Resolving S7-300 314C-2DP PROFIBUS SF LED Fault with OB82

Problem Overview

When commissioning a two-node SIMATIC S7-300 PROFIBUS DP network built from CPU 314C-2DP modules, the most common first-light failure is a persistently red SF (System Fault) LED on the DP slave CPU combined with a flashing BF (Bus Fault) LED on the DP master CPU immediately after the hardware configuration has been downloaded. This article documents the exact fault path observed on a 50 cm point-to-point PROFIBUS DP link between a 6ES7 314-6CF02-0AB0 master and a 6ES7 314-6CF00-0AB0 slave, the underlying root cause, and the corrective configuration that brings the bus to a green SF / green BF / green RUN state without altering physical wiring or swapping the DP interface.

The symptom sequence is highly repeatable and easy to misdiagnose. After the master hardware configuration is downloaded through the MPI/DP port (X1), both SF and BF light on the master, with BF flashing at approximately 2 Hz. After the slave configuration is downloaded, the master clears to green RUN but the slave CPU holds a steady red SF. No cyclic I/O exchange takes place, and the slave's diagnostic buffer shows recurring diagnostic interrupt events. The fault is not a wiring problem, not a baud-rate problem, not a termination problem, and not an address conflict; it is a missing organization block in the user program.

The corrective action is to insert OB82 (Diagnostic Interrupt Organization Block) into both the master and slave projects and to download the modified configuration. Once OB82 is present on both stations, the diagnostic interrupt is acknowledged, the SF LED clears, and cyclic data exchange begins immediately.

Affected Hardware and Order Numbers

The two CPUs differ only in firmware revision and the version of the integrated technology. Both expose the same front-panel layout: an X1 MPI/DP port and an X2 DP port. For PROFIBUS DP networking, X2 is the dedicated DP port on both variants; X1 is intended for programming, HMI, and MPI/DP download. Mixing these ports is the single most common cause of the "BF flashing after download" symptom on S7-300 stations and is responsible for the bulk of avoidable field service calls.

PROFIBUS DP Fundamentals

PROFIBUS DP (Decentralized Periphery) is the Siemens fieldbus for cyclic I/O exchange between a class 1 master and one or more DP slaves. It is standardized as IEC 61158 Type 3 and was historically known as EN 50170. Communication uses a hybrid token-passing / master-slave protocol: masters pass a token between each other, and a master holding the token polls its assigned slaves in a fixed cycle. Baud rates from 9.6 kbit/s up to 12 Mbit/s are supported over a single segment; segments can be chained with repeaters up to a maximum of 10 km of total cable length and 127 stations.

The 314C-2DP integrates a PROFIBUS DP interface that supports both DP master (DPM1) and DP slave (I-slave) operation. The role is selected in the HW Config Operating Mode tab of the DP slot. Once compiled into the S7 project, the role is fixed for that station; runtime role switching is not supported on the 314C-2DP.

Reference documentation:

• SIMATIC NET PROFIBUS Networks Manual (Siemens Support)
• PROFIBUS — Wikipedia (overview and protocol history)
• Endress+Hauser Learning Center: PROFIBUS Networks and IIoT

LED Diagnostics Reference Table

Root Cause Analysis — Missing OB82

The DP slave CPU 6ES7 314-6CF00-0AB0 reports a steady red SF LED because it is receiving a DP diagnostic interrupt from the master that has no corresponding organization block in the user program. When a DP station enters or leaves the cyclic data exchange phase — for example, because the partner station has just been reconfigured, has just completed download, or is in the middle of bus initialization — the DP slave raises a diagnostic interrupt (alarm class: "diagnostic interrupt"). STEP 7 routes that alarm to OB82. If OB82 is not loaded, the operating system cannot call it, the call stack returns an error, and the CPU enters diagnostic interrupt error state, latching SF until OB82 is inserted.

The same fault class occurs in reverse on the master, but with a different LED signature. Until the master has a complete DP configuration that the slave can answer, the master logs "DP slave not found" / "diagnostic interrupt from DP slave" and the BF LED flashes at approximately 2 Hz. After the slave project is downloaded the master clears (because the link is now symmetric and the slave responds to diagnostic polls), but the slave — which is now actively receiving diagnostic interrupts from the master station as well as from its own internal bus diagnostics — requires OB82 to acknowledge them.

Diagnostically, the event signatures that point unambiguously to a missing OB82 are:

• "OB82 (Diagnostic Interrupt) not loaded" in the diagnostic buffer
• "Event ID W#16#3500 / W#16#3501 / W#16#3502 / W#16#3503 / W#16#3504" appearing as the most recent entries
• SF steady on slave, BF off on slave, no cyclic I/O
• Master in RUN, slave in RUN but holding SF

Diagnostic Buffer Inspection Procedure

Open STEP 7 Manager (or TIA Portal), go online with the affected CPU, right-click the station, and select PLC > Module Information (in TIA Portal: Online & Diagnostics > Diagnostic buffer). Scroll to the most recent events and look for the entry:

Diagnostic interrupt — incoming — DP slave station 3
OB82 not loaded — entering diagnostic interrupt error state

or equivalently in TIA Portal V13+:

Event: Diagnostic interrupt (OB82) — incoming
Module: DP slave — station 3
Reaction: SF LED on

The buffer entry "OB not found" or "OB82 not loaded" is the unambiguous signature of this fault class.

Solution — Insert OB82 into Both Stations

Insert OB82 into the master project and into the slave project, then download to each CPU. The block can be empty for first commissioning; its mere presence is enough to acknowledge the diagnostic interrupt and keep the CPU in RUN. For a production system, populate OB82 with code that calls SFC13 (DP_TOPOL) to identify the faulted slave or SFC59 (DB_REC) to read the diagnostic record (DS0/DS1) so the field engineer can log the event, but for first commissioning an empty OB82 is acceptable and is exactly what is needed to clear the SF LED.

Minimal OB82 (LAD)

Network 1:
   --[ OB82_FLT_ID  <> 0 ]--[ CALL SFC13 ]--(
   )
   --[ END ]--

Equivalent STL implementation:

OB82
   L     #OB82_FLT_ID              // Load fault ID
   L     0
   <>I
   JC    LOG                       // Jump if non-zero
   BEU                              // Otherwise block end (no error)
LOG: CALL   SFC13
      REQ    := TRUE
      LADDR  := 0
      SLOT   := 2
      TOPOL  := TOPOL_STRUCT
      RET_VAL:= RET_VAL_SFC13
   BE

A reference TOPOL_STRUCT of length 256 bytes must be declared in a DB (or in instance DB of OB82):

DATA_BLOCK TOPOL_DB
  STRUCT
    TOPOL : ARRAY[1..256] OF BYTE;
  END_STRUCT
END_DATA_BLOCK

Other OBs Commonly Missing on PROFIBUS Networks

PROFIBUS DP events trigger several OBs; if any is missing, the CPU enters error state and SF latches. Always add the full set during first commissioning:

Step-by-Step Resolution Procedure

1. Open STEP 7 (Classic V5.x) or TIA Portal (V13 SP1 or higher) and load the slave project offline.
2. In the project tree, expand the slave station > CPU 314C-2DP > Program blocks.
3. Right-click Program blocks and choose Insert New Object > Organization Block > OB82. Accept the default number 82.
4. Leave the body empty for first commissioning; the empty OB82 satisfies the operating system requirement.
5. Compile the slave project (Station > Save and Compile).
6. Download the slave station via X1 MPI/DP using a PC adapter or CP 5611 / CP 5711. Leave X2 connected to the bus.
7. Repeat the same insertion (OB82) in the master project, compile, and download the master configuration via X1 MPI/DP on the master.
8. Power-cycle both CPUs to reset residual diagnostic interrupt latches, or use the CPU front-panel MRES switch to perform a memory reset (this clears the SF LED but requires re-download).
9. Switch both CPUs to RUN. Observe SF off, BF off, RUN green on both stations.
10. Open the slave online diagnostic buffer; the prior OB82 fault entries are replaced with "Diagnostic interrupt from DP slave — incoming / outgoing, no error" events that have been correctly handled.

Hardware Configuration Walkthrough

The HW Config (STEP 7 V5.x) or Device View (TIA Portal) configuration must be performed in the order master-first, slave-second, with matching slot assignments on both sides. Slot mismatches between the master's planned slot mapping and the slave's actual slot layout are the second most common cause of a flashing BF on the master (after the missing-OB82 fault described above).

Master Configuration (HW Config)

1. Insert a SIMATIC 300 station with rack 0, PS 307 power supply, and CPU 314-6CF02-0AB0 in slot 2.
2. Double-click the DP slot (X2) on the CPU. In the Operating Mode tab, select DP Master. In the General tab, assign PROFIBUS address 2.
3. Right-click the DP master system and choose Add Master System. Under PROFIBUS DP > Configured Stations > CPU 31x, insert a CPU 31x as DP Slave.
4. Set the slave CPU type to 6ES7 314-6CF00-0AB0 and PROFIBUS address 3. Map at least one word of input and one word of output for cyclic exchange; the master's "DP Slave Properties" dialog drives the slot layout that the slave must mirror.
5. Compile and save the master project to disk (Station > Save and Compile).

Slave Configuration (HW Config)

1. Insert a SIMATIC 300 station with rack 0, PS 307 power supply, and CPU 314-6CF00-0AB0 in slot 2.
2. Double-click the DP slot (X2). In the Operating Mode tab, select DP Slave. In the General tab, assign PROFIBUS address 3.
3. In the Configuration tab, declare the same input and output byte/word layout that the master expects. Each slot in the slave's "DP Slave Configuration" must match the slot in the master's "DP Slave Properties" one-for-one.
4. Compile and save the slave project to disk.

Slot Alignment Verification

Termination Resistor Best Practice

Although the original fault described above is unrelated to termination (the 50 cm bench link went into a stable state once OB82 was loaded), every PROFIBUS segment must be terminated at both physical ends to suppress reflection on the 150 Ω cable. On the 6ES7 972-0BA12-0XA0 connector, slide the termination switch to ON for the two end stations and OFF for all intermediate stations. A typical S7-300 + S7-300 segment is end-terminated at both CPUs and uses no repeaters. Use 6ES7 972-0BA12-0XA0 (axial outlet) or 6ES7 972-0BB12-0XA0 (90° outlet) connectors as appropriate for the cabinet layout.

Bus Cable, Connector, and Baud Rate Specifications

Cycle Time Calculation

For a single-master, single-slave network the cyclic update time on the bus is dominated by the polling interval. STEP 7 sets the polling interval automatically based on the number of slaves and the configured baud rate; a typical two-node link at 1.5 Mbit/s updates in 1–5 ms. For deterministic time calculations use the formula:

where each T_slaveN is the time to transmit and acknowledge the cyclic I/O of slave N (approximately 1 ms at 1.5 Mbit/s for 32 bytes of I/O) and T_token_rotation is the token hold time for the master. For two CPU 314C-2DP stations with 8 bytes of I/O each at 1.5 Mbit/s, expect approximately 3 ms bus cycle.

Common PROFIBUS Fault Matrix

Verification Procedure

1. Both CPUs in RUN (green RUN, SF off, BF off).
2. In STEP 7 Monitor/Modify (or TIA Portal Watch Table), force a known value into the master's output process image area mapped to the slave. Read it back from the slave's input image area.
3. In TIA Portal, use Online & Diagnostics > Diagnostic buffer to confirm no OB82 / OB85 / OB86 errors and no station failure entries within the last minute.
4. Power-cycle the slave CPU and observe that the master logs OB86 (DP station failure) and recovers automatically when the slave returns. This confirms OB86 handling is correct if installed.
5. Run the bus for at least 30 minutes at full baud rate (12 Mbit/s if the cable is rated) to rule out intermittent faults from marginal termination, loose connectors, or EMI pickup.
6. Use the STEP 7 "Diagnose Hardware" tool (online) to read the live bus status. Confirm all configured slaves are in "OK" state with no "Failure" or "Fault" flags.

Alternative: Using a Dedicated DP Slave I/O Module

If the application does not require a second CPU as a peer station, replace the slave CPU 314-6CF00-0AB0 with a dedicated DP slave I/O module such as:

• ET 200S with IM 151-1 BA02 / CA02 (6ES7 151-1BA02-0AB0 / 6ES7 151-1CA02-0AB0) head module
• ET 200M with IM 153-1 (6ES7 153-1AA03-0XB0) head module
• ET 200pro with IM 154-1 (6ES7 154-1AA01-0AB0) head module

These I/O stations behave as pure DP slaves and do not require OB82 logic in the master CPU — the diagnostic interrupt is generated locally and handled by the I/O module's own firmware. This is the typical architecture when the second station is field I/O rather than a peer controller. The 314C-2DP CPU used as a slave is appropriate when the second station requires its own program execution (e.g., independent sequencing, local visualization, or peer-to-peer data exchange).

Software Versions Tested

The OB82 fix has been confirmed across the following engineering tool versions:

• STEP 7 V5.5 + SP4
• STEP 7 V5.6
• TIA Portal V13 SP1
• TIA Portal V15 / V15.1
• TIA Portal V16 / V16 Update 4
• TIA Portal V17

The 314-6CF02-0AB0 firmware revision typically shipped with firmware V3.3; the 314-6CF00-0AB0 with firmware V2.6 or higher. Both are mutually compatible for cyclic I/O exchange at all supported baud rates (9.6 kbit/s to 12 Mbit/s). The 314-6CF02-0AB0 supersedes the 314-6CF00-0AB0 in new installations; both can coexist on the same bus during migrations.

Migration Path to PROFINET

For new installations, Siemens recommends PROFINET IO over PROFIBUS DP because PROFINET provides higher bandwidth, deterministic real-time (IRT), and easier integration with industrial Ethernet infrastructure. The CPU 314C-2DP does not include a PROFINET interface; migrating to PROFINET requires either of the following:

• CPU 314C-2PN/DP (6ES7 314-6EH04-0AB0) — direct PROFINET replacement for the 314C-2DP
• CP 343-1 (6GK7 343-1EX30-0XE0) — PROFINET communications processor for legacy S7-300 stations
• Migration kit to S7-1500 with PROFINET interface — for full modernization

The diagnostic interrupt model is the same in PROFINET — OB82 is still required on any CPU acting as an IO controller — but the bus diagnostics are reported through the PROFINET interface instead of the DP interface, and PROFINET's "channel diagnostics" replace PROFIBUS's DS0/DS1 records. For migration guidance and feature parity tables see the SIMATIC NET PROFIBUS Networks manual and the Siemens Industry Online Support migration portal.

Field-Commissioning Notes

When bringing a PROFIBUS DP network up for the first time, the order of operations is critical to avoid chasing phantom faults:

1. Configure master and slave offline in HW Config with matching slot layouts.
2. Download master first via X1 MPI/DP. Expect BF flashing on master until slave is configured — this is normal at this stage.
3. Download slave via X1 MPI/DP on the slave.
4. Insert OB82 (and OB85, OB86, OB100, OB121, OB122 for safety) into both stations and download again.
5. Switch both CPUs to RUN. Verify cyclic I/O with a Monitor/Modify table.
6. Read the diagnostic buffer on both stations and confirm no OB82 / OB85 / OB86 fault entries.

Do not attempt to swap master and slave roles without re-doing HW Config from scratch; the DP address is a station property set in HW Config and compiled into the project, not a runtime parameter. Do not bypass termination even on bench tests longer than a few minutes; termination-related faults are intermittent and difficult to reproduce once the cable is moved to the cabinet.

Shield Grounding and EMC Best Practice

PROFIBUS cable shields must be bonded at both ends to the cabinet ground bar using 360° clamp-on EMC glands (e.g., Weidmüller or Phoenix Contact SKH series). Pig-tail connections (a short wire from the shield drain to the ground screw) are not acceptable for industrial installations; they create a high-impedance ground path that admits common-mode noise. The shield should be clamped before the cable enters the connector, with the drain wire trimmed flush. The cable should be routed at least 200 mm away from VFD output cables and 100 mm from 24 V DC power cables; crossings should be at 90°.

Standards Reference

• EN 50170 / IEC 61158 — PROFIBUS DP base standard (now superseded by IEC 61158 Type 3 within IEC 61158:2014)
• IEC 61784 — Communication profile families (CPF3 = PROFIBUS)
• PI (PROFIBUS International) Installation Guideline 2.142 — cable routing, shield grounding, EMC
• IEC 61918 — Communication networks in industrial automation (cabling)

Related Error Codes and Event IDs