Resolving SM 326F F-DO Module Periodic Output Pulses

Problem Overview

When wiring Siemens SM 326F DO 8xDC24V/2A P-M switching safety-related output modules to external optocouplers or other light loads, integrators commonly observe the load flickering or pulsing on a roughly 10-second cycle. The behavior persists even when the F-CPU is in STOP, when the safety program is bypassed, or when standard writes to the output image are explicitly disabled in the user program. The only condition under which the pulsing stops is the physical removal of the SM 326F from the hardware configuration (HW Config) of the F-CPU.

This is one of the most frequently misdiagnosed issues on S7-300F and ET 200M safety installations because the symptom mimics a hardware fault or noisy power supply, but it is in fact a mandatory and documented diagnostic function of fail-safe digital output modules. The pulsing is not a software defect, a wiring error, or a defective module.

Affected Hardware Identification

The behavior is specific to fail-safe (F-) digital output modules in the SIMATIC S7-300 and ET 200M families. The configuration discussed in the field report uses the following components:

SM 326F modules in the 6ES7326-2BFxx and 6ES7326-1BFxx families all implement internal dark/light test cycles. The 8-channel 24V/2A P-M switching variant (6ES7326-2BF10-0AB0) and the older 6ES7326-1BF02-0AB0 exhibit the test pulse behavior. The 10-channel 6ES7326-1BK02-0AB0 variant is functionally similar but is sometimes confused with the 8-channel part because of the close order number prefix. All three are documented in the S7-300 Module Data manual, entry ID 8859629.

The SM 326F must be paired with an F-CPU in the S7-300F, S7-400F, ET 200S, or ET 200M F-system family. A standard S7-300 CPU cannot host a fail-safe module in safety mode, and the diagnostic pulse is part of the F-runtime running on the F-CPU, not just firmware on the SM 326F itself.

Root Cause: F-Module Light Test Mechanism

Every fail-safe digital output channel in a SIMATIC F-system is implemented internally as two redundant output drivers. On the SM 326F the architecture is a 1002 (one-out-of-two) configuration, meaning two independent semiconductor switches drive the same output terminal and the F-runtime compares their behavior continuously. To detect faults such as:

• Wire break on the load circuit
• Short circuit of the output to P24 (cross-circuit high)
• Stuck-at fault on either of the two internal drivers
• Soldering or contact failure in the load path
• External cross-circuit between two output channels

...the F-runtime instructs the SM 326F to perform a periodic light test (also called the readback test or dark test) on each channel. The module briefly de-energizes the output for a few hundred microseconds, then reads the actual voltage at the output terminal. If the readback does not match the expected level — for example, the output reads high when the test forces it low — the module reports a wire break or stuck-at diagnostic to the F-CPU, which in turn triggers a diagnostic interrupt (OB 82) and, if configured, passivates the affected channel.

This test is governed by the F-CPU's safety program and the F-parameter set downloaded to the SM 326F. It is documented in detail in the S7-300 Automation System, F-CPU manual, entry ID 19026151, in the section titled "Light test / dark test of fail-safe outputs" within the chapter on F-I/O. Siemens specifically notes that the light test cannot be fully disabled while the F-CPU operates the module in safety mode, because the test is the mechanism by which the module achieves its declared diagnostic coverage (DC) and SIL rating.

Light Test Pulse Timing and Waveform

The default test cycle for an SM 326F channel is approximately 10 seconds under typical configuration. The pulse itself is short, on the order of 200 to 500 microseconds, but the exact value depends on:

• The F-parameter "Test interval" configured in HW Config or in the TIA Portal safety editor
• The channel's "Discrepancy time" setting (the window in which the two redundant drivers must agree)
• The number of channels being polled (the cycle is staggered across the 8 outputs to avoid simultaneous dips)
• The configured switching frequency for the load (continuous vs. pulsed operation)

The pulse is staggered across channels; you will not see all 8 outputs dip at the same instant. Instead, you will see one channel pulse approximately every 1 to 2 seconds, which gives the appearance of continuous activity when viewing the output status at a coarse time scale on a panel meter or slow logic indicator. The 10-second figure is the per-channel cycle, not the per-module cycle.

The pulse is too short to actuate most electromechanical loads (contactors, solenoid valves, indicator lamps) but is long enough to be detected by:

• Optocouplers with low input current and fast response
• High-impedance voltage measurement (oscilloscope, logic analyzer, panel meter with >1 MHz bandwidth)
• Solid-state relays (SSRs) with sub-millisecond switch-off time
• LED indicators connected directly to the output (visible as brief flicker)

Standard SM 322 DO modules do not exhibit this behavior, because they are not fail-safe and have no diagnostic test pulse requirement. This is the single most useful differentiator when troubleshooting: replace the SM 326F with an SM 322 in the configuration and the pulsing stops, confirming the F-runtime is the source.

P-M Switching Architecture

The "P-M switching" suffix on the 6ES7326-2BF10-0AB0 means the module can drive loads in either high-side (P-switching) or low-side (M-switching) configuration per channel. Internally, both driver halves are present and tested, and the light test is performed on whichever half is the currently active switching element.

If the load is wired between the output terminal and M (the typical P-switching case), the light test briefly turns off the high-side driver and verifies the output terminal drops to near 0 V. If the load is wired between P and the output terminal (M-switching), the test briefly turns off the low-side driver. In both cases, the brief dip is what the customer-side optocoupler is observing.

The customer's optocoupler input typically draws 5 to 20 mA, well within the SM 326F's 2 A per-channel rating but well below the 0.5 A minimum load Siemens recommends for clean switching. Optocouplers are particularly sensitive to the test pulse for three reasons:

1. Their input impedance is high, so the small current change during the test pulse is significant relative to the operating current.
2. Their response time is microseconds, so 300 us is plenty of time for the LED inside to extinguish and re-illuminate.
3. Most optocouplers have no input filter capacitor at the LED, so the pulse passes directly to the next stage.

If the optocoupler is part of a downstream safety isolation barrier, the brief output dip may appear at the next stage as a momentary "off" state. This is normal and is the reason the original poster's optocoupler was observed "flashing." The flash is the F-system working correctly, not a fault.

Safety Mode vs Standard Mode Behavior

Siemens distinguishes two operating modes for the SM 326F:

The key question the field report raises — "is there a possibility to turn off these short-circuit tests, at least for testing purposes?" — has a clear answer from the Siemens F-system documentation: in safety mode, no. The light test is not an option that the integrator can disable; it is part of the F-module's contract for diagnostic coverage. Removing the light test would drop the module from SIL 3 to a lower category and would not be certifiable under IEC 61508 SIL 3 or ISO 13849-1 Category 4.

If the application does not require safety functionality, the simplest workaround is to use a standard SM 322 DO 8xDC24V/0.5A (e.g. 6ES7322-1BF01-0AA0) instead of the SM 326F. The standard module has no test pulse, costs less, and behaves like a conventional PLC output. This is the recommended approach for non-safety demonstration hardware and is documented as a valid substitution in the SIMATIC Safety Configuring and Programming manual, entry ID 54110126.

Diagnostic Codes and Buffer Entries

The SM 326F reports a defined set of diagnostic events to the F-CPU. The events most relevant to a pulsing output are listed in the table below; all entries are documented in the S7-300 Module Data manual (entry ID 8859629) and in the F-CPU manual (entry ID 19026151).

Reading the diagnostic buffer of the F-CPU is the fastest way to confirm the light test is running and that the channel is not actually faulty. If the buffer contains no entries for the SM 326F, the test is being executed without finding faults, which is the expected steady-state behavior. If the buffer is full of wire break entries, the load wiring is the problem and the light test is correctly reporting it.

Confirming the Diagnosis

Use this sequence to confirm the pulsing is a light test and not a true fault:

1. Check the module order number. Only fail-safe modules (SM 326F, SM 336F, ET 200S F-modules, ET 200SP HF modules) execute light tests. Standard SM 322 / SM 332 / SM 334 modules do not. If the module is a 6ES7326-xxx order number, it is F-class.
2. Monitor the output with an oscilloscope on a single channel. You should see the level remain steady at 24 V with periodic dips to 0 V of duration 200 to 500 us. If you see longer dips, irregular pulses, or noise patterns, the issue is not the light test.
3. Trigger a wire break and observe the diagnostic interrupt. Disconnect the load on one channel. The SM 326F should report a wire break to the F-CPU and passivate the channel within one test cycle. If the diagnostic interrupt (OB 82) does not fire, the test is not running as expected and the module may be in standard mode or have a parameter error.
4. Read the diagnostic buffer of the CPU. Look for entries containing "wire break," "short circuit," or "test pulse" related to the SM 326F. Entry IDs vary by firmware but the codes are documented in entry ID 8859629.
5. Verify the F-shutdown group configuration. An F-shutdown group that is incorrectly assigned can cause a channel to be commanded off periodically by the F-runtime, which is a different fault mode than a test pulse. Confirm the F-shutdown group assignment in the safety program using the F-Configuration Pack tools (entry ID 15208817).
6. Temporarily swap the SM 326F for an SM 322. If the pulsing stops with the standard module, the F-runtime is confirmed as the source. The original SM 326F can be reinstalled and the workaround applied.

If all of the above confirm the light test, the integrator can be confident the system is operating as designed. The pulsing is not a fault and the safety function is intact.

Configuration in STEP 7 and TIA Portal

If the hardware must remain an SM 326F but the application does not require safety, the module can be operated in standard (non-safety) mode by removing it from the F-runtime configuration. In STEP 7 V5.5 with the F-Configuration Pack installed:

1. Open HW Config and locate the SM 326F in the rack.
2. Open the module's object properties (right-click -> Object Properties).
3. Navigate to the "Parameters" tab. Set the module operating mode to "Standard" (the field is labeled differently in older STEP 7 versions but is found in the F-parameters group).
4. Confirm that the F-CPU is still running in standard mode, not F-mode. A CPU that is licensed and configured for F-mode will refuse to operate the module in standard mode and will report an inconsistency in the diagnostic buffer.
5. Recompile and download the hardware configuration.

For TIA Portal V15 and later, the equivalent procedure is to open the device view, select the SM 326F, and in the properties under "F-parameters" set the operating mode. The TIA Portal safety configuration is documented in the SIMATIC Safety Configuring and Programming manual, entry ID 54110126. The same entry also covers the discrepancy time, test interval, and F-shutdown group assignment for safety-mode operation.

For PROFINET-based F-stations (S7-315F-2PN/DP or S7-317F-2PN/DP with PROFINET F-modules), the test interval and discrepancy time are configured in the device view of the PROFINET device, not in the central rack. The PROFINET variant of the SM 326F is 6ES7326-2BF40-0AB0 or 6ES7326-2BF41-0AB0, which behaves identically to the PROFIBUS DP variant in terms of the light test.

Hardware Workarounds for Non-Safety Testing

If the integrator needs the SM 326F for genuine safety but the test pulse interferes with a non-safety demonstration load (for example, an optocoupler on a customer-facing panel), several hardware-level mitigations are possible without violating the safety function:

The capacitor-based filtering is the most common in the field. A 47 uF electrolytic at 35 V across the optocoupler input typically suppresses the 300 us pulse into a sub-1 V dip, which most optocouplers ignore. The trade-off is a slight delay (typically 1-5 ms) in optocoupler turn-on and turn-off, which is irrelevant for status indication but unacceptable for fast safety signaling. The time constant is approximately t = R x C, where R is the input impedance of the optocoupler and C is the filter capacitor; for an optocoupler with 1.5 kOhm input impedance and a 47 uF capacitor, t = 70 ms, which is well above the 500 us pulse and provides clean suppression.

Verification Procedure and Field Commissioning Checklist

After applying any of the workarounds above, verify the system is operating as intended:

1. Power cycle the station and confirm the F-CPU comes up in RUN with no SF (safety fault) LED.
2. Force a wire break on a test channel and confirm the F-CPU reports a wire break diagnostic and passivates the channel. The light test must still be running for this to be detected.
3. Restore the channel and acknowledge the passivation. The F-CPU should reintegrate the channel within one test cycle (10 s or less for the 6ES7326-2BF10-0AB0).
4. With the workaround in place, observe the optocoupler (or other load) for at least 60 seconds. No visible flicker or relay click should occur.
5. Capture an oscilloscope trace of the SM 326F output with the workaround in place. The trace should show 24 V continuous, with the light test pulse attenuated to a sub-1 V ripple.
6. Re-run the F-CPU's safety diagnostics via the safety view in STEP 7 or TIA Portal and confirm no new fault entries.
7. Confirm the discrepancy time, test interval, and F-shutdown group assignment match the safety validation report.

If the system passes all checks, the workaround is in place and the safety function is preserved. Document the workaround in the safety validation report and note that the SM 326F is still operating in F-mode and the light test is still running — the load simply no longer responds to it.

For every S7-300F or ET 200M station with SM 326F outputs, include the following in the commissioning documentation:

• Module order number (MLFB) and firmware version
• F-CPU order number and firmware version
• F-Configuration Pack version installed in STEP 7 or TIA Portal
• Test interval configured per channel (default is 10 s; configurable 1 to 100 s)
• Discrepancy time setting (for redundant channels, typically 100-500 ms)
• F-shutdown group assignment for each channel
• Light test pulse width measured at the output terminal (oscilloscope trace attached)
• List of loads that are sensitive to the test pulse and the workaround applied
• Confirmation that the workaround does not violate the safety function (signed by the safety engineer)

The default test interval of 10 s is a Siemens design choice that balances diagnostic coverage (shorter = better detection) against output disturbance (longer = less load disturbance). For applications where the test pulse is problematic, the interval can be lengthened up to 100 s in the F-parameter set, but the safety validation report must be updated to confirm the new diagnostic coverage still meets the required SIL or PL category. Lengthening the interval reduces the probability of fault detection within a given mission time, which is part of the PFHd (probability of dangerous failure per hour) calculation required by IEC 61508.

Frequently Asked Questions