S7-300 CPU 312C Data Logging: OB35 FIFO Buffer to SIMATIC NET OPC

1. Problem Overview: Logging Position Data from a CPU 312C

The hydraulic-cylinder position-regulating application described in the source uses a Siemens SIMATIC S7-300 CPU 312C as the controller. The CPU runs an SFB 41 "CONT_C" continuous PID loop against a position transducer (typically a 4-20 mA / 0-10 V analog input or SSI absolute encoder), and the operator needs a sampled log of the loop variables for later review in Excel. Sample period is 100 ms or faster; total captured length is on the order of 10 seconds (the source explicitly states 100 samples in 10 s, i.e. one sample per 100 ms).

The CPU 312C is the most resource-constrained member of the S7-300 compact family. Before any data-logging architecture is committed to, the engineer must understand the limits:

Reference: SIMATIC S7-300 CPU 312C / 313C Manual (Siemens Online Support).

2. Architecture: From Cylinder Sensor to Excel Cell

The reference implementation follows a five-stage pipeline:

1. Analog acquisition - Position transducer on AI channel of an SM 331 (e.g. 6ES7331-7KF02-0AB0) or onboard DI fast counters if an SSI / incremental encoder is used.
2. PID loop - SFB 41 "CONT_C" runs in OB35 (100 ms), outputs LMN (manipulated variable) to the proportional valve via SM 332 AO.
3. Acquisition buffer - A dedicated FIFO DB captures PV (process value), SP (setpoint), LMN, and a millisecond timestamp on every OB35 pass.
4. OPC mirror - When the FIFO reaches its trigger length, SFC 20 "BLKMOV" copies the buffer to a second DB exposed to the SIMATIC NET OPC server.
5. Excel sink - The OPC server is read by either an Excel VBA macro (OPC DA Automation wrapper) or by a third-party OPC client (MatrikonOPC, Kepware, Softing) that exports CSV.

3. Configuring OB35 for 100 ms Cyclic Sampling

The S7-300 family includes eight cyclic interrupt OBs with priorities 2-24 (OB 30 through OB 38). OB35 is the default 100 ms OB and is the natural choice for hydraulic position loops, which typically have a closed-loop bandwidth of 1-3 Hz.

OB35 properties to configure in HW Config > CPU 312C Properties > Cyclic Interrupts:

Reference: S7-300 Programmable Controller - CPU 31xC Technological Functions Manual.

3.1 Programmatic retrieval of OB35 execution count

To timestamp each sample without an external RTC, read the CPU system clock via SFC 1 "READ_CLK" and store the date/time in BCD format inside the FIFO entry. A tighter alternative is to use the OB35-PY (start time of OB35 invocation in milliseconds since last cold restart).

// STL - OB35 preamble
CALL SFC 1 "READ_CLK"
  RET_VAL := MW 100
  CDT      := DB100.SYSTIME[0]      // DATE_AND_TIME, 8 bytes

4. Designing the FIFO Data Block

Two DBs are required:

• DB100 "FIFO_BUF" - 8 KB maximum, internally organized as an ARRAY[1..N] of a UDT or STRUCT. This is the acquisition ring written by OB35.
• DB200 "OPC_MIRROR" - Same layout as DB100, exposed to the OPC server. The acquisition code never touches this directly; SFC 20 BLKMOV copies from DB100 to DB200 at the end of each capture window.

4.1 Sample UDT

// UDT 10 "SAMPLE"
TYPE UDT 10
STRUCT
  t_ms        : DWORD;        // OB35 PY time-stamp in ms
  t_ibt       : DATE_AND_TIME;// SFC 1 wall-clock (8 bytes BCD)
  pv_mm       : REAL;         // Process value (position, mm)
  sp_mm       : REAL;         // Setpoint
  lmn_pct     : REAL;         // Manipulated value -100..100 %
  err_v       : REAL;         // SP - PV
  pad         : WORD;         // 16-bit padding to DWORD boundary
END_STRUCT;
END_TYPE

One sample = 8 + 8 + 4 + 4 + 4 + 4 + 2 = 34 bytes (padded to 36 if the compiler inserts 2 alignment bytes). Plan capacity accordingly: for 100 samples = 3.4 KB; for 500 samples = 17 KB (over the 8 KB DB limit, so use two DBs).

4.2 Sizing math

The source confirms the typical workload: 100 samples in 10 seconds, well within the 8 KB DB ceiling.

5. Implementing the FIFO (TI-S7 Converting Blocks)

The Siemens STEP 7 Standard Library > TI-S7 Converting Blocks contains purpose-built FIFO functions, originally delivered with the technology CPUs (CPU 31xT, FM 350). They can be installed on any S7-300 / S7-400 and accept ANY pointer inputs, which is exactly the use case described in the source ("treat it as a FIFO stack, use the table functions FIFO-Load").

The two relevant calls are:

• FIFO_Load - pushes a value into the FIFO; returns status EMPTY / FULL / OVERFLOW in RET_VAL and the table-handle word.
• FIFO_Unload - pops the oldest value from the FIFO.

Reference: SIMATIC S7-300 / S7-400 Standard and System Functions - Reference Manual.

5.1 OB35 fragment - push to FIFO

// OB35 - capture PID snapshot
// Inputs from SFB 41 instance DB (e.g. DB50)
      L     DB50.DBD 24            // PV_IN scaled position, REAL
      T     MD 200                 // snapshot temp REAL
      L     DB50.DBD 28            // SP_INT
      T     MD 204
      L     DB50.DBD 56            // LMN
      T     MD 208
      L     DB50.DBD 0             // I_ITVAL (effective error)
      T     MD 212

      CALL SFC  1                  // READ_CLK
        RET_VAL := MW 220
        CDT     := MD 224

// Build one UDT 10 record at MW 200..MW 235
// Push it to DB100 via FIFO_Load
      CALL "FIFO_Load"
        DB_NO   := 100
        I_DATA  := P#M 200.0 BYTE 36
        RET_VAL := MW 250

      A     M 252.0                // status.FULL bit
      JC    COPY_AND_RESET

6. SFC 20 BLKMOV: Snapshot to OPC Mirror DB

When the FIFO returns FULL, OB35 transfers the entire DB100 to DB200 in a single block-move. SFC 20 "BLKMOV" copies up to 64 KB in one call on the S7-300, but is interruptible by higher-priority OBs. Because the source DB is in the work-memory page table, BLKMOV is fast (a 4 KB block typically completes in < 1 ms on a CPU 312C).

COPY_AND_RESET:
      CALL SFC 20                  // BLKMOV
        SRCBLK  := P#DB100.DBX 0.0 BYTE 3400
        RET_VAL := MW 260
        DSTBLK  := P#DB200.DBX 0.0 BYTE 3400

      JC   ERR_COPY                // RET_VAL 0=OK, !=0 error

Error codes:

7. SFC 21 FILL: Resetting the Acquisition Buffer

Immediately after BLKMOV, the FIFO must be cleared so the next 10-second capture window starts from zero. SFC 21 "FILL" writes a constant pattern across the entire DB.

      CALL SFC 21                  // FILL
        BVAL    := P#M 0.0 BYTE 4   // zero-pattern (MW0 = 16#0000)
        RET_VAL := MW 262
        BLK     := P#DB100.DBX 0.0 BYTE 3400

// Reset OB35 cycle counter (custom)
      L     0
      T     DB100.DBD 3400         // sample counter

8. SIMATIC NET OPC Server Configuration

The PC side requires:

1. A Siemens communications processor (CP 5611 PCI for PROFIBUS / MPI, or CP 5612 for SOFTNET-DP / SOFTNET-PB, or CP 1613 for Industrial Ethernet).
2. SIMATIC NET PC software (currently V18 / V19 / V20 depending on Windows 10 / 11). The install contains:
   • NCM PC Manager (PC station configuration)
   • OPC Scout V20 (browser for items)
   • OPC DA Server (the actual S7 data source)
   • S7 OPC Reducer (item filter)
3. Configured S7 connection: Station > Add > S7 Connection, partner = CPU 312C rack/slot, slot of OPC server = 1 (default).
4. OPC items: DB200,BYTE 0,N exposed as an array of UDT 10 equivalents.

Reference: SIMATIC NET OPC - FAQ and configuration overview (Siemens Online Support entry ID 23485970).

8.1 PC Station topology

[PC Station "DATA_LOGGER"]
   |-- [CP 5611]            Index 1 - PROFIBUS (MPI can run on CP 5611 with adapter)
   |-- [OPC Server]         Index 2 - S7 protocol
   |-- [Application]        Index 3 - WinCC / Excel / third-party
S7 Connection:
   Local End  = OPC Server
   Partner    = CPU 312C @ MPI 2 (or PROFIBUS address 2)
   Slot       = 2
   Rack       = 0

8.2 Configuring items in OPC Scout

1. Open OPC Scout V20.
2. Connect to OPC.SimaticNET.
3. Add a group "PID_LOG".
4. Add an item per DB200 field. Because the array is contiguous, expose it as S7:[DATA_LOGGER]DB200,BYTE0,3400 for bulk read, or expose per-offset items S7:[DATA_LOGGER]DB200,REAL0 for individual access from VBA.

9. Excel / VBA Integration via OPC DA Automation

Excel can read the OPC items in two common ways:

1. OPC DA Automation wrapper - the Siemens-supplied opcdaauto.dll exposes the OPC DA 2.0 spec as a COM object.
2. OPC Foundation .NET Standard - using OPCFoundation.NetStandard.Opc.Ua.Client or the UA COM wrapper if the gateway supports UA.

9.1 VBA skeleton (OPC DA)

' Add reference: OPC DA Automation 2.0
Option Explicit

Dim WithEvents opcServer As OPCServer
Dim WithEvents opcGroup  As OPCGroup
Dim opcItem                As OPCItem

Sub Connect_OPC()
    Set opcServer = New OPCServer
    opcServer.Connect "OPC.SimaticNET"
    Set opcGroup = opcServer.OPCGroups.Add("PID_LOG")
    opcGroup.IsActive = True
    opcGroup.IsSubscribed = True
    opcGroup.UpdateRate = 200

    Set opcItem = opcGroup.OPCItems.AddItem( _
        "S7:[DATA_LOGGER]DB200,REAL0", 1)

    ' Bulk-add all 3400 bytes if needed via AddItems()
End Sub

Private Sub opcGroup_DataChange( _
        ByVal TransactionID As Long, _
        ByVal NumItems As Long, _
        ClientHandles() As Long, _
        ItemValues() As Variant, _
        Qualities() As Long, _
        TimeStamps() As Date)
    Dim i As Long
    Cells(1, 1) = "t_ms"
    Cells(1, 2) = "pv_mm"
    Cells(1, 3) = "sp_mm"
    Cells(1, 4) = "lmn_pct"
    For i = 1 To NumItems
        Cells(i + 1, 1).Value = ItemValues(i)
    Next i
End Sub

9.2 CSV export from Excel

Once the spreadsheet contains the 100-sample window (rows 2-101, columns A-D), save it with ActiveWorkbook.SaveAs "C:\Logs\PID_" & Format(Now,"yyyymmdd_hhnnss") & ".csv", xlCSV. For continuous logging, add a timer-driven copy triggered every 10 s. CSV files open directly in Excel (or any text editor, as the source suggests Word too) without further conversion.

10. Alternative Data Transfer Methods

The OPC route is not the only option. The table below summarises each path against memory, hardware, and Excel-readiness criteria:

10.1 MMC card as a fallback

If the PC hardware is unavailable, the CPU 312C can be configured to write a binary log to its MMC using SFC 82 / SFC 83 / SFC 84 (CREATE / WRITE / CLOSE). The standard MMC limit is 8 MB on the 312C variant. Data is retrieved later via PLC > Copy RAM to ROM in STEP 7 or ProSave on the PC.

10.2 CP 343-1 IT FTP push

With a CP 343-1 IT (6GK7343-1GX31-0XE0) in slot 4, the controller can use FTP_GET / FTP_PUT from the IT-CP library to drop a CSV file on a Windows share or FTP server. The CPU writes the file from a DB image, the FTP block streams it over TCP/IP. This removes the OPC dependency entirely and answers the source's question about "simplest way" to move DB data to the PC.

11. Memory Budgeting for CPU 312C

Work-memory consumption of a typical data-logging build on the CPU 312C:

The remaining ~4 KB is consumed by Siemens system blocks (SFC / SFB copies in work memory). The CPU will tolerate the build but has no headroom for additional alarms, recipe DBs, or future expansion. If the project grows beyond this footprint, either upgrade to CPU 313C-2 DP (32 KB work) or CPU 314C-2 DP (48 KB work). The field report explicitly warns that "what you are asking is memory-intensive, and ressource intensive. It might very well be impossible to do with your machine." - this table quantifies that concern.

12. Verification and Commissioning Checklist

1. OB35 trigger rate - Watch CPU Diagnostics Buffer in STEP 7; OB35 entries should appear every 100 ms. Verify the start time delta between consecutive OB35 calls is 95-105 ms (allow jitter).
2. FIFO fill rate - Monitor DB100 sample counter; expect +1 every 100 ms.
3. FULL flag transition - Set a VAT breakpoint on M 252.0; should pulse every 10 s for a 100-sample FIFO.
4. BLKMOV return value - Watch MW 260 in OB35; must read 0000 after the copy.
5. FILL return value - Watch MW 262; must read 0000 after the reset.
6. OPC Scout connectivity - Browse S7:[DATA_LOGGER]DB200,BYTE0; quality should be GOOD and value should change every 10 s.
7. Excel DataChange - Single-step the opcGroup_DataChange event; verify Cell(2,1) updates.
8. File output - Open the saved CSV in Excel; columns A-D populated with 100 rows, pv_mm monotonically tracking cylinder position.
9. CPU run-time - Online > CPU > Scan cycle time > OB35; must stay below 50 ms (50% headroom) to avoid trigger skip.
10. Free work memory - Module Information > Performance Data > Work Memory > free; must remain > 1 024 bytes after download.

13. Troubleshooting Matrix

14. Summary of Recommended Stack

For the originally requested "100 samples in 10 seconds, exported to Excel" workload on a CPU 312C with SFB 41 PID on a hydraulic axis, the canonical architecture is:

• OB35 at 100 ms sampling
• DB100 FIFO_Load / FIFO_Unload from TI-S7 Converting Blocks (UDT 10)
• SFC 20 BLKMOV at FIFO-FULL to mirror DB200
• SFC 21 FILL to reset DB100
• CP 5611 / CP 5612 + SIMATIC NET OPC DA
• Excel VBA DataChange event to dump rows
• Periodic CSV save with timestamped filename

If the CPU 312C work-memory budget is exhausted, the next preferred alternative is CP 343-1 IT FTP push of the CSV file, which removes the OPC layer and the PC-side SIMATIC NET dependency entirely. Word-format export is also straightforward - the source asks "how can I open it with Excel or Word" - and a CSV file imports cleanly into both applications via File > Open.

FAQ