Configuring Siemens FC125 for PROFIBUS DP Diagnostics

Application context. FC125 (and its companion FB125 DP_DIA) is the legacy SIMATIC S7-300 / S7-400 block for reading PROFIBUS DP slave diagnostic data from a DP master system. It is supplied as part of the STEP 7 optional package and is intended to be called cyclically from OB1, with the activation flag driven from OB82, OB86, and OB100. This reference resolves the most common parameter confusion (specifically DP_MASTERSYSTEM) and documents the input/output contract, integration pattern, visualization block (FB126), and the migration path to PNIODiag for PROFINET systems.

1. Block Identification and Package Layout

The two blocks (FC125 and FB125) are functionally equivalent in their I/O contract. FB125 simply adds an instance DB so that the diagnostic data is retained across calls; FC125 returns the data in a target area pointed to by DATA_FIELD. Newer STEP 7 V5.x service packs prefer FC125 because it has no instance-DB overhead and integrates cleanly with the global diagnostic DB that almost every STEP 7 PROFIBUS project already owns.

2. Formal Interface Contract

3. Resolving DP_MASTERSYSTEM in HW Config

Three reliable ways to read the index you must pass to DP_MASTERSYSTEM:

1. HW Config property dialog. Right-click the PROFIBUS subnet line (the orange rail) and open Properties > General. The first tab shows the configured name and the internal number used by STEP 7 for this DP master system. This number is the value to load into DP_MASTERSYSTEM.
2. NetPro subnet list. In NetPro select the PROFIBUS subnet; the bottom status bar displays the configured Subnet number which matches the HW Config value for the same subnet.
3. System data block (SDB) inspection. Open PLC > Module Information > System Data on a live online connection and read the DP master system row of the diagnostic buffer; SDBs are tagged with the same index.

4. EXTERNAL_DP_INTERFACE Decision Logic

The boolean simply tells FC125 whether to query SFCs that target the local CPU's integrated DP port or SFCs that target an external CP/IM module in the rack. The matrix is:

Setting EXTERNAL_DP_INTERFACE = FALSE when a CP is actually the master causes the CPU to look at its own integrated interface (which in a CP-only rack does not exist) and returns RET_VAL = 8092. Setting it TRUE for an integrated CPU returns the same error because FC125 then tries to read a CP slot that is empty.

5. DATA_FIELD Sizing and Layout

The diagnostic data block must be at least 50 bytes. The minimum layout that FC125 expects is:

// DB125 - PROFIBUS DP diagnostic target
DATA_BLOCK DB125
  STRUCT
    STATE       : BYTE;    // Byte  0 - overall state (0=OK, <>0=fault)
    SLAVE_ADDR  : BYTE;    // Byte  1 - failing slave PROFIBUS address
    RESERVED    : ARRAY[2..5] OF BYTE;
    IDENT       : WORD;    // Byte  6/7 - DP ident (0x0000 = DP)
    LEN_DIAG    : BYTE;    // Byte  8 - length of standardized diag
    DIAG_DATA   : ARRAY[9..49] OF BYTE; // up to 41 bytes diag payload
  END_STRUCT;
END_DATA_BLOCK

6. Wiring FC125 into OB1, OB82, OB86 and OB100

The classic integration pattern from the STEP 7 online help for the diagnostic functions for PROFIBUS is event-driven with a polled refresh:

// OB1 - cyclic refresh
CALL FC 125
  CHECK_ACTIV         := M 100.0    // latched TRUE
  EXTERNAL_DP_INTERFACE := FALSE    // CPU 315-2 DP
  DP_MASTERSYSTEM     := 1          // index from HW Config
  DATA_FIELD          := P#DB125.DBX0.0 BYTE 50
  RET_VAL             := MW 102

// OB82 - diagnostic interrupt (slave diagnostic change)
       SET
       S     M 100.0                 // activate FC125 evaluation
// OB86 - rack/IO fault
       SET
       S     M 100.0
// OB100 - warm restart
       SET
       S     M 100.0
       R     M 100.0                 // single-shot, will clear in OB1

Why three OBs? OB82 fires when a slave changes its diagnostic state (channel fault, module pull, hot-swap on IM 153-1). OB86 fires on rack failure, which for a DP master includes loss of an entire DP segment. OB100 ensures that after a restart the diagnostics are rebuilt from scratch. OB1 then evaluates RET_VAL and the first byte of DATA_FIELD to drive HMI tags.

7. Visualization with FB126

FB126 was added to the same diagnostic package so that the raw byte array returned by FC125 can be rendered in WinCC flexible / ProTool without writing custom parsing code. FB126 takes the same DATA_FIELD pointer and produces a structured UDT with one element per known slave:

// FB126 - visualization driver
CALL FB 126, DB126
  DP_MASTERSYSTEM := 1
  DATA_FIELD      := P#DB125.DBX0.0 BYTE 50
  SLAVE_TABLE     := P#DB126.DBX0.0 BYTE 200   // pre-allocated 200 B
  RET_VAL         := MW 110

Each entry in SLAVE_TABLE is a fixed 20-byte slot: bytes 0…1 = PROFIBUS address, byte 2 = state, byte 3 = ident byte 1, bytes 4…19 = diagnostic text. WinCC reads the table via raw area pointer and displays it in a status screen.

8. Return Codes and Troubleshooting Matrix

9. Diagnostic Data Flow (Inline Diagram)

10. Commissioning Checklist

1. Install the STEP 7 optional package that contains FC125 / FB125 / FB126. Without it the blocks are not in the standard library and Blocks folder will be empty.
2. Compile HW Config and download SDBs so that the master system has its internal index written to the CPU.
3. Insert DB125 (or your chosen diagnostic DB) with at least 50 bytes of BYTE storage at offset 0.0.
4. Open OB1 and insert a call to FC125 with the parameter values from Sections 3 and 4.
5. Insert SET + S instructions in OB82, OB86 and OB100 that flip CHECK_ACTIV to TRUE.
6. Download all blocks to the CPU and place it in RUN. Force CHECK_ACTIV = TRUE and verify RET_VAL = 0.
7. Pull a slave connector and confirm that OB86 fires and that DB125.DBX0 becomes non-zero within two OB1 cycles.
8. Insert FB126 if WinCC visualization is required, then rebuild the WinCC tags that point at SLAVE_TABLE.
9. Save the project with the optional package version stamped in the project properties for reproducibility.

11. Migration to PNIODiag for PROFINET

FB125 / FC125 and the FB126 visualization were retired for new PROFINET IO projects. Siemens recommends the PNIODiag package, which exposes equivalent diagnostics over the standard PROFINET record reads and integrates with TIA Portal. For mixed PROFIBUS / PROFINET cells, keep FC125 on the PROFIBUS master system and run PNIODiag on the PROFINET IO controller side; the two coexist and their diagnostic DBs are independent.

12. Frequently Asked Questions