1. CPU Identification and the Role of the MMC
The 6ES7 315-2EH13-0AB0 is a member of the SIMATIC S7-300 family, marketed as the CPU 315-2 PN/DP. It carries firmware release V2.6 and combines two fieldbus interfaces in a single module:
- Port 1 (X1): PROFINET interface, 2-port switch, 100 Mbit/s full-duplex, supports PROFINET IO Controller (max. 128 IO devices) and PROFINET I-Device.
- Port 2 (X2): combined MPI / PROFIBUS DP master/slave, 12 Mbit/s, supports DP-V0/V1 master or slave.
The CPU is shipped without an MMC (Micro Memory Card). Because S7-300 CPUs have no internal non-volatile program memory, the MMC is the only legal load memory for the STEP 7 project, firmware image, system data blocks (SDBs), and the diagnostic buffer archive. Without an MMC inserted, the CPU powers up with the diagnostic buffer entry "No MMC inserted, CPU goes to STOP" and will not enter RUN.
Identifying your hardware revision is the first step in selecting a compatible MMC. The complete order number is read as follows:
| Field | Value | Meaning |
|---|---|---|
| 6ES7 | 3 | SIMATIC S7-300 / ET 200 product group |
| 315-2 | CPU 315-2 | Mid-range CPU, two interfaces |
| E | E | PROFINET + PROFIBUS variant |
| H13 | Hardware release 13 | Identical to 6ES7 315-2EH14 for MMC purposes |
| 0AB0 | Firmware V2.6 | Latest released firmware for this MLFB |
Cross-reference on the Siemens Industry Online Support: support.industry.siemens.com → entry "SIMATIC S7-300 CPU 315-2 PN/DP". The operating instructions for the S7-300 CPU 31xC and 31x, manual A5E00105492-11, is the canonical reference for the MMC requirements discussed below.
2. Work Memory Architecture of the 6ES7 315-2EH13
The CPU provides 256 KB of integrated work memory, split internally between code and data. For firmware V2.6 the partitioning is fixed:
| Memory Area | Size | Volatile? | Backed by MMC? |
|---|---|---|---|
| Program memory (OB / FC / FB / SFC / SFB code) | ~192 KB | Yes | Yes, full load image stored |
| Data memory (DBs, instance data) | ~64 KB | Yes | Yes, full load image stored |
| Load memory (STEP 7 project + symbols) | Equals MMC size (minus firmware image) | No | Yes, native medium |
| System memory (bit memory, timers, counters, I/O image) | 2 KB bit / 256 timers / 256 counters | Yes | Retentive range only |
On a power-down, the complete work memory is lost unless a corresponding load image exists on the MMC. Retentive flags and DB instances defined with the RETAIN attribute are restored from a backup area on the MMC at the next startup (this backup is the 64 KB retentive area on the MMC, not a function of the MMC size).
Conceptually the memory hierarchy looks like this:
3. The 1.5x Work-Memory Sizing Rule
Siemens' standard recommendation for the load memory on an S7-300 MMC is to oversize the work memory by a factor of 1.5. The reason is that the load image is compressed in work memory, but on the MMC the same blocks are stored uncompressed with the diagnostic data, comments, SDBs, and STEP 7 housekeeping. The compressed-to-uncompressed expansion factor is typically 1.3 to 1.7 depending on the project:
MMC_min_for_load = 1.5 x Work_Memory
For the CPU 315-2EH13-0AB0 this gives:
MMC_min = 1.5 x 256 KB = 384 KB => next available step = 512 KB
That is why a 512 KB MMC (order number 6ES7 953-8LF00-0AA0) is technically sufficient to store the entire load image of a fully loaded CPU 315-2 PN/DP. The next smaller size (128 KB MMC) is not produced for this CPU and is not on the compatibility list.
4. The Firmware-Update MMC Requirement (4 MB Minimum)
The firmware image of an S7-300 PN/DP CPU is not stored on the MMC by default; it is held in a separate Flash area on the CPU itself and is updated through a Siemens service procedure. To perform a firmware update you must place the firmware file on the MMC and instruct the CPU to flash it on the next power-up.
The combined size of the firmware file plus the still-resident load image forces a higher MMC minimum for the update operation. For the CPU 315-2EH13 the official minimum is 4 MB:
| Operation | MMC Minimum | Siemens Reference |
|---|---|---|
| Normal program operation, empty project | 512 KB (6ES7 953-8LF00-0AA0) | Operating Instructions, Section 4.5 |
| Typical project, DP + PN, with GSDML | 2 MB (6ES7 953-8LJ00-0AA0) | Operating Instructions, Section 4.5 |
| Firmware update of CPU 315-2 PN/DP | 4 MB (6ES7 953-8LL00-0AA0) | FAQ "Operating system update for CPU 315-2 PN/DP" on Siemens Industry Online Support |
| Full PROFINET project + device descriptions + archive | 8 MB (6ES7 953-8LM00-0AA0) | Operating Instructions, Section 4.5 |
The 4 MB threshold is a hard minimum: with a 2 MB card the firmware update cannot be carried out because the image does not fit alongside the existing load image. If the new firmware is downloaded to an empty MMC (for example as part of a CPU replacement) the update may succeed on a 2 MB card in some cases, but Siemens explicitly does not support this path.
5. Why the Manual Mentions 8 MB for PROFINET
The phrase "minimum 8 MB if PROFINET is used" that appears in the S7-300 CPU 31xC and 31x operating instructions is often read as a strict lower bound. In practice the figure comes from two sources:
- GSDML device descriptions. Each PROFINET IO device referenced in HW Config stores its GSDML (typically 30-200 KB) on the MMC. A 50-device project consumes 1.5-10 MB on its own.
- STEP 7 project archive. Engineers routinely keep a copy of the STEP 7 source project (.zip) on the same MMC for field service. A moderately commented STEP 7 project for the 315-2 PN/DP is 1-4 MB.
Siemens adds the 1.5x factor on top of those two storage consumers, which is where the 8 MB recommendation originates. The recommendation is not a hardware minimum for the CPU itself, it is a recommendation for a comfortable field-deployment configuration.
Decision rule in plain language:
- Just to run the program: 512 KB suffices (1.5x rule).
- To update the firmware: 4 MB is the hard minimum.
- To operate PROFINET and keep a project copy on the MMC: 8 MB is the comfortable size.
6. MMC Order Numbers and Compatibility
Only the MMC family listed in the operating instructions is guaranteed to work in the CPU 315-2 PN/DP. Third-party SD-to-MMC adapters frequently fail the CPU's presence detection and the CPU will refuse to start.
| Order Number (MLFB) | Size | Spare-part code | Recommended Use |
|---|---|---|---|
| 6ES7 953-8LF00-0AA0 | 512 KB | S7-300 MMC 512K | Minimal load, no firmware update |
| 6ES7 953-8LJ00-0AA0 | 2 MB | S7-300 MMC 2M | DP-only projects, low PN device count |
| 6ES7 953-8LL00-0AA0 | 4 MB | S7-300 MMC 4M | Firmware update capable, most projects |
| 6ES7 953-8LM00-0AA0 | 8 MB | S7-300 MMC 8M | PN projects + project archive on MMC |
| 6ES7 953-8LP00-0AA0* | 64 MB | Not released for 315-2EH13 | Not supported on this CPU |
*The 64 MB MMC and all larger cards target the S7-400, S7-1200, and S7-1500 families. They are physically the same form factor but will not be recognized by a 6ES7 315-2EH13 and may cause a continuous MMC-fault cycle.
For an application that will run for ten years in a brownfield installation, prefer the 4 MB MMC (6ES7 953-8LL00-0AA0) because it covers firmware updates without oversizing the part. The incremental cost between 4 MB and 8 MB is rarely justified unless the project stores archives on the card.
7. Step-by-Step: Selecting and Installing the MMC
The following procedure ensures the MMC you select is correctly recognised and the CPU boots cleanly with the new card.
- Confirm the CPU MLFB and firmware. Open the module door and read the label, or read Online → Accessible Nodes in STEP 7 / TIA Portal. The diagnostic buffer entry "Module identification" lists the full MLFB.
- Estimate the load-image size. In STEP 7 (classic), open PLC → Module Information → Memory with the existing project loaded. The Load memory row gives the byte count actually consumed. Multiply by 1.5 to obtain the minimum MMC size.
- Cross-check against the firmware update requirement. If you plan to perform an OS update in the life of the CPU, the 4 MB minimum applies regardless of the load image size.
- Order the MMC from Siemens Industry Mall. Use the order numbers in Section 6. Spare-part codes are accepted by the Siemens quotation system.
- Insert the MMC with the CPU powered down. The slot is located behind the lower flap on the front of the CPU. Push the card in until the ejector spring clicks. The label side faces left (LED side), contacts face down.
- Power on and observe the LEDs. MMC LED must extinguish within 5 s. STOP LED must be steady on or flashing; RUN must come on within 30 s if the project on the MMC is consistent.
- Verify in STEP 7. Online → Accessible Nodes → Module Information → Memory should show the new MMC size and the same load memory in use.
8. Verification Procedure After a New MMC is Inserted
After any MMC swap, run the following verification matrix before returning the machine to production:
| Check | Tool / Path | Expected Result | Action if Failed |
|---|---|---|---|
| CPU goes RUN | Front-panel RUN LED | Steady green within 30 s | Re-seat MMC, check project CRC |
| MMC detected | Module Information → Memory → "Load memory" | Reports new size in KB | Replace MMC, suspect counterfeit |
| PROFINET devices come online | Online → PROFINET IO → Topology | All IO devices green | Check device name assignment |
| Retain data restored | Compare tag values vs last recipe | Equal | Re-load retain variables |
| Diagnostic buffer clean | Module Information → Diagnostic Buffer | No "MMC fault" entry | Replace MMC, check contact spring |
| Free load memory | Module Information → Memory → "Free load memory" | > 1 MB for future updates | Re-evaluate MMC size |
9. Troubleshooting MMC-Related Faults on CPU 315-2EH13
| Symptom | LED Pattern | Diagnostic Buffer Event ID | Likely Cause | Remedy |
|---|---|---|---|---|
| CPU in STOP, MMC LED steady red | STOP + MMC red | 16#02_3A_0001 | No MMC inserted | Insert MMC |
| CPU in STOP after MMC swap | STOP flash, MMC red | 16#02_3A_0100 | MMC write-protected or corrupt | Power down, re-seat, replace card |
| Firmware update aborts | STOP flash 1 Hz, then STOP steady | 16#02_3F_0001 | MMC < 4 MB or insufficient free space | Use 4 MB (or larger) MMC, free > 2 MB |
| Retain lost after power cycle | RUN, no fault | None, value reset to initial | RETAIN attribute not set on DB | Edit DB properties → Non-Retain → Retain |
| PROFINET devices fail to come up | BUSF1 + BUSF2 flashing | 16#02_3E_0010 | Topology mismatches the saved SDBs | Recompile HW Config, re-download |
| CPU cycles STOP/RUN every 60 s | STOP / RUN alternating | 16#02_3A_0200 | MMC counterfeit or contact issue | Replace with Siemens-sourced MMC |
10. Cost vs Risk Trade-off
Price per megabyte on S7-300 MMCs is nearly flat; the price gap between a 2 MB and an 8 MB card is small in absolute terms. The engineering cost of a single field service call to change a 2 MB card for a 4 MB card because a firmware update is required several years later is an order of magnitude larger. The default recommendation is therefore:
| Scenario | Recommended MMC | Order Number |
|---|---|---|
| New machine, expected lifetime > 5 years, PN + DP used | 4 MB | 6ES7 953-8LL00-0AA0 |
| New machine, archive copy of project stored on MMC | 8 MB | 6ES7 953-8LM00-0AA0 |
| Spare CPU held in stock, no project | 512 KB | 6ES7 953-8LF00-0AA0 |
| Planned firmware update of an existing fleet | 4 MB | 6ES7 953-8LL00-0AA0 |
11. Frequently Asked Questions
What is the minimum MMC size for CPU 315-2EH13-0AB0?
The minimum MMC for normal program operation is 512 KB (6ES7 953-8LF00-0AA0), applying the 1.5x work-memory rule (1.5 x 256 KB = 384 KB rounded up to the next commercial step). This is sufficient only when no firmware update is planned and no extra project files are stored on the card.
Does the S7-300 manual really require 8 MB when PROFINET is used?
No, the 8 MB figure in the operating instructions is a recommendation for a comfortable field-deployment configuration, not a hardware minimum. The strict minimum for PROFINET operation is the 1.5x rule (512 KB), and the strict minimum that also allows firmware updates is 4 MB (6ES7 953-8LL00-0AA0).
Why does the firmware update require 4 MB and not 512 KB?
The firmware image of the CPU 315-2 PN/DP plus the existing load image on the MMC must coexist on the card during the update. The combined size does not fit on a 2 MB card, which is why Siemens specifies 4 MB as the hard minimum for an OS update.
Can I use a 64 MB or larger MMC from an S7-1500 in the 315-2EH13?
No. The CPU 315-2EH13 only recognises MMCs from the 6ES7 953-8Lxxx-0AA0 family, up to 8 MB. Larger cards (64 MB, 256 MB) are physically identical but are not in the compatibility list and will not be detected, leaving the CPU in STOP with an MMC-fault entry.
What happens if I power the CPU without an MMC?
The CPU goes to STOP with the diagnostic buffer entry "No MMC inserted" and the MMC LED is lit steady red. The CPU cannot enter RUN without a valid MMC; the load image, SDBs, and firmware backup are all stored exclusively on the card.
How do I know if a fitted MMC is genuine Siemens?
Genuine S7-300 MMCs have a Siemens logo and the order number laser-etched on the rear surface. Counterfeit cards frequently report the correct size in the operating system but fail write-verify after several hundred power cycles; if a CPU is cycling STOP/RUN with a valid-looking card, the MMC is the first suspect.