Diagnose Frozen FOUNDATION Fieldbus Values on a Redundant Compact FF Link

When FOUNDATION Fieldbus measurements freeze on a redundant S7-400H/Compact FF Link installation, capture link, DP and device diagnostics before switching CPUs or resetting the link. Siemens documents a factory-reset function that erases stored Compact FF Link parameters and stops cyclic FF communication; on redundant stations it resets both links. The source does not establish why a CPU switchover temporarily restores values, so use the documented diagnostics to localize the failure.

What the symptom does and does not show

The source reports that FF measurements occasionally freeze and that an S7-400H master/standby CPU switch restores them, with no visible error. It provides no timing, LED state, diagnostics, FF device identity, firmware or configuration. This observation does not identify whether the cause is the FF device, FF segment, Compact FF Link, DP master path or AS processing.

What a Compact FF Link reset actually does

Siemens Compact FF Link Operating Instructions (05/2022) document “Restore default state” via SIMATIC PDM. It deletes parameters stored in the link's flash memory and stops cyclic FF communication; on redundant stations both Compact FF Links are reset. This is a factory-reset operation, not a harmless runtime refresh. Do not use it as a routine response to frozen process values.

Controlled diagnostic sequence

During the next occurrence, record the time, affected FF tags/devices, input status, DP/FF diagnostics and LEDs on both links before switching anything.
Determine whether the link is redundant and confirm both links' article number, hardware revision and firmware match; check the configured DP master and FF segment.
Compare the frozen device values/status with other devices on the same segment and with AS/DP diagnostics to localize the failure domain.
Follow the diagnostics and recovery procedure for the identified device/link/segment fault; do not issue factory reset without a reviewed re-commissioning plan.
If a CPU failover is required by the site response procedure, record what changed and verify data recovery and redundancy afterward.

Expected redundancy behavior

The 2022 Compact FF Link instructions state that, without changes to the FF configuration, input/output states are retained during redundant link switchover and the maximum switchover time is 70 ms. That device-specific switchover statement does not explain an intermittent freeze or justify switching the redundant AS CPUs.

Verification

Confirm that affected FF inputs update with valid status, diagnostics are clear, and the event does not recur during the agreed observation period. Verify both redundant links return to the intended active/standby condition using the documented indications; do not assume one refreshed value proves sustained recovery.

Limitations and Operational Precautions

The ticket has no replies and lacks a diagnostic record, device list, Compact FF Link firmware and PCS 7 version. No documented operation was found that safely “resets” only frozen values while preserving configured communication. Factory reset disrupts cyclic FF traffic and resets both links in redundant stations.

Engineer FAQ

Should I use SIMATIC PDM Restore default state when a measurement freezes? Not as a routine reset. It erases stored link parameters and stops cyclic FF communication; both links are reset in a redundant station.

Does link redundancy guarantee no interruption? The manual gives up to 70 ms under the stated no-configuration-change condition; it does not guarantee resolution of unrelated device or network faults.

Technical References

Compact FF Link Operating Instructions, 05/2022, A5E35651552-AC, sections 4.2, 9.1 and 9.4 (redundancy and Restore default state). Official Siemens PDF opened and read.
PCS 7 FOUNDATION Fieldbus Commissioning Manual, 05/2017, A5E36184959-AA. Search result confirms redundant DP master requirement for Compact FF Link on S7-400H; direct PDF retrieval was not available in this review.