S7-300 CPU 315-2DP CPU Replacement: MMC Transfer Procedure

Overview

The SIMATIC S7-300 CPU 315-2DP (order number 6ES7315-2AFxx-0AB0 or 6ES7315-2AHxx-0AB0 family) stores the entire STEP 7 user program, the system data, and the configurable parameters on the plug-in Micro Memory Card (MMC). Because the program and project data live on the MMC and not in internal flash, replacing the CPU with an identical (or firmware-compatible) part number is essentially a card-swap operation. No STEP 7 download to the new CPU is required, provided the MMC is the same physical card and the new CPU is healthy.

This procedure covers: identification of the existing CPU and MMC, pre-replacement backup, safe card insertion rules, the RAM-to-ROM function for persisting live actual values to the MMC, and post-replacement verification with STEP 7 V5.x or TIA Portal.

Prerequisites

• Replacement CPU with identical or higher firmware than the original. Common order numbers:
  • 6ES7315-2AF03-0AB0 (FW ≤ 2.0.x)
  • 6ES7315-2AF04-0AB0
  • 6ES7315-2AH10-0AB0
  • 6ES7315-2AH14-0AB0 (latest 2-series variant)
• PG/PC with STEP 7 V5.4 / V5.5 / V5.6 (classic) or TIA Portal V13 SP1 or higher, plus a PC adapter (USB-MPI or USB-PROFIBUS) or an Ethernet CP for online access.
• Original MMC card already populated with the running project (preferred path).
• A second, empty MMC of equal or larger capacity as a working spare (e.g., 6ES7953-8LG31-0AA0, 8 MB).
• Documentation of the configured MPI/PROFIBUS node address and the IP address of the integrated PN/DP interface (where applicable).
• Operator present, controlled shutdown authorized, process I/O in safe state.

CPU 315-2DP Hardware Identification

Before pulling the card, record the existing module’s identity. The order number is printed on the front label, but you can also read it from STEP 7 via PLC → Module Information → Order Number.

The mode switch positions are standardized on every S7-300 CPU:

• RUN – program execution, read-only from PG.
• RUN-P – program execution, PG can write (default after power-up on a project-bearing MMC).
• STOP – program halted, all outputs frozen to configured substitute values.
• MRES – memory reset (used only for clearing a CPU; do not use it during a card swap).

MMC Types and Compatibility

The S7-300 family uses the proprietary Siemens MMC. They are not interchangeable with SD cards or PCMCIA cards of other platforms.

Pre-Replacement: Backup the Project

1. Connect the PG to the CPU via MPI/PROFIBUS or the PN port. In STEP 7 classic choose PLC → Upload Station to PG; in TIA Portal choose Online → Upload device as new station (hardware and software).
2. Save the uploaded project under a date-stamped name (e.g., PlantA_CPU315_Backup_2024-05-17.zip). This is a defensive backup only; the new CPU will boot from the original MMC, not from this archive.
3. Open PLC → Module Information → Diagnostic Buffer. Export the buffer to a text file — this becomes the baseline for post-swap fault comparison.
4. Record CPU → Clear/Reset protection status. If the CPU is password-protected, capture the password out of band; the new CPU will prompt for it on first STOP→RUN transition if the project is encrypted.
5. Note the IP / PROFIBUS address. The S7-300 stores network parameters with the project on the MMC, so they will travel with the card, but document them for sanity-checking after the swap.

Physical Swap Procedure

1. Bring the controlled process to a safe state. Energize machine interlocks, coast drives, and post operator warning signs.
2. Toggle the CPU mode switch to STOP. Verify the ST LED is solid yellow.
3. Open the front cover of the old CPU. The MMC slot is at the top of the module behind a small ejector flap. Press the ejector gently and pull the card straight out. Do not bend; the contact edge is fragile.
4. Visually inspect the card contacts for contamination, oxidation, or bent pins. If dirty, wipe with a dry, lint-free ESD cloth. Do not use solvents.
5. With the new CPU unpowered (or in STOP), insert the MMC into the new CPU’s slot with the label facing outward. Push firmly until the ejector snaps flat.
6. Apply power to the PS 307. The new CPU performs a startup test, reads the MMC, and transitions to RUN if the mode switch is on RUN or RUN-P and the project is consistent.
7. Verify the SF (group error) and BF (bus fault) LEDs are off. A flashing BF on the first start is normal during DP slave enumeration.

RAM-to-ROM: Persisting Actual Values

The MMC stores two distinct sets of data:

• Initial values — the snapshot at last download or last Copy RAM to ROM operation. These are the values that re-appear after a memory reset or power-up without remanence.
• Actual values — the live runtime values currently in work memory, including any modifications made by the program (counters, setpoints, recipes, data block edits from the HMI).

If the plant has been running long enough that the actual values diverge from the initial values (e.g., tuned PID gains, set recipe data, runtime-modified tags), and you want those values to survive the next overall reset or the next cold start, you must copy them to the MMC before pulling the card.

Procedure in STEP 7 V5.x:

1. PLC → Copy RAM to ROM…
2. Confirm the dialog. The runtime-relevant data blocks are copied from main memory to the MMC. The progress is shown in the diagnostic buffer as event 0x4561 (HMI: "RAM to ROM completed").
3. After completion, open any DB online and verify that the value in the Actual value column equals the value you expect. This column will now be the new Initial value after a memory reset.

Procedure in TIA Portal:

1. Online → Snapshot of the actual values (TIA V15+), then Download to device with the option Download actual values to MMC.
2. Alternatively, right-click the CPU → RAM to ROM.

Reference: the official Siemens FAQ "What should I watch out for with "RAM to ROM" in STEP 7 V5.x and in the TIA Portal?" covers the diagnostic buffer events, the requirement that the MMC be present, and the password implications.

First Power-Up of the New CPU

1. With the MMC seated and the mode switch in STOP, apply 24 V to the PS 307.
2. Watch the LED sequence:
   • All LEDs on briefly → self-test.
   • SF red / BF red flash during DP startup is acceptable for the first 5–15 seconds while slaves are parameterized.
   • RUN green solid + ST yellow off = healthy RUN.
3. Go online with STEP 7 / TIA Portal and confirm:
   • Operating mode: RUN
   • Cycle time: matches the original within ± 5 %
   • Diagnostic buffer: only the new POWER ON events and the expected RAM to ROM history; no SF events

If the CPU stays in STOP with SF lit, see the troubleshooting matrix below.

Verification Checklist

Troubleshooting Matrix

Field-Proven Caveats

• Pull the card last. Even if the documentation lets you swap power-on, technicians report far fewer fault entries when the card is moved with the CPU unpowered.
• Don't confuse MMC with MC. The S7-300 uses MMC. The S7-400 uses MC (Memory Card). They are mechanically and electrically incompatible.
• Real-time clock battery. The CPU 315-2DP has no integrated battery — the clock is buffered by a supercapacitor for a few days at most. After a multi-day outage expect the clock to be wrong; re-set via PLC → Set Time of Day or via NTP through a CP.
• Remanence. The CPU 315-2DP supports remanent bit memory, counters, and timers backed by the MMC. Verify the remanence settings in HW Config → CPU properties → Retentive Memory; otherwise data will be lost on every power-down.
• Keep the old CPU’s MMC as cold spare. Even after a successful swap, label and archive the old MMC; it is the fastest path to a fallback if the new unit is later diagnosed as defective.

FAQ