Resolving PCS 7 SFC TRIG_CPU_RESTART Errors After CPU Restart

Resolving PCS 7 SFC TRIG_CPU_RESTART Errors After CPU Restart

Sequential Function Chart (SFC) instances in SIMATIC PCS 7 frequently fail to resume execution after the Automation Station (AS) returns from STOP, RUN, or warm/cold restart. The most common symptom is a latched TRIG_CPU_RESTART output mapped to the tag TrigCPUR, accompanied by a green "consistent data" indication on the SFC faceplate, even though the chart never advances past the start step. This technical reference covers root cause, status output interpretation, configuration of CPU operating parameters, recovery procedures, and verification methods for SFC instances in PCS 7 V8.x through V9.1.

1. Problem Description

After a CPU restart event (warm restart, cold restart, or power-fail restart), SFC instances derived from SFC types may exhibit one of the following behaviors:

• The SFC instance displays a red returning arrow indicating inconsistent data.
• The SFC instance displays a green returning arrow indicating consistent data but the chart does not advance past the start transition.
• The output TRIG_CPU_RESTART (tag: TrigCPUR) is set TRUE and remains latched.
• Writing the RESUME command to the SFC internal control block has no effect.
• The chart can only be recovered through the SFC Control Panel "Resume" button, manual mode switching, or a full project download.

The root symptom is that the SFC operating mode and execution state are not properly re-initialized when the AS comes back to RUN, leaving the chart in a suspended state that cannot be cleared by simple user-level commands.

2. SFC State and Operating Mode Fundamentals

Each SFC instance in PCS 7 has two orthogonal state machines that must be understood to troubleshoot restart behavior:

2.1 Operating Mode (OM)

2.2 Run State (RS)

During a CPU restart, both state machines must be re-initialized. If the SFC was in the middle of a transition evaluation at the moment of stop, the saved state can be inconsistent because no step was active and no transition was complete.

3. Status Output Reference

The SFC faceplate in the OS WinCC Runtime exposes the following Boolean outputs from the SFC instance DB. These correspond to the I/O flags generated by the SFC compiler in the AS:

When a CPU restart occurs and the SFC property Retain SFC state is enabled, the instance preserves the active step in its instance DB. On the next AS startup the chart is re-initialized to that saved state, but the CPU_REST and TRIG_CPU outputs are forced TRUE to signal that operator intervention is required.

4. Root Cause: Consistent vs Inconsistent SFC Data

Siemens officially documents two restart-data states in the PCS 7 SFC manual (PCS 7 SFC Manual):

4.1 Consistent Data

The AS stopped at a defined point: a step was active, or the chart was in Idle. The instance DB can be re-loaded and the chart resumes from that state. The faceplate shows a green returning arrow.

4.2 Inconsistent Data

The AS stopped during a transition evaluation, while a step change was in progress, or while a sequencer was being modified. The instance DB cannot be unambiguously restored. The faceplate shows a red returning arrow. A RESET is mandatory before any further command is accepted.

The "Consistent Data / Green Arrow" appearance combined with a latched TrigCPUR is misleading: although the data is structurally consistent, the operator policy of the system requires an explicit Resume to clear the restart latch. The Resume cannot be issued programmatically through standard I/O writes in some AS firmware versions; the operator must use the SFC Control Panel or the SFC Visualization faceplate.

5. SFC Properties and Retain State Behavior

Open the SFC type in the ES, right-click the chart, and select Object Properties. Three tabs affect restart behavior:

5.1 Retain SFC State

When enabled: The active step is saved to the retentive area of the instance DB. After CPU restart, the SFC resumes from the saved step but requires Resume/Start acknowledgement.

When disabled: The SFC returns to Idle after CPU restart, regardless of prior state. The instance DB does not store the last active step.

6. Resolution Procedure: Recovering SFC After CPU Restart

Use the following decision-tree procedure on the OS WinCC Runtime faceplate.

6.1 Identify the State

1. Open the SFC faceplate (double-click the SFC symbol in the OS picture).
2. Observe the resume icon in the upper-right corner:
   • Green returning arrow = consistent data (go to step 6.2).
   • Red returning arrow = inconsistent data (go to step 6.3).
3. Note the current operating mode from the QAUTMAN indicator.

6.2 Recovery from Consistent Data

1. Click the Standard view of the SFC faceplate.
2. Switch the chart to Manual mode using the MAN/AUTO button.
3. Click Resume in the command group. The chart should clear CPU_REST and TRIG_CPU.
4. Switch back to Auto mode. Verify RUN = 1 within one cycle.

If Resume has no effect, the SFC is in a protected state. Proceed to step 6.4.

6.3 Recovery from Inconsistent Data

1. Open the SFC faceplate.
2. Click Reset. The chart transitions to Idle and clears all error latches including TRIG_CPU.
3. Click Start to begin execution. The chart will run from the initial step.

6.4 Programmatic Recovery (ES Side)

For automated recovery, use SFC control block commands via the SFC library FB 245 (SFC_CONTROL) or by writing the command word of the SFC instance DB at the symbolic address <SFC_Instance>.-CMD:

// Force Resume on SFC instance "REACTOR_1"
"REACTOR_1".-CMD := 16#0002;  // RESUME command code
// Wait one OB1 cycle, then clear the command
"REACTOR_1".-CMD := 16#0000;

Command codes for the SFC instance command word:

On S7-400 CPUs with firmware V6.x and AS 410 with firmware V8.x, the command word must be pulsed for exactly one OB1 cycle. Holding the command word HIGH will cause the SFC to re-enter the command state on the next scan, creating an oscillation.

7. Configuring CPU Operating Parameters for Autostart

To prevent the manual Resume requirement on routine CPU restarts, configure the SFC to autostart:

1. In the PCS 7 ES, right-click the SFC type in the component view.
2. Select Object Properties → CPU Operating Parameters tab.
3. Set the following parameters:
   • Operating mode: AUTO
   • Cyclic execution: Enabled (checked)
   • Autostart: Enabled (checked)
4. Compile and download the SFC to the AS.

With Autostart enabled, the SFC will begin executing from its saved or initial step automatically on the next AS RUN transition, and the TRIG_CPU output will be cleared by the system without operator intervention.

8. AS-OS Communication and Status Word Considerations

The SFC status outputs reach the OS through the AS-OS connection configured in NetPro. The relevant PCS 7 OS connection attributes are:

The status word of an SFC instance is mapped in the instance DB at offset +0 (WORD type). Bit assignments are documented in the PCS 7 SFC reference manual (SFC Reference Manual). The relevant bits for restart diagnosis are:

9. Common Error Scenarios and Recovery Matrix

10. Verification and Commissioning Checks

After applying the recovery procedure, verify the SFC has been correctly restored by executing the following checks in order:

1. Output verification: Confirm in the OS faceplate that RUN = 1, CPU_REST = 0, and TRIG_CPU = 0.
2. State consistency: Watch the active step indicator in the SFC faceplate for 5 to 10 seconds; it should not flicker between two steps.
3. Transition logging: In WinCC Alarm Logging, search for the SFC instance's transition messages; the latest message should correspond to a step change, not to "CPU restart acknowledged."
4. Diagnostic buffer: In the ES, open the AS diagnostic buffer (PLC → Diagnostic/Setting → Diagnostic Buffer) and confirm no SFC-related errors are recorded after the recovery.
5. Cyclic monitoring: Monitor the SFC instance DB online for 5 minutes; the active step number should change at least once if the process is live, or remain stable if the chart is intentionally idle.

11. Version-Specific Behavior and Firmware Notes

The SFC behavior discussed in this article was reproduced and verified on PCS 7 V9.0 SP1 with AS 410 firmware V9.0. The reference to the Siemens support article 48933314 (Siemens Support Article 48933314) remains valid as the canonical procedure for SFC restart handling.