Resolving CPU 313C-2 DP STOP with SF and BF Faults in S7-300

Symptom Profile and Field Configuration

A redundant pair of SIMATIC S7-300 PLCs has been deployed with two CPU 313C-2 DP units (MLFBs typically 6ES7 313-6CF04-0AB0, 6ES7 313-6CF14-0AB0, 6ES7 313-6CG04-0AB0, or 6ES7 313-6CG14-0AB0), both configured through the SIMATIC S7-300 Software Redundancy (SWR) library. After energising the cabinet, one of the two CPUs (CPU2) fails to enter RUN. The reported front-panel indication on CPU2 is:

• SF (System Fault) — RED, solid ON
• BF (Bus Fault, PROFIBUS DP) — RED, flashing at ~1 Hz
• MAINT — OFF
• DC 5V — GREEN, solid ON
• FRCE — OFF
• RUN — OFF
• STOP — ORANGE/YELLOW, solid ON

The downstream ET 200M station connected to the shared (or paired) PROFIBUS DP segment of CPU2 reports simultaneously:

• SF — RED, flashing
• BF1 — RED, flashing
• BF2 — RED, flashing
• ACT — ORANGE, flashing
• ON — GREEN, flashing

The combined signature — solid STOP+SF+BF-flashing on CPU2 with every fault indicator flashing on the IM 153 — is a textbook reading of a CPU that has been unable to complete startup or maintain its PROFIBUS master role and has been forced to STOP by its own diagnostics. The downstream IM 153 has lost its logical DP master, which is the expected downstream behaviour when its assigned master drops.

CPU 313C-2 DP LED Reference

The following table summarises the front-panel LED meanings on a CPU 313C-2 DP (6ES7 313-6xx04-0AB0 and 6ES7 313-6xx14-0AB0 families). It maps the standard Siemens notation to operating-state behaviour.

Reading SF=solid, BF=flashing, STOP=solid, RUN=off, DC5V=on against the table narrows the cause to one of three families — a PROFIBUS DP master-side failure detected during startup, a required OB missing or in error, or a configuration / SD-card checksum error. The DC 5V LED being GREEN rules out the internal 5 V rail and the 24 V backplane. The MAINT LED OFF rules out maintenance-diagnostic interventions. FRCE=OFF rules out a forced I/O lockup. The remaining candidates all relate either to the PROFIBUS DP segment or to the SWR sync link.

ET 200M IM 153 LED Reference

The ET 200M in question uses either IM 153-1 (single PROFIBUS interface, MLFB 6ES7 153-1AA03-0XB0 or later) or IM 153-2 (dual PROFIBUS interface for redundant masters, MLFB 6ES7 153-2AB02-0XB0 or later). The presence of BF2 in the panel reading confirms an IM 153-2 dual-port device.

A reading of SF+BF1+BF2=flashing, ACT=flashing, ON=flashing on an IM 153-2 indicates that the IM is powered (ON flashes during AR negotiation by firmware design) but cannot complete arbitration with either master — typical when both CPU sides are at fault, when the DP segment from CPU2 was disconnected at the time of switch-over, or when the SWR sync link between the two CPUs is broken. The combination rules out a single-segment incident and points instead to a fault affecting both masters simultaneously — a configuration the IM recognises as a complete loss of arbitration partner.

Probable Root Causes Ranked by Probability

The following matrix lists the most common root causes observed for this front-panel combination, ranked by frequency across the entire S7-300 installed base. Each row ties the diagnostic-buffer evidence to a concrete first-remedy on site.

All nine above reproduce the observed LED combination; the order is empirical and shifts on any specific site depending on cabinet age, vibration exposure, and connection-resource budget. The diagnostic flowchart below ties the most likely causes to ordered actions.

Software Redundancy (SWR) Architecture Implications

Two CPU 313C-2 DP units running S7-300 Software Redundancy form a one-master/one-standby pair. The SWR library is installed into the STEP 7 catalog; the standard calls include SWR_START, SWR_DIAG, SWR_ZYK, and SWR_SYC, instantiated through their FBs in OB 1, OB 100, OB 86, and OB 87. Their work and instance DBs (DB 101..DB 104 or higher, project-specific) hold the data image that the standby keeps aligned with the master. The pattern of LEDs after energise reveals the operating state:

• If only one CPU is in RUN and the other is in STOP without SF or BF, this is the standby in normal quiescent state.
• If the standby CPU is in STOP with SF/BF, the SWR pair has not been able to form, because the standby is signalling an I/O or sync fault that prevents it from entering the SWR ready-to-takeover state.
• If both CPUs are in STOP after energise, SWR has failed on both halves and the application is not running anywhere.

The fact that only CPU2 is in the abnormal state confirms that CPU1 is the active master. CPU2 should normally either be a passive standby with no fault LEDs or be carrying its own dedicated segment with the IM 153 reporting clean. In SWR, the sync channel is typically MPI between the two CPUs (a dedicated cable with two 9-pin D-sub connectors, terminators OFF) or an Ethernet pair on the integrated PN interface (CPU 313C-2 PN/DP variant). If the sync link between CPU1 and CPU2 is broken, both CPUs independently treat each other as missing; each one attempts to start as master and may go to STOP once the application is rebuilt and a duplicate master role is detected.

STEP 7 Diagnostic Buffer Extraction Procedure

Open STEP 7 V5.x and reach the diagnostic buffer. The buffer is the only authoritative source for the cause of the STOP transition; it stores a chronological ring of hex-coded events with timestamps and OB references.

1. Connect the PG to the surviving CPU (CPU1) over MPI/PROFIBUS adapter, e.g. PC Adapter USB 6GK1 571-1AA00-1AA0, with the PG/PC interface set to PC Adapter (MPI) or PC Adapter (PROFIBUS).
2. In SIMATIC Manager select PLC > Accessible Nodes and locate both CPU 313C-2 DP nodes. CPU1 shows the master status; CPU2 should also respond, even in STOP.
3. Right-click CPU2 > PLC > Module Information.
4. Open the Diagnostic Buffer tab. Set the filter to Errors and Events only.
5. Scroll to the bottom (most recent events). Expand each entry by clicking Detail — Siemens stores the full event, secondary event, and informational event in the same record.
6. Click the Save As… button on the toolbar; export the entire buffer to *.txt. Repeat for CPU1.
7. Open the text exports in a text editor and search for the following hex event codes first.

The full dictionary of error messages returned at the STATUS outputs of instructions is published in the Siemens documentation "Error messages S7-300, S7-400": Error messages S7-300, S7-400 — Siemens documentation portal. Cross-reference any STATUS hex code returned by an instruction against this list before declaring the CPU hardware at fault.

PROFIBUS Physical Layer Verification Procedure

Once the diagnostic buffer confirms a DP-bus cause (event ID 0x0A41 or similar), verify the physical layer top-down — segment, connectors, terminators, shield, and slave power.

1. Disconnect power and de-energise the segment.
2. Confirm cable shield continuity end-to-end; shields are bonded at both ends through clamps.
3. Confirm the topology is bus (line), not star. Repeaters off the trunk form a chain only up to the manufacturer specification for the relevant IM 153 firmware generation.
4. Verify termination: exactly two terminators across the segment, both ON, located at the two physical ends of the trunk. On SIMATIC PROFIBUS connectors the terminator slides into one of three positions: ON, OFF, or "RC" branch. Leaving a middle-node terminator ON is the single most common cause of intermittent DP faults.
5. Measure the voltage between pins 3 and 8 (the data lines) at each end. With both ends terminated and the master powered but no other traffic, the voltage is approximately 1.1 V (logic "1" level 5 V idle on the bus); an open end reads ≈5 V DC; a shorted end reads ≈0.4 V.
6. Measure the bus termination resistance across pins 3 and 8 with master and slaves powered off; expect 220 Ω in series with each parallel 390 Ω pull-up/pull-down at the two end nodes (a Thévenin equivalent of ≈110 Ω).
7. Replace the connector at the trouble device first; bent pins 3 and 8 are common in cabinet-shifted hardware.
8. Re-energise and observe BF. The flashing BF transitions to OFF or SOLID as the physical layer is repaired; only a configuration mismatch continues a flashing BF.

Configuration and GSD Verification

If the physical layer verifies cleanly but the diagnostic buffer still shows DP-slave failure, the next surface is the HW Config. A configuration drift after a memory-card replacement is a frequent root cause.

1. Open HW Config for the SWR station in STEP 7 V5.x.
2. Compare the IM 153 MLFB and order number against the GSD catalog. Right-click the IM and re-select the precise article number. For an S7-300 SWR installation both CPU 313C-2 DP MLFBs must match exactly: different firmware suffix letters (e.g. -6CF04 versus -6CF14) are not interchangeable in HW Config without warning.
3. Check the PROFIBUS address of each IM. SWR-pair CPUs are typically master address 1 (CPU1) and 2 (CPU2), each with slaves on unique addresses; no slave can share an address with another on the same segment.
4. Verify the slots for digital input/output modules: missing slots cause CPU startup delay and, with required OBs absent, force a STOP.
5. Compile and download to CPU2 with CPU > Download to Target System; select Reset if HW Config does not yet match the CPU.
6. Cross-check that the GSD file revision loaded into HW Config is at least as high as the IM 153 firmware revision actually installed on the field device (read with PG/MPI).

Connection Resources and PG Adapter Limits

Older S7-300 CPUs have a hard connection-resource budget. The CPU 313C-2 DP family is constrained to small fixed numbers; SWR itself consumes four connections per direction, plus one connection to each HMI panel and one PG connection for commissioning. With low FW and several HMIs the budget is exceeded easily. Older S7-300 processors provide a connection-resources view under Online > Module Information > Communication; read this view for the live count.

Persistent resource codes 0x0050, 0x005C, or 0x0070 in the diagnostic buffer confirm a resource-limit event; the recovery is to close any opened Online windows in STEP 7 (each PG window locks a connection) and to verify that the HMI panel is still talking and not holding orphan sockets.

Corrective Action Matrix

Re-commissioning and Verification Checklist

1. Connect STEP 7 to CPU1 (the surviving master). Read its diagnostic buffer; confirm SWR is stable.
2. Power down CPU2. Disconnect the DP connectors, look for bent pins or backshell strain.
3. Power up CPU2 only. Observe LED transition: BF flashing → OFF, MAINT OFF, FRCE OFF, STOP solid (expected — CPU2 in solo STOP is its cold state).
4. With STEP 7 connected to CPU2, reset and download the SWR project. Wait for the buffer to clear ERROR entries.
5. Add CPU1 to the partner list under SWR configuration; both CPUs should now reflect SWR ready states, with one CPU in RUN and the other in STOP without fault LEDs.
6. On the ET 200M, confirm SF OFF, BF1/BF2 OFF, ACT ON (if dual-port), ON solid.
7. Trigger a manual failover from STEP 7 (SWR > Start Switchover). The roles must swap cleanly without SF/BF on the side taking over.
8. After a successful failover, repeat SWR switchover three times to validate that the SWR library has received its inputs correctly.

Preventive Hardening and Spares Strategy

Recurrence of this exact combination is reduced through a small set of lifecycle measures:

• Spare CPU 313C-2 DP at the cabinet: one MLFB-aligned unit with the firmware pre-loaded to the field level, ready for cold replacement.
• Spare IM 153-2 at the cabinet with the same MLFB generation and firmware revision.
• Pre-terminated spare PROFIBUS cable assemblies (5 m, 10 m, 15 m) with shielded D-sub end-shells.
• PG/PC adapter cable (e.g. 6GK1 571-1AA00) on the operator desk, with the PG/PC interface preset to PC Adapter (MPI) and the binding to CPU1.
• Diagnostic buffer exports archived daily to a versioned STEP 7 project directory.
• Connection-resource budget tracked in the project documentation; reserve at least two OP/PG resources for commissioning.
• Annual PROFIBUS physical-layer audit (resistance check, terminator survey, shield-clamp tightening).

For broader control-system standards and PROFIBUS DP reference material, refer to the Siemens Industry Online Support pages and the IEC 61158 / EN 50170 PROFIBUS standards.

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