S5 AWL :AS Instruction: Process Alarm Disable for S7-1500

Overview of the S5 :AS Instruction

The Siemens SIMATIC S5 Anweisungsliste (AWL) statement :AS is a CPU-level control instruction that disables the process alarm (interrupt) path of the S5-135U and S5-155U operating system. The mnemonic AS is the German abbreviation for Alarme Sperren (disable alarms). Its direct counterpart is :AF (Alarme Freigeben, enable alarms). The instruction was originally defined for the S5-135U and S5-155U family where the IP (Interrupt-Processor) and IA (Interrupt-Acknowledge) signal modules in the central rack can raise a hardware process alarm to OB2. When the CPU has executed :AS, the alarm-enable bit is cleared and OB2 will not be entered on the next process alarm from the IP/IA module.

In an S5-95U, S5-100U, or S5-115U, the same token may appear but the underlying mechanism differs. S5-95U and S5-100U do not have the S5-135U/155U interrupt-module infrastructure; the :AS opcode, when present in the S5-95U instruction set, masks a different class of interrupts. S5-115U from CPU 941B / 942 / 943 / 944 upwards uses the S5-135U alarm model and :AS/:AF behave as described here.

Reference Code from a STEP 5 Migration

The following S5 AWL segment was extracted from a real STEP 5 to S7-1500 migration. The code latches a start condition into M71.1, mirrors it to M80.0, and then calls FB31. The :AS line sits between the latch logic and the conditional block call, which is the standard S5 idiom for protecting a critical FB against process-alarm pre-emption.

NETWORK 1
:U   E   32.7     // AND  input 32.7 (start command)
:UN  M   71.1     // AND  NOT flag 71.1 (latch state)
:UN  T   15       // AND  NOT timer 15 (timeout)
:S   M   71.1     // SET  flag 71.1 (latch)
:=   M   80.0     // assign result to M 80.0

NETWORK 2
:UN  E   32.6     // AND  NOT input 32.6 (stop)
:U   M   80.0     // AND  M 80.0
:=   M   80.0     // result back to M 80.0 (self-hold)

NETWORK 3
:AS               // DISABLE process alarm (Alarme Sperren)
:SPB FB 31        // conditional jump to FB31

The :AS opcode is one network by itself and consumes no operand. The next line, :SPB FB 31, is the conditional block call (Springe bedingt to FB31 if RLO = 1). The CPU enters FB31 with the alarm-enable bit cleared. Inside FB31, the user program can read/write IP/IA modules, manipulate timers, or perform sequential control without the risk of an OB2 pre-emption. The alarm-enable bit is set again only when the program reaches a corresponding :AF opcode, or when the CPU performs a cold restart.

S5 Process Alarm Architecture

To map :AS correctly, you must understand the S5-135U/155U interrupt model. The CPU services several event classes through dedicated organization blocks.

The :AS / :AF pair controls the alarm-enable flag in the CPU system data area (SZD). When the bit is 0, OB2 is suppressed. When the bit is 1, OB2 is called on the next process alarm. The default after COLD RESTART is enabled. After WARM RESTART, the state from before the stop is preserved.

Operation: :AS versus :AF

The :IRA opcode is the matching acknowledge. It is typically placed at the end of OB2 to re-arm the IP/IA module. Without :IRA, the IP/IA module fires one interrupt and then holds its alarm line asserted until acknowledged, regardless of :AS/:AF state.

S5 CPU Family Compatibility

Not every S5 CPU implements the :AS / :AF pair.

If the source listing contains :AS, the original CPU is almost certainly an S5-115U, S5-135U, or S5-155U. S5-95U programs occasionally carry the same mnemonic but the semantics differ; always cross-check the OB2 body and the signal-module list before migration.

Alarm Enable State Machine

The S5-135U/155U alarm-enable state machine is a 1-bit toggle with a counter for nested calls.

Two consecutive :AS calls require two :AF calls before OB2 is re-enabled. This counter behaviour also exists in the S7 SFC 39 / SFC 40 implementation; treat the SFC pair as balanced calls in your code review.

Interaction with Block Calls (FB / PB / SB)

The :AS opcode modifies a CPU-level flag, not a block-local variable. Its effect persists across block calls. The migration source code demonstrates the canonical pattern:

:AS
:SPB FB 31

Inside FB31, OB2 will not be entered on a process alarm. The IP/IA module holds the alarm asserted (single-shot queue depth) until either the user program executes :AF or the IP/IA module's alarm buffer overflows. In S5-135U/155U the IP/IA module typically queues one alarm; a second alarm arriving while :AS is active is lost unless the module is configured for interrupt accumulation via the COM 135U parameterisation tool.

This persistence is a common bug source. If the developer forgets to execute :AF on every code path, the system stays alarm-disabled. Code-review checklist for S5 programs that contain :AS:

1. List every :AS in the program (cross-reference).
2. Verify a matching :AF exists on every code path that does not end in BE or BEA.
3. Verify the matching :AF is reachable when :AS is taken (consider conditional jumps).
4. Verify FB31 and any other block called under :AS do not assume OB2 is running.

Migration to S7-1500

The S7-1500 (and S7-300/400) CPUs do not have a textual :AS instruction. The S7 replacement uses System Function Calls (SFCs) from the standard library.

The standard replacement for the S5 :AS opcode is a call to SFC 39 "DIS_IRT" with MODE = 0 (disable all interrupt OBs). For the S5 :AF opcode, call SFC 40 "EN_IRT" with MODE = 0 (enable all). If only one interrupt class must be disabled, MODE = 1 with OB = 40 disables only OB40.

S7-1500 STL / SCL Replacement Code

The S7-1500 replacement for the S5 source segment is shown below in STL and in SCL. In TIA Portal the call sites are inserted as standard FBs from the "Standard library → System function blocks" folder.

// S7-1500 SCL replacement for the S5 source segment
// Network 1: start-latch logic
IF "E32_7" AND NOT "M71_1" AND NOT "Timer15".Q THEN
    "M71_1" := TRUE;
    "M80_0" := TRUE;
END_IF;

// Network 2: stop-self-hold
IF NOT "E32_6" AND "M80_0" THEN
    "M80_0" := TRUE;
END_IF;

// Network 3: replace :AS with SFC 39
"DIS_IRT_DB"(MODE := 0, OB := 0);     // SFC 39 disable all

// Network 4: replace :SPB FB 31 with conditional call
IF "M80_0" THEN
    "FB31_DB"(Start := TRUE);
END_IF;

// Network 5: replace :AF with SFC 40
"EN_IRT_DB"(MODE := 0, OB := 0);      // SFC 40 enable all

STL version (kept close to the S5 source):

// S7-1500 STL replacement for the S5 source segment
U   "E32_7"
UN  "M71_1"
UN  "Timer15".Q
S   "M71_1"
=   "M80_0"

UN  "E32_6"
U   "M80_0"
=   "M80_0"

CALL "DIS_IRT"          // SFC 39
 MODE := 0
 OB   := 0

A   "M80_0"
SPB FB31

CALL "EN_IRT"           // SFC 40
 MODE := 0
 OB   := 0

The SFC instance DBs are created automatically by TIA Portal on first compile. Verify the instance DBs are present in the project tree under "Program blocks → System blocks".

S5 versus S7-1500 Alarm Architecture

The structural differences between the S5 and S7-1500 alarm models are summarised in the table below.

Note the difference in the warm-restart behaviour. In S5 the alarm-enable state is preserved across a warm restart, so an S5 program that left :AS active at STOP will resume with alarms disabled. In S7, SFC 39 is re-armed on STOP→RUN. Migrated code that depends on persistent alarm disable must call SFC 39 in OB100 (warm restart) as well.

TIA Portal Implementation Procedure

1. Open the migrated S7-1500 project in TIA Portal.
2. Add the SFC 39 call site. From "Instructions → System function blocks" drag "DIS_IRT" into the FB that contains the equivalent of the S5 :AS.
3. Set MODE = 0 (disable all) or MODE = 1 with the target OB number.
4. Compile the project. TIA Portal creates the instance DBs for SFC 39 and SFC 40.
5. Download to the S7-1500 CPU.
6. Go online and open the watch table for the alarm-disable flag. Force the disable call and verify OB40 is not entered on a forced hardware interrupt.
7. Force the enable call and verify OB40 is entered on the next hardware interrupt.

The S7-1500 hardware-interrupt source is configured in the device configuration: select the DI module → Properties → "Hardware interrupt" → assign a process tag to OB40. Common S7-1500 / ET 200SP signal modules that raise OB40 include the ET 200SP digital-input modules (e.g. 6ES7131-6BF00-0BA0, 8DI 24V DC) and the S7-1500 central DI modules (e.g. 6ES7521-1BH00-0AB0, DI 16x24V DC).

Edge Cases and Field Notes

• Nested disable: two :AS calls require two :AF calls. The same counter rule applies to SFC 39 / SFC 40. Code review must count pairs.
• Cold-restart state: in S5, :AS state is reset on COLD RESTART (OB20) but PRESERVED on WARM RESTART (OB21). In S7, SFC 39 state is reset on every STOP→RUN transition.
• OB82 diagnostic alarm: S5 :AS suppresses only OB2 (process alarm). S7 SFC 39 with MODE = 0 suppresses OB82 as well. If the S5 program relied on OB19-OB22 to fire even when process alarms were disabled, S7 must use MODE = 1 with OB = 40 to disable only OB40.
• Time-of-day interrupt: S5 :AS does NOT affect OB10-OB13. S7 SFC 39 with MODE = 0 affects OB10-OB17 in some S7-1500 firmware versions. Confirm with the target CPU's system manual.
• OB priority: in S5, OB2 has a fixed priority. In S7-1500, OB40-OB47 each have a configurable priority. The priority of OB40 must be set higher than the calling block's priority, otherwise OB40 will not pre-empt the block that called SFC 39.
• Calling SFC 39 from OB1: if SFC 39 is called unconditionally in OB1 and SFC 40 is missing, the entire program will run alarm-suppressed. Migrated code that does this in a control loop will never service OB40. Review all SFC 39 call sites for matching SFC 40.
• Calling SFC 39 from OB100: acceptable for setting initial alarm state, but the matching SFC 40 must be reachable from the same restart class.

Safety Considerations

If the original S5 program is part of a safety chain (e.g. a burner control or a chemical-reaction sequence), the migration must preserve the equivalence between :AS suppression and S7-1500 SFC 39 suppression. Add a code-review entry: "Verify SFC 39 / SFC 40 do not break PROFIsafe / F-CPU alarm path".

Troubleshooting Matrix

Verification Procedure After Migration

1. Open the S7-1500 project in TIA Portal.
2. Mark the location where :AS was in the S5 source. Confirm SFC 39 is present with MODE = 0 (or MODE = 1 with the specific OB).
3. Mark the location where :AF was. Confirm SFC 40 is present with the same MODE / OB setting.
4. Go online with the S7-1500 CPU.
5. Force a hardware interrupt on a digital-input module that is mapped to OB40.
6. Confirm OB40 is NOT entered while the SFC 39 call site is enabled (online → watch table → DIS_IRT_DB.OK = TRUE).
7. Confirm OB40 IS entered after the SFC 40 call site is reached (online → watch table → EN_IRT_DB.OK = TRUE).
8. Remove a module to trigger a diagnostic alarm. Confirm OB82 is suppressed (if MODE = 0) or fired (if MODE = 1 with OB = 40).
9. Check the diagnostic buffer of the S7-1500 CPU for any SFC 39 / SFC 40 event entries.
10. Document the test in the migration report; attach the watch-table screenshot.

Cross-Reference Strategy for S5 to S7-1500 Conversion Tools

The STEP 5 to S7-1500 migration path is not natively supported by TIA Portal. The common route uses the S5 converter in STEP 7 (Classic) to produce an S7-300/400 STL project, then a TIA Portal migration to S7-1500 STL/SCL. The :AS / :AF opcodes are flagged as unmappable by every mainstream converter because they are CPU-control instructions, not logic operations. The post-migration manual step is:

1. Open the S7-300/400 project after the STEP 5 import.
2. Search for the literal token "AS" in the AWL source.
3. For each occurrence, replace with an SFC 39 call block (call to FB0 / FC0 wrapper that calls SFC 39 with the right MODE).
4. For each "AF" occurrence, replace with an SFC 40 call block.
5. Compile the S7-300/400 project; resolve any type errors in the wrapper blocks.
6. Migrate the S7-300/400 project to S7-1500 in TIA Portal.
7. Verify the wrapper blocks are still present in the S7-1500 project (TIA Portal may move or rename them).

An alternative is to use the Siemens SIMATIC Migration Tool add-on for TIA Portal, which maps a wider set of S5 opcodes; check the tool's manual for the current :AS coverage.

Frequently Asked Questions