Diagnosing SM326F 6ES7 326-1BK02-0AB0 Internal Power Supply Faults

The SIMATIC SM326F 6ES7 326-1BK02-0AB0 (F-DI24×DC24V) is a fail-safe digital input module used in S7-300F / S7-400F safety controllers. This article documents the field procedure for isolating intermittent "Internal power supply failed" diagnostics logged in the CPU diagnostic buffer, fired as Event ID 3842 with OB82 diagnostic-interrupt execution, and mirrored in the WinCC / Process Historian as a failure event for the connected field devices.

1. Problem Description

Two physically identical SM326F modules installed in the same PROFIBUS-DP slave (DP address 3) generate the following sequence inside the S7 CPU diagnostic buffer:

1. The CPU raises Event 3842 - "Module/submodule has triggered a diagnostic interrupt: diagnostic status has changed."
2. OB82 executes (diagnostic interrupt OB). The incoming and outgoing event buffers in OB82's start information indicate a transition between "module OK" and "module fault".
3. The diagnostic byte reports Internal power supply failed on the affected FDI module.
4. Within milliseconds the same module logs an outgoing diagnostic interrupt, indicating the fault has cleared.
5. The corresponding PROFIBUS slave appears in the Process Historian as page 35340 with a device-side failure message for the wired sensors.

A third identical SM326F (installed on a different DP slave) does not produce this entry, which rules out a CPU-level or project-wide configuration error and points the investigation toward the local power environment of DP slave 3.

2. Module Identification and Key Specifications

3. Diagnostic Buffer Mechanics on S7-300/400

The CPU diagnostic buffer is a ring buffer in non-volatile RAM (or MMC, depending on CPU) that stores the most recent diagnostic events in chronological order. Event 1 is always the newest entry. Each entry contains:

• Time stamp (CPU time, resolution 1 ms)
• Event ID (decimal code, see STEP 7 help or TIA Portal Web Server - Diagnostics Buffer)
• OB that was (or would have been) executed
• Module logical address (base address of the F-module)
• Channel / submodule number
• Plain-text description (language-dependent on the engineering station)

For Event ID 3842 the standard interpretation is:

"A module/submodule has triggered a diagnostic interrupt to indicate that its diagnostic status has changed. There is no disruption, no maintenance requirement and no maintenance request."

This event is the trigger, not the actual fault. The actual fault description is contained in the Channel Diagnostics record (DS0 / DS1) that the CPU reads from the module in the same OB82 call. The text "Internal power supply failed" is therefore decoded from DS1 byte 0 / 7 of the SM326F.

3.1 OB82 Start Information Layout

Reading these fields in OB82 with SCL/ST makes it possible to filter power-supply-failure events from real channel faults, and to write them to a discrete alarm buffer that is not mixed with safety-related process alarms.

4. Why the SM326F Reports "Internal Power Supply Failed"

The SM326F supervises two internal rails:

1. L+ / M - the 24 V fed to the front connector and routed through an internal PTC + EMC filter.
2. Sensor supply Vs1 / Vs2 - two separately switched 24 V outputs used to feed the connected NAMUR / 24 V sensors (SIL2/SIL3 inputs use both Vs rails cross-wise).

The module generates a "Internal power supply failed" diagnostic when any of the following conditions is true for longer than the configured debounce time (typ. 2–5 ms):

• L+ drops below ~18 V (brown-out detection).
• Vs1 or Vs2 short-circuit current exceeds the internal limit (typ. 1.1 A per rail).
• Internal 5 V logic rail collapses due to inrush from a parallel module on the same ET200M backplane.
• EMC event on the backplane induces a watchdog reset of the module's diagnostic ASIC.

Because the trigger is debounced and re-evaluated at the next diagnostic scan (typ. 10 ms), the module can self-clear the diagnostic the instant the rail recovers. The CPU sees a paired in/out sequence tens of milliseconds apart - exactly the symptom described in the source case.

5. Root-Cause Matrix

Rank the following candidates by frequency observed in field service reports for the 6ES7 326-1BK02-0AB0.

6. Pre-requisites for Diagnosis

• STEP 7 V5.5 SPx or TIA Portal V15.1+ with the F-Configuration Pack matching the F-library used in the safety program (Distributed Safety F-Configuration Pack or F-Configuration Pack for TIA Portal).
• Online connection to the S7 CPU (Ethernet or MPI/DP).
• Read access to the Process Historian / WinCC archive that recorded page 35340.
• Digital multimeter (true-RMS, ≥ 10 MΩ input) and a portable oscilloscope (≥ 20 MHz, isolated channels).
• Spare SM326F 6ES7 326-1BK02-0AB0 module with the same FW revision as the installed one.
• Pinout of the front connector (24-pin) and the ET200M wiring diagram.

7. Step-by-Step Diagnostic Procedure

Step 1 - Capture the diagnostic buffer

1. Go online with the CPU.
2. Right-click the CPU → Diagnostics Buffer.
3. Filter for events with ID 3842 and incoming/outgoing pairs.
4. Note the timestamps; correlate with the Process Historian event (page 35340) and with the plant operating state (motor start, valve actuation, etc.).

Step 2 - Decode the channel diagnostics

1. From the diagnostic buffer entry, double-click the line. STEP 7 opens the Module Information dialog.
2. Switch to the Channel Diagnostics tab.
3. Confirm that the active diagnostic is "Internal power supply failed" rather than "Sensor supply Vs1 short circuit" or "Sensor supply Vs2 short circuit".

Step 3 - Verify the 24 V supply on the module

1. Open the front cover of the SM326F.
2. Measure L+ / M on the front connector (pins 1 and 20) with the multimeter. Expect 24.0 V ± 5 % (20.4 V – 28.8 V).
3. Repeat on the second affected module and on the third (working) module.
4. Switch the multimeter to MIN/MAX and leave it latched through one full production cycle.

Step 4 - Oscilloscope capture

1. Connect a differential probe (or two passive probes with common ground referenced to the PE bar) across L+ / M at the front connector.
2. Set time base to 100 ms/div, vertical to 2 V/div, single-shot trigger on a falling edge below 20 V.
3. Run the plant. The dip that correlates with the OB82 entry is typically 18–19 V and 1–4 ms long.

Step 5 - Cross-check the ET200M power budget

1. Open the ET200M station in HW Config.
2. Sum the power consumption from backplane bus for every module on the same IM153-x.
3. Confirm the total does not exceed the IM153-x budget (1.5 A for IM153-2 BA, 2.0 A for IM153-2 AA, 5.0 A for IM153-2 BA / BX with the active backplane).

Step 6 - Compare the two affected modules with the good one

1. Physically swap a faulty module with the good one on the same ET200M.
2. Power-cycle the station.
3. Monitor the diagnostic buffer for 1 hour of normal operation.
4. If the diagnostic now appears on the new slot, the module is defective - replace it.
5. If the diagnostic stays with the original slot, the power environment is at fault - inspect wiring, fuses, and the upstream 24 V supply.

Step 7 - Inspect the power supply path

1. Trace the 24 V feed from the SITOP / power supply to the ET200M terminal block.
2. Check the protective device: miniature circuit breaker (MCB) rating, electronic fuse trip curve, or PTC reset state.
3. If the MCB is rated close to the calculated load (e.g. 4 A for a 3.2 A load), nuisance trips and foldback events are common during inrush. Replace with a C-curve MCB or an electronic fuse with 1.5× headroom.
4. Verify that the same 24 V feed is not shared with a DC motor drive or a heating element that has its own inrush characteristic.

Step 8 - Review grounding and shielding

1. Confirm that the ET200M subrack is bonded to the cabinet PE bar with a short, low-impedance strap.
2. Confirm that the PROFIBUS cable shield is clamped at both ends (EMC practice) and that the shielding bus is isolated from signal conductors at the sensor end.

8. Verification

After the corrective action, monitor for at least 24 hours of production with the diagnostic buffer and Process Historian both live.

9. Optional: Suppress the Diagnostic Interrupt

If the diagnostic is non-critical and reappears after root-cause analysis has been exhausted, the diagnostic interrupt can be masked in HW Config:

1. Open the SM326F in HW Config → Object Properties → Diagnostics.
2. Uncheck "Internal power supply failure".
3. Recompile and download the HW configuration.

An alternative is to keep the interrupt enabled but ignore it in the F-runtime group. The F-Configuration Pack provides a wrapper that filters incoming diagnostics before they reach the F-shutdown logic.

10. Sample OB82 Filter in SCL

The following SCL block separates "internal power supply" events from channel-level events so they can be archived independently.

{S7_block := 'FB500', S7_family := 'OB82_FILTER'}
FUNCTION_BLOCK FB500
VAR
    cntInternal : INT;  // counter of internal supply diagnostics
    cntChannel  : INT;  // counter of channel-level diagnostics
END_VAR
BEGIN
    // Called from OB82
    IF #OB82_INT_FAULT AND NOT #OB82_MDL_DEFECT THEN
        // Pure internal supply event - do not passivation
        cntInternal := cntInternal + 1;
        // write to log buffer or HMI tag for Process Historian
    ELSIF #OB82_PNT_INFO THEN
        // Channel-level event - hand to F-runtime group
        cntChannel  := cntChannel + 1;
    END_IF;
END_FUNCTION_BLOCK

11. Sample WinCC / Process Historian Tag Update

To record the OB82 timestamp and slot number in the historian, expose a small tag block from the OB82 handler:

// Tag name in WinCC: 'FDI_Slave3_PSU_EventCount'
// Update source: DB500.DBD0 (DWORD) - count
// 'FDI_Slave3_LastTime' - DATE_AND_TIME stamp
// 'FDI_Slave3_LastSlot' - INT slot number

On the Process Historian side, create a query that filters page 35340 by tag prefix FDI_Slave3_ and correlates with OB82 timestamp to surface the actual cause. This is more robust than reading the page number directly because the page number is an internal archive pointer.

12. When to Open a Siemens Support Request

Open a SIMATIC Support Request (SIOS) when one of the following is true after completing Steps 1–8:

• Both the slot and the module have been swapped and the fault follows the module, but the replacement module reproduces the fault within 24 h.
• The L+ supply is within spec and the power budget is in headroom, yet the diagnostic reappears.
• The diagnostic is intermittent and not correlated to any plant event that the engineering team can identify.
• Multiple ET200M stations on the same DP master report the same event at the same timestamp.

Attach the exported diagnostic buffer (CSV), the oscilloscope capture (CSV from the scope), and the HW Config screenshot of the affected ET200M station. Reference the F-library version and the STEP 7 / TIA Portal version. This will let the Siemens F-Application Support team reproduce the issue on a test bench.

13. Summary Field Procedure

1. Capture and decode the diagnostic buffer; confirm the text "Internal power supply failed" on the SM326F.
2. Verify L+ / M on the front connector with a multimeter and oscilloscope.
3. Audit the ET200M power budget and the upstream 24 V fuse / MCB rating.
4. Swap the suspect module with the known-good unit on the same station to localise the fault.
5. If the fault follows the module, replace it. If the fault follows the slot, repair the power environment.
6. Verify with a 24-hour live test that OB82 / OB121 is no longer triggered and that the Process Historian page 35340 stays clean.

This sequence covers the three most common root causes for the symptom pattern described in the source case: under-sized 24 V supply, mixed hardware revisions, and a defective F-DI module.

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