Resolving S7-GRAPH S_NO Not Matching Active Step Number in TIA Portal

The S_NO output of an S7-GRAPH function block is the most commonly used tag for tracking the active step in a sequencer. Engineers routinely connect S_NO to external HMI displays, condition-handling logic, interlock routines, and downstream equipment. When S_NO no longer matches the step that the operator sees in the GRAPH editor or in the watch table, the entire process visualization breaks. The typical symptom is that toggling the mode switch from Automatic to Manual and back to Automatic leaves S_NO stuck on a step that is no longer active, and only a CPU restart or program reset returns the sequencer to a coherent state.

This article consolidates the Siemens support information, the GRAPH FB interface specification, and field diagnostics into a single troubleshooting reference. It explains why the S_NO appears to drift, how to determine which of the three documented causes is responsible, and how to correct the display without resetting the working memory.

1. Problem Description and Observed Symptoms

Engineers running S7-GRAPH in TIA Portal V16, V17, V18, V19, or V20 on S7-300, S7-400, or S7-1500 CPUs report the following behaviour pattern:

• The GRAPH function block runs in Automatic mode (mode switch SW_AUTO active).
• The S_NO output reports a step number that does not correspond to the step highlighted in the GRAPH online view.
• The output parameter S_MORE is TRUE, indicating that the FB has more than one active step internally.
• The output parameter S_ACTIVE is FALSE, meaning the step currently shown in S_NO is not flagged as the canonical "active" step of the sequencer.
• When the operator toggles the mode from Automatic to Manual and back to Automatic in the GRAPH Testing > Sequence Control > Mode panel, the S_NO remains stuck on the wrong value.
• A CPU restart, a PG online re-download of the FB, or a memory reset of the instance DB restores correct behaviour, but this is unacceptable in a running plant.

The issue is reproducible only under specific conditions, and the symptoms look identical to a programming bug, which makes the root cause elusive without consulting the GRAPH interface specification.

2. Root Cause Analysis

Siemens documents three independent mechanisms that cause S_NO to display a step number that does not match the visual "active step" the engineer expects:

2.1 Multiple Active Steps Inside the FB

According to the official Siemens Knowledge Base article Step number display in S7-Graph (ID 10103460), the S_NO parameter always reports the currently active step regardless of which sequence it belongs to. When the GRAPH function block contains more than one sequence, or when the block contains a simultaneous (parallel) branch, the FB can have multiple active steps at the same time. S_NO then displays only one of them; which one is displayed depends on the internal sorting order. This is the most common root cause and the one that produces the S_MORE = TRUE signal in the status display.

2.2 Manual Mode Selection of a Non-Active Step

The S_NO output can be commanded to a different step number when the FB is in Manual or Step mode. The user uses the input parameter S_SEL (INT) to select a step number, then pulses S_ON or S_OFF to enable or disable it. Per the official Input parameters (S7-300, S7-400, S7-1500) reference, S_SEL is specifically the STEP_SELECT input for the S_NO output parameter in manual mode. If S_SEL is wired to a non-zero value when the FB returns to Automatic mode, the FB may continue to display that commanded step number rather than the step that is actually executing.

2.3 Display Mode Configuration of the FB

The GRAPH FB exposes three static parameters in the instance DB that govern what S_NO shows:

If SALL_DISP is inadvertently set to TRUE, the value of S_NO cycles through the active steps on every OB1 scan. An engineer observing the tag in the watch table sees a "stuck on the wrong step" symptom; in fact the FB is rotating through multiple legitimate active steps.

3. Diagnostic Procedure

Before applying any fix, isolate which of the three causes is in effect. The watch table below is the minimum set of tags to monitor online.

1. Open the GRAPH instance DB in Monitor / Modify and force the visibility of all instance tags.
2. Add the following tags to a watch table:
   • iDB.S_NO — the step number output.
   • iDB.S_MORE — BOOL, more than one step is active.
   • iDB.S_ACTIVE — BOOL, the step displayed in S_NO is flagged active.
   • iDB.MOP.SACT_DISP
   • iDB.MOP.SEF_DISP
   • iDB.MOP.SALL_DISP
   • iDB.S_SEL — INT, manual step selection input.
   • iDB.S_SELOK — BOOL, manual selection acknowledged.
3. Toggle the GRAPH FB to Manual, then back to Automatic, and observe which of the three values flips:
   • If S_MORE goes TRUE at the moment of mismatch, the cause is Section 2.1 (multiple active steps or parallel branch).
   • If S_SEL is non-zero at the moment of mismatch, the cause is Section 2.2 (manual selection left active).
   • If S_NO cycles between several values on every PLC scan, the cause is Section 2.3 (display mode).
4. Cross-check against the GRAPH online view. The rightmost step in any simultaneous branch is the one GRAPH displays first; use S_PREV and S_NEXT output toggles in the instance DB to step through all active steps.

4. Static Parameter Configuration

The static parameters in the MOP (Mode Of Operation) structure are the canonical lever for correcting the step display. They are read once at FB initialization and at the transition between Automatic and Manual mode. The following matrix maps the symptom to the required parameter state.

To set the static parameters in TIA Portal:

1. Right-click the GRAPH instance DB in the project tree and select Open in editor.
2. Navigate to the MOP structure. By default TIA Portal hides the structure; enable Show all tags in the toolbar.
3. Expand MOP > SACT_DISP, SEF_DISP, SALL_DISP.
4. Set the values according to the matrix above. Initial values defined in the DB take effect on the next OB1 scan; values modified online via the watch table take effect immediately.
5. Compile and download the project. A consistent download is acceptable; a full memory reset is not required.

5. Parallel Branches and Multiple Sequences

If the diagnostic procedure identifies S_MORE = TRUE but the program source visibly has no simultaneous branch, the FB likely contains more than one sequence. Each sequence is a separate sequencer state machine sharing the same FB and the same S_NO output. The Siemens Knowledge Base ID 10103460 states that when several steps are active across multiple sequences, the S_NO output cycles through them and the calling program must filter by the sequence index. The S_PREV and S_NEXT outputs (BOOL) are the standard Siemens mechanism to step through the active steps one at a time.

Sample STL code that walks every active step in a multi-sequence GRAPH FB:

// Determine the count of active steps and step through them
// iDB = instance DB of the GRAPH FB
// FirstStep = BOOL output, TRUE for one cycle when the first active step is at S_NO
// LastStep  = BOOL output, TRUE for one cycle when the last active step is at S_NO

A  iDB.S_MORE           // More than one step is active
JC MULTI                // Jump to multi-step handler

L  iDB.S_NO             // Single step active, S_NO is the answer
T  #ActiveStep
JU DONE

MULTI: SET
      A  iDB.S_NEXT     // Pulse S_NEXT to advance to the next active step
      JC ADVANCE
      L  iDB.S_NO       // First active step is already in S_NO
      T  #ActiveStep
      JU  DONE
ADVANCE: L  iDB.S_NO
         T  #ActiveStep
DONE:   NOP 0

If the calling program needs the step from a specific sequence, the standard pattern is to compare S_NO against the expected range and gate downstream logic. For example, if Sequence 1 owns steps 1 through 20 and Sequence 2 owns steps 21 through 40, the calling logic checks S_NO > 20 to identify the sequence.

6. Manual Mode and S_SEL Handling

The S_SEL input parameter is the engineering tool for selecting a step in Manual mode. It is the root of the second mismatch scenario. The official Siemens documentation specifies that S_SEL is an INT (the step number to select), that S_ON and S_OFF are the BOOLEAN commands that enable or disable the step chosen by S_SEL, and that S_SELOK (BOOL) is the acknowledgement output. The common mistake is to wire S_SEL from a non-volatile tag (for example, an HMI tag that retains its last value across power cycles). When the FB returns to Automatic mode, the last commanded step number from S_SEL is still in the input buffer and the FB continues to display it.

Recommended engineering practice:

1. Initialize S_SEL to 0 (no selection) on every transition from Manual to Automatic. Use the SW_AUTO mode bit as the gating condition.
2. Verify S_SELOK = FALSE after the transition. A TRUE value indicates a selection is still pending.
3. Do not bind S_SEL to retain tags. Use a temporary tag that is reset in the FB's own OB1 call.

Sample STL for a safe Automatic / Manual handover:

// On the rising edge of SW_AUTO, clear S_SEL and force S_OFF
A   M 0.0                 // SW_AUTO rising edge (one-shot)
JC  CLEAR_SEL
L   0
T   iDB.S_SEL
CLR                          // Pulse S_OFF to release the manual selection
S   iDB.S_OFF
CLEAR_SEL: SET
A   iDB.S_SELOK             // Wait for acknowledgement
R   iDB.S_OFF               // Release the manual step
NOP 0

7. Avoiding the Memory Reset Workaround

The source observation is correct: a memory reset is not required, and it is destructive. The correct non-destructive sequence to recover the sequencer state is:

1. Stop the CPU via the PG (online > stop).
2. Open the GRAPH instance DB. Correct the offending static parameter (Section 4) or the S_SEL input (Section 6).
3. Download the modified DB to the CPU as a consistent download in RUN mode (TIA Portal will prompt). The retentive bits of the instance DB are preserved.
4. Bring the CPU back to RUN. Verify S_NO matches the active step in the GRAPH online view.

If the CPU is in STOP because the sequencer entered an unrecoverable state, perform a restart rather than a memory reset. The S7-1500 CPU's restart initialization clears the working memory but preserves the retentive tags you have configured. The S7-300/400 restart is equivalent to an S7-1500 restart for the purposes of recovering a GRAPH FB.

8. Verification Procedure

After applying the fix, the following acceptance test must pass before the program is released to production:

1. Place the GRAPH FB in Automatic mode and run the sequencer through every transition at least once.
2. Monitor S_NO in a watch table and confirm it tracks the highlighted step in the GRAPH online view at every cycle.
3. Toggle the FB to Manual, select a step that is not the current active step with S_SEL, then toggle back to Automatic. Verify S_NO returns to the actual active step and that S_SEL is cleared.
4. Confirm S_MORE = FALSE for the entire run. If it ever flips to TRUE, a parallel branch or multi-sequence scenario is still present and Section 5 must be revisited.
5. Confirm S_ACTIVE = TRUE for the entire run. This is the canonical confirmation that the FB agrees the step displayed in S_NO is the active one.
6. Save the GRAPH online trace and the watch table screenshot to the project change-log.

9. Troubleshooting Matrix

10. Compatibility Notes Across S7-300, S7-400, and S7-1500

The interface described in this article is stable across the documented platforms:

• S7-300 / S7-400: S7-GRAPH runs as a separate option package on STEP 7 V5.x and on TIA Portal up to V20. The MOP structure, S_NO, S_MORE, S_ACTIVE, S_PREV, S_NEXT, and S_SEL are present in every GRAPH FB from version 5.3 onward.
• S7-1500: S7-GRAPH is part of the standard TIA Portal installation. The interface is the same, but the display parameters are also exposed through the GRAPH FB's Properties dialog under Settings > Display. Configuration there is equivalent to writing to the MOP structure manually.
• Firmware sensitivity: The display logic has been stable across S7-1500 CPU firmware V2.5 through V3.1. The Siemens Knowledge Base article ID 10103460 documents the S_NO display behaviour and applies to all listed firmware versions.

11. Field-Commissioning Checklist

Before sign-off, verify the following items on the live CPU:

1. The GRAPH instance DB is in the program and matches the FB type in the project.
2. MOP.SACT_DISP = TRUE, MOP.SEF_DISP = FALSE, MOP.SALL_DISP = FALSE unless the application explicitly needs the alternate behaviour.
3. The HMI is bound to S_NO through a non-retain tag; the S_NO value is not stored across cold restarts.
4. No ladder / STL / SCL code forces S_SEL to a non-zero value when SW_AUTO is active.
5. The sequencer is exercised through every transition at commissioning, and a watch-table screenshot of S_NO matching the GRAPH online view is archived.
6. The operator console's Testing > Sequence Control > Mode function is restricted to authorized personnel to prevent accidental Manual/Auto toggles that could expose the mismatch condition.

12. Frequently Asked Questions