Resolving S7-300 CPU 312 STOP LED Blinking with MMC Card Error

Problem Statement

On a SIMATIC S7-300 system built around the CPU 312, MLFB 6ES7 312-5BD00-0AB0, the following fault is reported after commissioning or a hot-swap of the Micro Memory Card (MMC):

• CPU is in STOP immediately after the MMC is inserted into the slot.
• The STOP LED blinks at the slow 0.5 Hz / 2 Hz error rate, not a steady STOP.
• STEP 7 / SIMATIC Manager reports "No online partner is active" or "The partner is not reachable" when the MMC is in place.
• Removing the MMC allows an MPI/PG connection and the project can be downloaded; behaviour returns to normal.

Operation without the MMC is acceptable only for diagnostics because the CPU 312 has no integrated load memory – the MMC is the load memory. The platform must be returned to a state in which the card is loadable, formatted correctly, and accepted by the firmware.

CPU 312 Hardware Identification (MLFB 6ES7 312-5BD00-0AB0)

Before any corrective action, confirm the exact hardware version. The 312-5BD00-0AB0 differs from sibling 312-C CPUs and from later 312-5BE/5BF revisions in work memory size, instruction set, and supported MMC types.

Two consequences flow directly from the table:

1. The CPU cannot run any user program without a valid MMC. Removing the card allows online access for engineering, not for production operation.
2. Any MMC formatted on a different CPU family (e.g. an S7-400 MMC, an S7-22x MMC, or a consumer SD card re-labelled) is rejected at startup, producing the blinking STOP pattern described above.

Cross-reference: SIMATIC S7-300 CPU 31xC and CPU 31x Operating Instructions – section 4.2 "Memory concept of the CPU 31x".

STOP LED Blink Patterns and Diagnostic Meaning

The CPU 31x family uses a small grammar of LED states. Reading the exact pattern avoids chasing the wrong fault.

The reported symptom (CPU works without the card, blinks with the card) is consistent with the slow 0.5 Hz blink: the CPU is signalling that the inserted load memory is invalid and is asking the operator to perform a memory reset before it will accept a fresh download.

To read the same information in software, connect the PG with the MMC removed, switch the interface to Accessible Nodes, navigate to the CPU, and open PLC → Diagnostics/Setting → Diagnostic Buffer. Typical entries that match this scenario:

• "MMC error – no readable content" (event ID 0xE201 / W#16#E201)
• "Memory card check sum error" (event ID 0xE202)
• "STOP due to MMC read error"
• "User program not executable – OB missing"

Each entry has a time stamp and the OB / FC / DB address that triggered the request, which is invaluable when multiple engineers are swapping the card.

Root Cause Analysis: MMC Corruption and Mismatch

The blinking-STOP-with-MMC symptom is almost always one of the following five root causes. The fix path differs for each – identify first, then act.

Solution Path A – Delete / Format the MMC from SIMATIC Manager

The cleanest path when you have a Field PG with STEP 7 installed. This path does not require a live CPU connection – the MMC is read/written directly through the PG's MMC slot (any SIMATIC Field PG M, Premium, or Advanced has an MMC slot on the left side, MMC label facing down).

Step-by-step procedure

1. Power the CPU down and remove the MMC. Label it immediately – in a plant with many spare MMCs this prevents cross-contamination.
2. Insert the MMC into the Field PG's MMC slot. The text/label side of the card faces down; the chamfered corner is at the front.
3. Open SIMATIC Manager. On the left-hand project tree you will see an icon representing the removable card (the SIMATIC Memory Card icon – a small card symbol above the project tree).
4. Right-click the icon and select "SIMATIC Memory Card → Display contents" (German: Speicherkarte S7 → Inhalt anzeigen).
5. Select all blocks with Ctrl+A and press Del. If the card was used on a foreign CPU this is the only way to remove the project-specific SDBs.
6. Alternative menu path: File → S7 Memory Card → Delete. This issues a low-level format that rebuilds the Siemens MMC header. Wait for the progress bar to complete – do not remove the card while the red LED on the Field PG is lit.
7. Eject the card via the SIMATIC Manager icon (right-click → Remove Memory Card) before physically pulling it.
8. Insert the MMC into the powered-down CPU 312 and power up.
9. Verify with the Accessible Nodes view that the CPU now reports a normal hardware-OK state with no diagnostic buffer entries.

If SIMATIC Manager reports "The card is write-protected", the card has a mechanical write-protect slide (rare on Siemens MMCs; common on no-name cards). Slide it to the unlocked position and retry. Genuine Siemens 6ES7 953-8LG00-0AA0 (4 MB) and 6ES7 953-8LJ00-0AA0 (8 MB) cards do not have a write-protect tab.

Solution Path B – MRES Memory Reset on the CPU

When the MMC contains a project the CPU will not run (wrong CPU type, password locked, corrupted) and the field engineer does not have a PG, the CPU itself can perform the equivalent of a low-level format. This is the MRES (Memory Reset) operation, executed via the mode selector switch on the front of the CPU 312.

Mode selector positions

MRES procedure for a 312-5BD00-0AB0

1. Insert the MMC that is suspected of being bad.
2. Turn the mode selector to STOP. Wait for the STOP LED to come on steady.
3. Turn the selector to MRES and hold for ~3 seconds, then release. The STOP LED goes out briefly.
4. Within 3 seconds, turn the selector to MRES again and hold for 9 to 11 seconds exactly. The STOP LED blinks at 2 Hz during the hold and goes steady when complete.
5. Release the selector – it springs back to STOP. The MMC is now formatted and the CPU is ready for a fresh download.

After MRES, the diagnostic buffer retains its previous entries (MRES does not clear the buffer) – clear it manually with PLC → Clear/Reset Diagnostic Buffer from a connected PG if you need a clean log.

Solution Path C – External EPROM Prommer

When the MMC is unreadable to the CPU and to the Field PG – typically after an under-voltage event on the card – the only remaining option is to reformat the flash through a Siemens external prommer.

Hardware required: SIMATIC S5/S7 External Prommer, MLFB 6ES7 792-0AA00-0XA0 (USB variant) or the legacy 6ES7 792-0AA00-0AA0 (parallel). The 0XA0 version supports:

• S7-300 MMCs (all sizes 64 KB – 8 MB)
• S7-400 memory cards (RAM and Flash)
• S7-200 memory cards
• S5 memory cards and EPROMs (legacy)

Procedure

1. Install the prommer driver and the accompanying S7PROMM software on a Windows 7/10/11 PC. The driver package is shipped with the device; current builds are available from the Siemens Industry Online Support portal under article ID 16666712.
2. Insert the MMC into the prommer's slot, label side up.
3. Launch S7PROMM, select File → Format → MMC → Format. Acknowledge the warning that all data will be erased.
4. After the format completes (typically 30–90 s for a 4 MB card), the prommer writes a fresh Siemens MMC header that the CPU 312 will accept.
5. Insert the card into the CPU. The CPU should now boot to STOP-without-blink. Proceed with a normal project download.

Field PG and HSP Prerequisites

Before any of the three paths above, verify the following so that the engineer is not chasing ghosts on a green-field bench or during a hot callout.

If Accessible Nodes does not show the CPU when the MMC is in place, but does show it when the MMC is removed, the firmware is refusing to start the communication stack because the load memory is unreadable. This is the exact symptom of the original report and is solved by paths A, B or C above.

Verification and Re-commissioning

After any of the three solutions, perform the verification sequence below before handing the system back to operations. Each step has a pass criterion – do not skip.

1. Power cycle with MMC inserted. CPU should come up in STOP (steady LED), not blinking. The SF, BF and FRCE LEDs should be off.
2. Read the diagnostic buffer. Confirm that the previous MMC errors are no longer being added. The most recent entry should be "Power on" or "STOP requested by user".
3. Download the project. Use PLC → Download. The transfer should complete in seconds (a 4 MB MMC over MPI 187.5 kbit/s will take ~30 s; a DP/USB adapter 1.5 Mbit/s will take ~5 s).
4. Switch to RUN. The RUN LED should illuminate within 1 s. The STOP LED must go off completely – no residual blink.
5. Force table read. If the project uses forces, confirm PLC → Display Force Values shows the expected values and the FRCE LED is lit only when active forces exist.
6. Process signal sanity check. With the machine in a safe state, toggle one digital output via VAT or watch table, confirm the physical actuator responds, and reset.

If after these steps the STOP LED still blinks, re-read the diagnostic buffer for the most recent entry. A repeat checksum error points to a hardware fault on the MMC itself; replace it with a new Siemens 6ES7 953-8LG00-0AA0 (4 MB) or equivalent before continuing.

Preventing Future MMC Corruption

Three engineering practices reduce the probability of recurrence and the MTTR when it does.

• Apply the write filter. In the CPU 312's object properties (HW Config → CPU → Properties → Diagnostics/Clock), enable the Operating Mode setting to keep the CPU in RUN-P and avoid hot-swap of the MMC under power. The CPU 312 MMC is rated for 100 000 write cycles – PLCs that frequently write setpoints or recipes to the card will exceed this within a few years of three-shift operation.
• Label and segregate MMCs by project. A small plastic case with dividers and project labels prevents the single most common cause of the blinking STOP: an MMC carrying a foreign SDB set being inserted into the wrong CPU after maintenance.
• Keep a hot spare. A formatted, project-loaded spare MMC in a static-shielded bag at the panel door restores MTTR from hours to minutes. Spare cards should be refreshed annually because flash data retention is 10 years typical but degrades with temperature and bit-errors.

Troubleshooting Matrix – Quick Reference

FAQ