S7-1200 Memory Card: Load/Work Memory Architecture Guide

Overview

The S7-1200 CPU family does not behave like an S7-300/400 with respect to data block (DB) placement: a DB declared in the project is always loaded into work memory at runtime, and there is no instruction equivalent to SFC 82 / SFC 83 / SFC 84 ("Read from load memory", "Write to load memory") that can page a large DB in and out of the SIMATIC Memory Card (SMC) on demand. Understanding the three-tier memory model, the role of the SMC, and the firmware-specific size limits is mandatory before declaring an Array [0..30000] of Byte or planning a 1 MB+ data store on a 1211C/1212C/1214C/1215C/1217C.

This reference consolidates the system manual constraints, firmware version deltas, and the field-proven workarounds (DataLog, recipes, OPC UA, external storage) that let you push beyond the native work memory ceiling without re-architecting the PLC.

S7-1200 Memory Architecture

Every S7-1200 CPU contains three logically distinct storage areas, described in the S7-1200 Programmable Controller System Manual:

On a download from TIA Portal, the entire project is written to the SMC (or, in older firmware, to internal load memory) and the CPU copies the executable portions into work memory at start-up. The DB instances your program reads and writes are physically located in work memory; the SMC only retains them in compressed project form for power-up reload.

Role of the SIMATIC Memory Card (SMC)

The SMC is a pre-formatted SD-card style device (some early CPUs used MMC). Its functions are:

1. Project storage: When a card is inserted, the project is downloaded to the card; without a card, project storage falls back to internal flash on supported CPUs.
2. CPU firmware update: The card can be used to update the CPU's firmware (front switch or automated).
3. Transfer of a working project between CPUs: Insert a programmed SMC into a new CPU of the same family; the CPU boots from the card.
4. Data storage for DataLog/Recipe files: CSV-formatted data created at runtime by the user program.
5. Service data backup: Optional, when the card contains a project file with the same article number.

The 24 MB card the user observed is the standard 6ES7954-8LF02-0AA0 (or the legacy 6ES7954-8LE02-0AA0 for 12 MB). Newer firmware levels accept the 256 MB (6ES7954-8LL02-0AA0), 2 GB (6ES7954-8LP02-0AA0), and 32 GB (6ES7954-8LT03-0AA0) variants. The card is not user-partitioned - the entire card is one Siemens-proprietary file system.

Can a DB be Placed in Load Memory Only?

Short answer: No. The S7-1200 instruction set does not include SFC 82 (S7-300/400) or the S7-1500 READ_DBL/WRIT_DBL instruction family (SFC/FB) that would allow a DB to live in load memory and be paged on demand. Every DB declared in TIA Portal - including an Array [0..30000] of Byte - is statically allocated in work memory after the program is loaded.

This is a deliberate platform design decision. The S7-1200 targets mid-size discrete control and is not designed for large structured-data handling; the S7-1500 is the platform that introduces load-memory-resident DBs through READ_DBL / WRIT_DBL and the "memory area in load memory" attribute on the DB.

Work Memory Limits by CPU Model

The most relevant constraint for declaring a 30,001-byte array or planning 1 MB+ of data is the work-memory budget. The values below are nominal totals; usable memory is smaller because the runtime system, system blocks, and the OB/FC/FB code also consume work memory.

A single Array [0..30000] of Byte consumes 30,001 bytes (~30 KB) just for the data section, before the compiler adds symbol table overhead and any structure alignment. On a 1211C with 50-100 KB of work memory, this single array can consume 30-60% of the budget and will trigger a download diagnostic if the residual memory is insufficient for code.

Memory Card File System Internals

The SMC uses a Siemens-proprietary FAT-like file system. Files visible from the user-program perspective are limited to:

• LOG_<DataLogName>.csv - created by the DataLogCreate / DataLogWrite instruction family.
• <RecipeName>.csv or .bin - created by the RecipeExport / RecipeImport family (V4.0+).
• S7_JOB.S7S - read/write/delete operations queued by the Web server or user program.
• The project archive: SIMATIC.S7S (or crashdump.bin on diagnostics event).

From TIA Portal you can browse the card via "Online > Memory card > [user-card]" and read/write CSV files. The user program accesses the file system through the extended instructions DataLog and Recipe in the program editor under Optional packages > Recipes and File handling.

Storing >1 MB of Data on S7-1200 - Practical Strategies

When the application demands more data than the work memory can hold, the following field-proven strategies apply. Each has different latency, persistence, and bandwidth characteristics.

Strategy 1 - DataLog to SMC

The DataLog instruction family (V2.0+) writes a CSV file directly to the SMC. The file is not loaded into work memory; the CPU streams records from the user program buffer (typically 1 KB to 4 KB) into the card file.

• Max records per file: limited by SMC free space (multi-MB).
• Access latency: ~5-20 ms per record write, depending on record size.
• Read-back: DataLogRead can pull N records into a target DB.

Best fit: trending, batch records, audit trails, event logs. Not suitable for fast random access.

Strategy 2 - Recipe DB + SMC file export

Create one or more "active" recipe DBs of the data you actively manipulate (must fit in work memory), and use RecipeExport to push the current DB contents to a CSV file on the SMC, then free the DB for a different recipe. The data on the SMC survives power-cycle, the in-RAM copy is replaced by RecipeImport when the user picks a different set.

Best fit: machine set-ups with dozens of named recipes but only one loaded at a time.

Strategy 3 - Store-and-forward over network

The user's original requirement ("store > 1 MB and send it by network") is a classic store-and-forward problem. On S7-1200 with firmware V4.4+ the native OPC UA server (DA profile) exposes selected DBs as readable nodes, with no additional hardware. A SCADA/IoT gateway pulls the data on schedule, freeing the PLC from the storage burden. Older firmware (V4.0-V4.3) requires a S7 connection (PUT/GET on PN, or TSEND/TRCV on TCP) and a partner PLC or HMI/PC that archives the stream.

Best fit: when the consumer of the 1 MB+ data is a host system, not the PLC program itself.

Strategy 4 - External storage via I/O-Link or Modbus TCP

For data that does not need to be in the PLC, a Modbus TCP register bank on an external gateway (Phoenix Contact, Wago, Beckhoff) or an SD-card-equipped I/O-Link master (e.g., Siemens SCALANCE + ET200 I/O-Link master) provides GB-scale storage. The S7-1200 exchanges only the working set in work memory.

Best fit: long-term historian data, video stills, large recipe libraries.

Strategy 5 - Migrate to S7-1500

If the data genuinely must live in the PLC and be randomly accessible, an S7-1500 CPU (e.g., CPU 1511-1 PN at 300 KB work memory, CPU 1515-2 PN at 500 KB, or 1518 with 3 MB) plus the load-memory DB attribute and the READ_DBL / WRIT_DBL instructions is the engineering-correct solution. A S7-1500 DB flagged "Memory area = Load memory" can exceed work memory; the runtime instructions page 4 KB segments in and out on demand. This is not available on S7-1200 and will not be added in any planned firmware.

Firmware-Version-Conditional Behavior

Memory-related behavior changed across S7-1200 firmware versions. The points below are the ones that affect the 1 MB+ data question:

The user's observation that the SMC is 24 MB is consistent with the 6ES7954-8LF02-0AA0 card; that article number is supported across all V4.x firmware. Larger 256 MB / 2 GB / 32 GB cards are usable on V4.4+, and the V4.5+ firmware added 32 GB support to support long DataLog retention in edge applications.

Worked Example - 30,001-Byte Array on a 1214C

To make the trade-off concrete, suppose the goal is to declare myDB.data : Array [0..30000] of Byte on a CPU 1214C (6ES7214-1AG40-0XB0) at firmware V4.2.

1. Data bytes required: 30,001 B ≈ 29.3 KiB.
2. Code + system block overhead typical: 25-40 KB (depends on OB/FC/FB count).
3. Work memory budget (V4.2): 150 KB.
4. Residual after array: ~80-95 KB for code and other DBs.
5. Verdict: Fits, but tight. If additional DBs or large FBs (PID_Compact instances, Motion blocks) are added, the compile will fail with diagnostic "Insufficient work memory in CPU" or the download will warn about > 80% utilization.

If the target were a 1211C (V4.0, 50 KB work memory), the same array would exceed 50% of the budget before code and is not advisable. Re-size the array or use a tighter element type (Array [0..30000] of Bool ≈ 3.8 KB, packed into a DB bit area).

Diagnostics and Verification

After deployment, verify the actual load on the CPU and the SMC via TIA Portal online diagnostics:

1. Right-click the CPU in the project tree > Online > Diagnostics. The "Memory" tab reports the current load memory usage, work memory usage, and retentive memory usage in bytes and as a percent of nominal.
2. Open Online > Memory card > [user-card] to list the file-system contents and free space on the SMC.
3. In the CPU's diagnostic buffer, the following events are memory-related and should be alerted in the HMI:

The diagnostic buffer can be exported as plain text and correlated to the application timestamp; many SCADA packages log it as part of plant historian. The SMC free-space should be monitored via the user program if DataLog is used heavily; a DataLogClose and DataLogDelete sequence at the end of each shift prevents card-full events.

Troubleshooting Matrix

Decision Path for the Original Question

Returning to the user's question: "I need to store more than 1 MB of information with the program on S7-1200 and send it by network. Do you know a way to store information that is not on work memory?" The decision tree:

1. If the data is structured and randomly accessed at runtime by the PLC program - it must be in work memory. A 1 MB+ structure is impractical on S7-1200 (largest work memory is 400 KB on a 1217C at V4.4+). Choose an S7-1500 CPU or external storage.
2. If the data is collected and forwarded - use the OPC UA server (FW V4.4+) or PUT/GET on PROFINET to push records to a host as they are created. Do not store them in the PLC at all.
3. If the data is appended and archived - use DataLog on the SMC. The 24 MB card is sufficient for tens of thousands of records; the 256 MB/2 GB/32 GB cards extend this to millions.
4. If the data is a set of named recipes, only one active at a time - keep one active DB in work memory and use the Recipe FB family to swap CSV files on the SMC.

Across all four cases the user's underlying assumption - that the SMC is a generic secondary memory that can be addressed like an S7-300 MMC - is incorrect for the S7-1200 platform. The card is project + DataLog + Recipe storage, and the user program can only manipulate it through the predefined instructions, not by direct symbol access.

Field-Proven Tips

• Always insert the SMC before powering the CPU; hot-swap is not supported and will generate diagnostic buffer events.
• Keep at least 5 MB free on the SMC at all times. DataLog performance degrades sharply when free space drops below this threshold.
• For firmware upgrades, copy the project to the SMC after the firmware update has been performed; the order matters to avoid an inconsistent project archive.
• If you must retain DB values across a power-cycle, configure the retentive area in PLC properties > Retain memory before downloading. A common mistake is to mark tags "retain" without increasing the retain budget - the compiler accepts it but the values are not actually saved.
• When planning a CPU upgrade, check that the new MLFB accepts the same SMC article number. Some early CPU revisions had SMC compatibility restrictions documented in the release notes.

FAQ