S7-300 Software Redundancy DB Area Length Error: FC100 SWR_START

Problem Description

A redundant pair of SIMATIC S7-315-2DP CPUs configured with the Siemens Software Redundancy (SWR) blocks transitions unexpectedly into STOP mode shortly after FC100 "SWR_START" initializes the redundancy link. The diagnostic buffer reports an "Area length writing error" (German: Bereichslängenfehler beim Schreiben) tied to a data block operation inside the SWR runtime. The fault appears only when the engineer changes DB_NO_LEN (the number of redundant user data blocks) from 1 to any larger value such as 2, 4, or higher.

When DB_NO_LEN = 1 the system runs for hours or days without incident. The instant a second redundant DB is referenced (or when the project is downloaded with DB_NO_LEN > 1), the standby CPU goes into STOP within one or two redundancy cycles and the active CPU follows on the next link update.

Affected Hardware and Software

The fault is independent of the CP type and reproduces on both the integrated DP interface of the 315-2DP and on external CP 342-5 modules. It is also independent of the VERB_ID used for S7 connection number 1.

FC100 SWR_START Parameter Reference

The following parameter assignment corresponds to the field program that triggered the STOP:

Root Cause Analysis

The "area length writing error" is raised by the SWR library while copying redundant user DBs from the active CPU to the standby. There are three independent root causes that can trigger it; in this incident two of them were present simultaneously.

Cause 1 — DB length below minimum

Siemens documentation states that every DB declared redundant via DB_NO and DB_NO_LEN must have a length of more than 2 words (i.e. > 4 bytes). A DB containing only a single BOOL or a few bytes under that threshold causes FC100 to perform an out-of-range SFC write. With DB_NO_LEN = 1 the engineer happened to assign a DB that just exceeded the threshold; the moment the second redundant DB was added it was sized below 4 bytes and triggered the fault.

Cause 2 — Stale DBs in the CPU image

FC100 "SWR_START" re-initializes DB_WORK_NO, DB_SEND_NO and DB_RCV_NO at each call. If the actual on-line image of those DBs was created in a previous program version with a different structure (e.g. older SWR library revision), the residual length on the CPU does not match the offline project. SWR's copy routine then writes past the runtime DB and the area-length check fires. The standard fix is to delete DB1, DB2 and DB3 on the CPU before downloading the new FC100 instance, then download the blocks in this order: FC100 → DB1 → DB2 → DB3 → DB5 (instance of FB 101) → FB 101 → remaining user DBs.

Cause 3 — Non-contiguous DB numbering

When DB_NO = 10 and DB_NO_LEN = 2, the SWR library expects DB10 and DB11 to both exist on both CPUs. If only DB10 is present in HW Config or in the offline program, the copy step writes to DB11 which is not allocated, and the CPU enters STOP. Verify in STEP 7 via PLC > Accessible Nodes that DBs DB_NO through DB_NO + DB_NO_LEN - 1 are present and identical on station A and station B.

Step-by-Step Resolution

1. Back up both projects. Archive the active project via File > Archive and read the on-line program from station A and station B to a separate project. Note the current SWR library version under PLC > Module Information > Blocks.
2. Install OB 121 (Programming Error OB). On each CPU, download OB 121, OB 122 (I/O access error), OB 80 (time error), OB 82 (diagnostic interrupt), and OB 85 (program cycle error). With these OBs in the CPU, transient access errors no longer drop the PLC into STOP; the user program is given a chance to log the error via SFC 36-39 instead.
3. Verify each redundant DB length > 4 bytes. In SIMATIC Manager, right-click each DB in the block folder, choose Properties and confirm the length shown in the Length in the work memory field is greater than W#16#0004. Recommended safe size: 16 bytes or more, even if most of it is unused.
4. Delete the three SWR system DBs in the CPU. In Accessible Nodes, mark DB1, DB2, DB3 (the values of DB_WORK_NO, DB_SEND_NO, DB_RCV_NO) and choose Delete. Do not delete DB5 (the instance of FB 101) — it will be re-written by the next download.
5. Re-download blocks in correct order. From the offline project, download to station A first, then station B:
   1. FC 100 "SWR_START"
   2. DB1, DB2, DB3 (system DBs, freshly created)
   3. FB 101 "SWR_DIAG" and its instance DB5
   4. User DBs in the range DB_NO … DB_NO + DB_NO_LEN - 1
   5. OB 100, OB 1, OB 35 (or whatever cycle OB triggers FC100)
6. Confirm DB contiguity. In Accessible Nodes > station A > Block folder, sort by number and verify that no DB number is missing between DB_NO and DB_NO + DB_NO_LEN - 1. Repeat for station B.
7. Cold restart (MRES). Perform a full restart on both stations so that OB 100 rebuilds the SWR working DB from scratch. Power down the standby first, MRES the active, restart, wait for standby to be promoted, then power up the standby and let it resynchronize.

Memory Layout and DB Numbering Rules

The SWR library claims three ranges of DB numbers on each CPU. They must not overlap with each other or with user DBs:

Flag bytes MB_NO through MB_NO + MB_LEN - 1 (in this case MB40..MB109) are also reserved. Any user write into this range will be silently overwritten by the SWR runtime; if the program reads flags that should have been written, the value will appear stuck. Avoid using MB40..MB109 for HMI handshakes or interlocking logic.

Error Code Reference (RETURN_VAL / EXT_INFO)

Diagnostic and Error OBs

Loading the full set of error OBs is best practice for any SWR system and is the single most effective way to keep a transient STOP from becoming a production outage:

• OB 80 – Time error. Triggered when OB 1 is overrun or by SWR's internal watchdog.
• OB 82 – Diagnostic interrupt. Captures IM 153-3 hot-swap events.
• OB 85 – Program cycle error. Reports a missing or wrong-length DB, including the SWR area-length fault.
• OB 121 – Programming error. Catches the access error at the instruction level (e.g. SFC 20 BLKMOV with a mismatched length).
• OB 122 – I/O access error. Triggered if SWR reads from a slave that is currently being swapped.

Each OB can call SFC 6 RD_SINFO to read the start event information and SFC 17 ALARM_SQ or SFC 107 ALARM_DQ to push a message to the HMI / WinCC. Logging the offending DB number, block number, and SW:OB85 fault code on every entry makes future root cause analysis far faster.

Verification and Commissioning Checks

1. Diagnostic buffer clean – On both CPUs, PLC > Module Information > Diagnostic Buffer should show no entries newer than the most recent MRES, apart from expected partner-link events.
2. FB 101 SWR_DIAG status – Call FB 101 in a low-priority OB (e.g. OB 82) and read the STATE, MASTER, STBY and LINKUP outputs. LINKUP = TRUE within 1 second of FC 100 completing on both stations.
3. Forced failover – Stop the active CPU. Within one redundancy cycle the standby must take over. Verify all PAA_FIRST..PAA_LAST outputs (QB0..QB5 in the example) hold their last value throughout the failover.
4. DB mirroring loop test – Write a known pattern (e.g. W#16#1234) into DB10 on the active CPU. Within one SWR cycle the same pattern must appear in DB10 on the standby. Monitor/Modify is the easiest tool for this.
5. Non-redundant channel – If DB_A_B_NO_LEN is non-zero, write to DB_A_B_NO on station A and confirm readback on station B. Ensure the two DBs are different numbers (here both are 112 — the example shows this as disabled with length 0, which is correct).

Common Pitfalls and Edge Cases

• Mixing SWR library revisions – Station A and B must run the same major SWR library version. A V1.x block on one and V2.x on the other causes a size mismatch in the system DBs and the same area-length error. Check PLC > Module Information > Block list for the timestamp on FC 100.
• Optimized DB access (S7-300 does not support this anyway) – On S7-300 the access mode is fixed to "standard"; however, when migrating blocks from an S7-400 project, an "optimized" attribute left in the S7-300 DB can corrupt the length. Always re-create the DBs in the S7-300 project.
• Reusing DB112 for both directions – The example uses DB_A_B_NO = DB_B_A_NO = DB112 with DB_A_B_NO_LEN = 0. That is allowed and effectively disables the non-redundant channel. Setting DB_A_B_NO_LEN > 0 with the same number on both directions is illegal and triggers the same STOP.
• DP master sys ID mismatch – DP_MASTER_SYS_ID = 1 must match the index in HW Config (not the Profibus address of the master itself). On the integrated 315-2DP this is usually 1; on a CP 342-5 it can be 2 or higher.
• Slot for SWR_START – Call FC 100 once in OB 100 (startup) and again in OB 1 (cycle). Calling it more than once per cycle causes the library to re-initialize and may trigger the area-length error on already-written DBs.
• MPI vs Profibus for the link – If MPI_ADR is used for the partner's MPI/DP port address, verify it physically matches the partner's port being used. Using 4 for a CP 342-5 port that is actually 5 will keep the link up intermittently and confuse the diagnostic.

Extended Sizing Reference

For planners who need to size the on-line memory of an SWR CPU, the rough formula is:

Total SWR DB memory (bytes) =
    sizeof(DB_WORK_NO) +
    sizeof(DB_SEND_NO) +
    sizeof(DB_RCV_NO) +
    sum(sizeof(DB_i) for i in [DB_NO .. DB_NO + DB_NO_LEN - 1]) +
    sum(sizeof(DB_i) for i in [IEC_NO .. IEC_NO + IEC_LEN - 1]) +
    sizeof(DB_A_B_NO) + sizeof(DB_B_A_NO)

The 315-2DP has 128 KB of work memory and 256 KB of load memory (for 6ES7 315-2AG10). Reserve at least 20% of work memory for SWR overhead — typically 24 KB is enough for the configuration in the example. A 315-2DP can mirror up to approximately 8,000 bytes of DBs per redundancy cycle; if your application is close to that, consider splitting across multiple DB_NO/DB_NO_LEN windows or upgrading to S7-300 SWR V2.x, which supports a longer per-cycle payload.

Related Siemens Documentation

• SIMATIC S7-300 Software Redundancy – Function Manual (entry ID 1137086)
• S7-300 CPU 31x-2 DP Operating Instructions (entry ID 12416006)
• S7-300/400 Standard and System Functions reference (entry ID 22254723)
• Software Redundancy for S7-300 and S7-400 – Library download and release notes (entry ID 30805151)
• ET 200M IM 153-3 manual (entry ID 10806878)