S7-1200 SIMATIC SD Card: Firmware Update & CM 1243-2 AS-i Setup

Overview: SIMATIC Memory Card, Firmware Update, and AS-Interface for the S7-1200

Engineers integrating a SIMATIC S7-1200 into an AS-Interface (AS-i) network face three coupled decisions: (1) which AS-i master module to mount on the left side of the CPU, (2) which AS-i power supply to wire into the yellow AS-i cable, and (3) which SIMATIC memory card to use for firmware updates. A common pitfall is the assumption that any SD card can be formatted to act as a Siemens memory card — it cannot. The S7-1200 boot loader checks the card's Master Boot Record and a Siemens-specific identifier file before accepting a firmware update payload. Off-the-shelf SD cards fail this check and produce a steady ON/OFF LED pattern on the CPU.

This reference consolidates the catalog numbers, the firmware update procedure, the differences between the CM 1243-2 AS-i master and the CM 1243-5 DP master, the DCM 1271 data decoupling module, and the third-party AS-i power supplies (Pepperl+Fuchs) that can substitute for the Siemens AS-i supply. All references link to official Siemens Industry Online Support and TIA Portal documentation.

SIMATIC S7 Memory Card Family and Selection Matrix

The S7-1200 accepts only SIMATIC S7 memory cards (also called SIMATIC MC) for firmware updates, program-card operation, and service data logging. The card is inserted into the slot located behind the removable cover on the front of the CPU. The card is keyed and labeled so that it cannot be confused with a standard SD card.

Source: Siemens SiePortal — 6ES7954-8LT03-0AA0 and Siemens SiePortal — 6AV2181-8XP00-0AX0.

Why a Generic SD Card Cannot Be Used for Firmware Updates

The S7-1200 firmware update mechanism relies on three checks that a standard SD card fails:

1. Master Boot Record (MBR) signature check. Siemens pre-formats the S7 memory card with a non-standard partition table that places the S7 firmware container in a hidden primary partition. A consumer SD card formatted with FAT32 places its first partition at sector 0, which the CPU's boot ROM does not recognize.
2. Vendor identification block. Each Siemens card contains a 512-byte vendor descriptor block in sector 1 that identifies the card as a SIMATIC MC. The descriptor is written at the factory with a Siemens private key and is checked at boot. Off-the-shelf cards return a vendor ID of "Generic" or the card manufacturer's ID, which the boot loader treats as a foreign medium.
3. Firmware container header. The actual update file (S7_FWUP.S7S or similar) is signed by Siemens. The card must be write-protected-compatible (it is not a true WP switch, but the descriptor prevents the CPU from writing a new container over the old one). Some aftermarket cards are pre-formatted with a fake "Siemens" header that the S7-1200 boot ROM rejects on the second pass.

When any of these checks fail, the CPU's STOP, RUN, and ERROR LEDs produce a diagnostic pattern:

Step-by-Step: S7-1200 Firmware Update via SIMATIC Memory Card

Prerequisites:

• CPU fully powered down.
• SIMATIC S7 memory card, 24 MB, 6ES7954-8LF03-0AA0 (or legacy 6ES7954-8LF00-0AA0).
• Firmware update file (.upd) downloaded from the Siemens support page for the exact CPU MLFB.
• PG/PC with SD card reader, Windows 10/11, and TIA Portal V13 SP2 or later (only required to write the file to the card; not required for the update itself).

Procedure:

1. Identify the CPU MLFB. Read the label on the side of the CPU. The MLFB follows the pattern 6ES721x-xxxxx-xxxx. Each CPU variant has a unique firmware.
2. Download the firmware update file. Navigate to the Siemens support entry for the specific CPU. The file is delivered as S7FWUPD.UP or <MLFB>_Vxx.upd. Confirm the file size is between 1 MB and 6 MB for an S7-1200 update.
3. Insert the SIMATIC MC into the PG's SD reader. Windows may show a "format disk" prompt. Do not format. Click Cancel. If Windows already formatted the card, the card is permanently damaged for Siemens use and must be replaced.
4. Copy the .upd file to the card root. Place the file in the top-level directory of the SIMATIC MC. Do not place it in a subfolder. Do not rename the file — the CPU looks for a specific filename pattern.
5. Safely eject the card. Use the Windows "Safely Remove Hardware" tray icon. A hot-unplug can corrupt the vendor descriptor block.
6. Power down the CPU. Move the mode switch to STOP. Power off. Wait 10 seconds for the internal capacitors to discharge.
7. Insert the card into the CPU. The card slot is behind the front cover. The card's beveled corner faces up. The slot is keyed to accept only the SIMATIC MC, not a standard SD card.
8. Power on the CPU. The CPU reads the card automatically. The sequence is: STOP LED ON, then STOP LED blinks, then RUN LED ON when the firmware is loaded. Total time: 30–90 seconds for an S7-1200.
9. Verify the firmware version. In TIA Portal, go online with the CPU, read the device identification, and confirm the firmware version field matches the version downloaded in step 2.
10. Remove the card. The card is not required for normal operation unless it is being used as a program card. Leaving the card in the slot can shorten its write-cycle life.

AS-Interface Master Selection: CM 1243-2 vs CM 1243-5

The CM 1243-x family is a common source of catalog confusion. The suffixes denote different protocol masters, not different AS-i revisions:

Rule: For AS-Interface, order the CM 1243-2 (3RK7243-2AA30-0XB0). The CM 1243-5 is a PROFIBUS module and will not bring up an AS-i network. The "-2" suffix in the Siemens catalog numbering indicates the second protocol family assigned to the 1243 form factor, which historically was AS-i.

Source: Siemens SiePortal — CM 1243-2 AS-i Master.

The CM 1243-2 supports AS-Interface Specification V3.0, allowing 62 digital AS-i slaves per network (A/B addressing). Per slave, the master exchanges 4-bit bidirectional process data. Cycle time for a full network is approximately 5 ms with 31 standard slaves or 10 ms with 62 A/B slaves. The module appears in the TIA Portal device tree as "AS-i master" once installed, and is configured via the CM 1243-2 wizard rather than the standard S7-1200 hardware configuration.

AS-i Power Supply: Siemens DCM 1271 vs Third-Party Units

An AS-i network requires a 30 V DC power supply with data decoupling. The data decoupling unit prevents the 2.4 V_pp AS-i communication signal from being shorted by the power supply's output capacitor. Siemens offers an integrated data decoupling module in the same form factor as a standard S7-1200 signal module:

Source: Siemens SiePortal — DCM 1271.

Substitution rule: If a third-party AS-i power supply (such as a Pepperl+Fuchs unit) is already on-site, it can replace the Siemens DCM 1271 only if it includes integrated data decoupling. A bare 30 V DC industrial power supply (Phoenix Contact, Meanwell, etc.) cannot be used because it will short the AS-i communication signal. Verify the data decoupling choke is present by reading the data sheet's "AS-i compatible" or "VAS-i" certification.

TIA Portal Hardware Catalog: Why CM 1243-2 May Be Missing

Older TIA Portal installations do not show the CM 1243-2 in the device catalog. The module's HSP (Hardware Support Package) is not bundled with the base install — it must be installed separately or supplied by a TIA Portal version that includes it.

Source: TIA Portal Documentation — SIMATIC Memory Cards (S7-1500) (the S7-1200 card section uses the same vendor descriptor mechanism).

Third-Party AS-i Components: Pepperl+Fuchs Gateways

Pepperl+Fuchs is the most common third-party supplier of AS-Interface components outside Siemens. Common models in a Pepperl+Fuchs AS-i architecture include:

When a Pepperl+Fuchs gateway is used in place of the CM 1243-2, the S7-1200 exchanges AS-i data with the gateway over PROFINET or EtherNet/IP rather than the dedicated AS-i backplane bus. The S7-1200 program must include the gateway's GSDML file, and the AS-i diagnostics become fieldbus-level (PROFINET diagnostics) rather than backplane-level. Plan the engineering effort accordingly.

SIMATIC Memory Card Service Life and Write Cycle Considerations

The S7 memory card uses a single-level cell (SLC) NAND flash designed for industrial temperature ranges (-25 °C to +70 °C operational). The approximate service life depends on the card capacity and the write volume. The Siemens datasheet provides a formula:

Service life [years] = (Card capacity [GB] × Write cycles per cell) / (Data write rate [GB/yr] × Wear-leveling factor)

For a 24 MB S7 memory card used purely for firmware updates (a few MB written per year), the service life is effectively unlimited. However, if the card is configured as a service data card (DataLog) with high-frequency trace recordings, the write rate can shorten the life to 3–5 years. Replace the card preventively every 5 years in high-write applications.

Source: Siemens SiePortal — S7 Memory Card 32 GB (service life section).

Verification: Confirming the Firmware Update and AS-i Master Operation

1. Firmware version check. In TIA Portal, go Online > Online & diagnostics > Diagnostics > Module information. The firmware version field must match the target version from the .upd file. If it shows the previous version, repeat the update procedure.
2. Card removal verification. Power down, remove the SIMATIC MC, and power up. The CPU must boot into RUN with the new firmware. If the CPU reverts to the old firmware, the .upd file was not written to the card correctly or the card is not a SIMATIC MC.
3. CM 1243-2 LED status. With the CM 1243-2 mounted and powered, the module's LEDs report: SF (red, off in normal state), AS-i (green, ON when configured), AUX (green, ON when 24 V backplane is good). All three green ON indicates the master is running.
4. AS-i slave detection. In TIA Portal, open the AS-i master wizard and run Configuration > Read actual configuration. The detected slaves must match the expected list. Detected slaves with a red icon indicate an address conflict or wiring error.
5. AS-i power supply voltage. Measure the yellow AS-i cable voltage at any slave. It must be between 29.5 V DC and 30.5 V DC. If the voltage is below 29 V DC under load, the data decoupling module is undersized for the network's continuous current draw.

Troubleshooting Matrix: Common Failure Modes

FAQ