M001:: Label.
2. A "M 230.1": Loads M 230.1 ("EKV").
3. AN "M 227.3": AND NOT with M 227.3.
4. O "M 230.2": ORs with M 230.2 ("ELPZ **** ELS BILDEN" - "ELPZ **** Form ELS").
5. = #DRON: Assigns the RLO to the output #DRON.
6. A "M 199.2": Loads M 199.2 ("EKS").
7. = "M 227.4": Assigns the RLO to M 227.4 ("EST").
8. A "M 199.1": Loads M 199.1 ("SBL").
9. A "m 211.4": ANDs with M 211.4.
10. AN "M 227.3": AND NOT with M 227.3 ("LBL").
11. S "M 227.4": If RLO is true, set M 227.4.
12. A "M 230.1": Loads M 230.1 ("ESF").
13. A "M 227.3": ANDs with M 227.3 ("ESB").
14. A "M 1.0": ANDs with M 1.0 ("EVO").
15. S "M 227.4": If RLO is true, set M 227.4 ("EKV").
16. The large nested A(...) block is a complex logical condition. It combines M 212.6, M 211.0, M 211.2, M 230.2, M 212.2, and M 230.3.
17. A "M 1.0": ANDs the result of the complex block with M 1.0 ("EGA").
18. S "M 227.4": If RLO is true, set M 227.4.
19. Another nested A(...) block combines M 230.1, M 230.2, M 212.2.
20. AN "M 227.3": AND NOT with M 227.3.
21. A "M 211.6": ANDs with M 211.6.
22. S "M 227.4": If RLO is true, set M 227.4.
23. A "M 227.4": Loads M 227.4.
24. = #RULA: Assigns the RLO to the output #RULA.

Section 13: WinCC Visualization

// --------------      WinCC visualisation ----------------;

     A     "M 227.3"                   //failure
     =     #failure                    // WinCC visualisation failure

     A(    
     O     "M 230.1"                   //drive shall run in autom
     O     
     A     "M 230.2"                   //drive run in manual
     AN    "M 212.2"                   //pulse drive off autom
     )     
     AN    "M 227.3"                   //drive failure
     A     "M 211.6"                   //feedback
     O     "M     199.2"               //lamp test
     O     
     AN    #REM
     A     #FEBA
     =     #on_off                     // WinCC visualisation On / Off

     A     "m 211.4"
     AN    "M 227.3"                   //drive pre selected
     AN    #on_off                     // WinCC visualisation On / OF
     =     #flash_slow                 // WinCC visualisation flash slow

     L     LB    10
     T     #Visu                       // WinCC visualisation
//-------------------------------------------------------------------

Explanation:

This final section is dedicated to preparing values for display on a WinCC HMI.

1. A "M 227.3": Loads M 227.3 (which is often a fault flag).
2. = #failure: Assigns this state to the output #failure for WinCC.
3. The complex A(...) block calculates the #on_off status:
   • It checks conditions like M 230.1 (drive in auto), M 230.2 (drive in manual), M 212.2 (pulse drive off auto).
   • It then AND NOTs with M 227.3 (drive failure).
   • It ANDs with M 211.6 (feedback).
   • It ORs with M 199.2 (lamp test).
   • It ORs with the previous RLO.
   • It AND NOTs with #REM.
   • It ANDs with #FEBA.
   • = #on_off: The final RLO is assigned to #on_off for WinCC. This likely controls the "on/off" state indication of a piece of equipment.
4. A "m 211.4": Loads M 211.4.
5. AN "M 227.3": AND NOTs with M 227.3 (drive pre-selected).
6. AN #on_off: AND NOTs with #on_off.
7. = #flash_slow: The RLO is assigned to #flash_slow for WinCC. This could be used to make an indicator flash slowly if the drive is pre-selected but not yet on, and not in a failure state.
8. L LB 10: Loads the value of LB 10 (Local Byte 10) into Accumulator 1.
9. T #Visu: Transfers this value to #Visu for WinCC visualization. This might be a general status byte or a specific visualization command.

Summary of Functionality (High-Level Guess):

This STL code appears to be a control block for a specific piece of equipment or a process step (e.g., a motor, a valve, a stage in a production line). It performs the following general tasks:

• Reads configuration/status: Retrieves status words from DB 5 and DB 6 based on an index NR.
• Processes input parameters: Takes various boolean inputs (#V24A, #REM, #HAON, #FEBA) and maps them to internal flags.
• Monitors changes: Detects changes in certain input words (likely MW 211) by comparing current and previous states stored in DB 4.
• Manages operational states: Contains complex logic with multiple conditional jumps (M001, M002, M003, M004, M005, M006) to handle different operational modes (e.g., automatic, manual, fault processing, VO-operation, staged drive).
• Implements fault detection and handling: Sets and resets various fault flags (M 227.3, M 230.1, etc.) and jumps to dedicated error processing routines.
• Controls timers: Uses an S5 timer (#_TIME) for timing-related operations.
• Updates status words: Saves updated status words back to DB 5 and DB 6.
• Generates outputs: Produces output signals like #DRON, #RULA.
• Prepares WinCC visualization data: Calculates and assigns values to #failure, #on_off, #flash_slow, and #Visu for display on an HMI.

Key Abbreviations (based on comments and common PLC practice):

• ZW: Zustandswort (Status Word)
• NR: Nummer (Number/Index)
• EKS: Error/Fault (Störung)
• ERM: Error Message / Error Monitoring
• EST: Status (Zustand)
• VO-BETRIEB: VO-Operation (likely a specific operational mode)
• EBVG: (Unclear, possibly related to "Betriebsverriegelung" - operational interlock or "Betriebsvorgabe" - operational setpoint)
• EUS: (Unclear, possibly "Einheit Störung" - unit fault, or "Ein/Aus Schalter" - on/off switch)
• ESB: (Unclear, possibly "Einzelschrittbetrieb" - single step operation, or "Störungsbetrieb" - fault operation)
• EVO: (Unclear, possibly "Ein/Aus Vorbereitung" - on/off preparation)
• EKV: (Unclear, possibly "Eingangsventil" - input valve, or "Ein-Kanal-Verriegelung" - single channel interlock)
• FEBA: Feedback
• REM: Remote
• HAON: Hand/Auto ON

This is a very detailed analysis of the provided STL code. Understanding STL requires careful tracing of the RLO and accumulator contents, especially with indirect addressing and jumps. If you have a specific part you'd like more detail on, or if you can provide context about the machine/process this code controls, I can offer an even more precise explanation!

The AI broke it down into steps. Do the same when trying to understand the code.

Remember, the AI had to guess the meaning of the code because you didn't provide any background information.

Kind regards
Stefan