Resolving Simatic S7-300 CPU 315-2 PN/DP All-LEDs-On Fault

Symptom Description

The Simatic S7-300 CPU 315-2 PN/DP (order number 6ES7315-2EH14-0AB0 and predecessor variants 6ES7315-2EH13-0AB0 / 6ES7315-2EH10-0AB0) fails to enter any defined operational state. The front-panel status LEDs — SF, BF, BF2, DC5V, FRCE, RUN, and STOP — remain illuminated simultaneously and continuously after power-on. Power cycling does not restore normal operation. The CPU does not respond to connection requests from STEP 7 (SIMATIC Manager), TIA Portal, or HMI stations such as WinCC Runtime, which report "CPU disconnected" over TCP/IP.

This simultaneous-LED state is not a documented normal operating mode of the S7-300 family. It indicates that the CPU firmware cannot complete its self-test phase, that the boot loader has failed to hand off control to the operating system, or that the Micro Memory Card (MMC) holding the runtime image is defective, unreadable, or has been corrupted by an interrupted firmware update.

CPU 315-2 PN/DP LED Reference

The CPU 315-2 PN/DP integrates two interfaces: a PROFIBUS DP master/slave (MPI-compatible) port X1 and a PROFINET interface (two-port switch) on X2. The following table summarizes the LED conventions used on the PN/DP variant; the SF, BF, DC5V, FRCE, RUN, and STOP behaviors are also documented in the Siemens CPU 315-2 DP / CPU 317-2 DP manual (Siemens Support document 12996906).

The simultaneous lighting of RUN and STOP together with SF and BF/BF2 is the diagnostic signature of a CPU that cannot complete its boot sequence. The DC5V LED being lit simultaneously confirms that power supply integrity is not the cause — the issue is at the MMC, firmware, or main CPU ASIC level.

Root Cause Analysis

Five causes account for the vast majority of "all-LEDs-on" reports on the S7-300 platform. Each cause is ranked by field frequency.

Pre-Diagnostic: Power Supply and Wiring Checks

Before touching the MMC or attempting a reset, eliminate power-side root causes. The S7-300 power supply module (PS 305 for AC, PS 307 for DC) must be healthy for the CPU DC5V LED to be valid.

1. Measure 24 V DC at the CPU power terminals (top-left L+ / M). Acceptable range: 20.4 V to 28.8 V DC under load.
2. Verify PS module status: the green DC 24 V OK LED on PS 307 should be lit solid. If it is off or flashing, the supply is at fault.
3. Confirm rack grounding at the functional earth (FE) terminal on the PS module using the Siemens grounding bus concept.
4. Check that the CPU is fully seated on the DIN rail and that the backplane connector is not oxidized.

Establishing a Programming Connection to a Non-Responsive CPU

The PROFINET interface may be unreachable if the IP configuration on the MMC is corrupted or the Ethernet ASIC has not initialized. The MPI/DP interface on port X1 remains available as a low-level service channel and is the recommended path for first contact.

Option A — MPI Connection (Recommended First Path)

1. Connect a Siemens PC Adapter USB A2 (6GK1571-1BA00) or a CP 5711 to the MPI/DP port X1 of the CPU.
2. In SIMATIC Manager: Options > Set PG/PC Interface; select S7ONLINE (STEP 7) → PC Adapter (MPI). Set the local MPI address of the PG to 0 and the highest MPI address on the bus to 31.
3. Default MPI address of an S7-300 CPU is 2 when no project has been loaded. Use Accessible Nodes to discover the CPU even if its address has changed.
4. If the CPU does not appear, repeat the scan with the highest station address set to 126.

Option B — PROFIBUS DP Connection

If the wiring supports DP, use the same PC Adapter on the DP port at 1.5 Mbps. The CPU's DP address is normally read from the MMC; on a corrupted MMC the CPU falls back to address 2 on the MPI subset.

Option C — PROFINET (Ethernet)

Use only after the CPU is partially recovered. With a corrupted MMC the PROFINET interface may still come up but report the CPU as unreachable to higher-level clients (the WinCC "CPU disconnected" symptom). Attempt only after MPI access succeeds.

Reading the Diagnostic Buffer via STEP 7 / TIA Portal

Procedure with SIMATIC Manager (STEP 7 V5.x)

1. Open the project that historically controlled the plant (if available). If the project file is unavailable, use Accessible Nodes to bypass project context.
2. Right-click the CPU in the project tree.
3. Navigate: PLC > Hardware Diagnostics > Module Information.
4. Select the Diagnostic Buffer tab.
5. Click Help on Event on the topmost (most recent) entry to obtain Siemens event ID interpretation.

Procedure with TIA Portal (V15 to V18)

1. Open the project. In the project tree, expand the PLC station.
2. Select the CPU and double-click Online & Diagnostics.
3. In the diagnostics dashboard, click Diagnostics buffer.
4. Read the topmost event. For events of type "STOP" with reason codes containing "MMC", "Memory card", or "Firmware", proceed to the MMC Reset Procedure below.

Common event IDs you may encounter when all LEDs are on include (non-exhaustive):

MMC Replacement Procedure

The Micro Memory Card for CPU 315-2 PN/DP is a Siemens-proprietary card. Third-party SD/MMC adapters will not work. Approved card sizes: 64 KB up to 8 MB for the 315-2 PN/DP. Typical field card: 6ES7953-8LF20-0AA0 (2 MB) or larger variants.

1. Power off the S7-300 rack or place the CPU mode selector in STOP. Wait 5 seconds.
2. Open the MMC slot cover on the front of the CPU.
3. Eject the existing MMC by pressing it inward until the spring releases.
4. Insert the candidate replacement MMC, label side facing left.
5. Power on. Watch the LED sequence:

• DC5V solid → STOP slow flash → STOP solid (no MMC content): this means the MMC is accepted but empty.
• DC5V solid → all LEDs on: the replacement MMC is also defective or incompatible.

6. If the new MMC is accepted, proceed to Project Reload Procedure.

Factory Reset via the Mode Selector (MRES)

If the diagnostic buffer indicates a user-program fault rather than an MMC fault, perform a factory reset to clear the on-MMC program image:

1. Set the mode selector to STOP.
2. Turn the selector to MRES and hold for 3 seconds. The STOP LED begins flashing slowly.
3. Within 3 seconds, briefly turn the selector back to MRES. The STOP LED begins flashing quickly — this is the active reset phase.
4. When the STOP LED stops flashing and goes solid, the reset is complete. The CPU retains IP / MPI / DP addresses if they were stored on the MMC; otherwise defaults are restored.

If the STOP LED does not transition to slow flash within 3 seconds of the first MRES press, the CPU is not processing the selector inputs and is likely halted at firmware level — escalate to MMC replacement.

Project Reload Procedure

1. Connect to the CPU via MPI using the procedure above.
2. Open the project's Hardware view (STEP 7) or Device configuration (TIA Portal). Confirm the order number matches the physical CPU: 6ES7315-2EH14-0AB0.
3. Select the CPU and choose Download to device > Hardware and software (only if different) in TIA Portal, or PLC > Download in SIMATIC Manager.
4. When prompted about the CPU in STOP, click Yes to download in STOP state.
5. After download completes, set the mode selector to RUN.
6. Verify the RUN LED is solid green, STOP LED off, and SF / BF / BF2 off.

HMI / WinCC Connection Recovery

After restoring the CPU, the WinCC client may continue to report "CPU disconnected" if its connection tag still points to the previous IP. Verify each side:

If WinCC uses a named connection (S7-connection by name), ensure the CPU's PROFINET device name is identical to the one programmed in the WinCC project. A factory reset via MRES may revert the PROFINET device name to the default empty string.

Hardware Replacement Decision Matrix

Verification Checklist After Restoration

1. RUN LED solid green, all other LEDs off (except possibly FRCE if forcing is intentionally active).
2. Online connection established within 5 seconds of Accessible Nodes scan.
3. Diagnostic buffer shows no new entries during a 30-minute soak with normal I/O activity.
4. All DP / PROFINET slaves report No fault in HW Config or TIA device view.
5. WinCC Runtime shows the configured tag values updating live.
6. Force table verified: if FRCE LED is on, confirm forcing is intentional.
7. Cycle power once; confirm CPU returns to RUN without operator intervention.

Preventive Recommendations

• Always store a backup of the project's MMC image using PLC > Upload Station to PG or TIA Portal's Backup function. Keep at least two generations of backups.
• Label each MMC with the project name, CPU order number, and date. Do not interchange MMC cards between CPUs of different order numbers without verifying compatibility.
• When upgrading firmware, use the Siemens Automation Tool on a dedicated laptop with the rack offline and a UPS-backed power supply. Never interrupt a firmware upgrade at the OS level.
• Add a UPS or at minimum a redundant 24 V DC supply to the PS module to prevent undervoltage events during plant faults.
• Schedule a quarterly diagnostic buffer review to surface latent errors before they escalate to all-LEDs-on conditions.

When to Escalate to Siemens Support

If the all-LEDs-on state persists after a confirmed-good MMC swap and a stable 24 V supply, the CPU's mainboard is at fault and the unit must be replaced. Capture the diagnostic buffer (if reachable via a secondary CPU on the same MPI segment), the order number, the firmware version (printed on the front label, e.g., V3.3), and the serial number before contacting Siemens Industry Online Support at support.industry.siemens.com. Reference document 12996906 for LED behavior validation when opening the case.