S7-1500 FileReadC FileWriteC: TIA Portal V15 MMC File Handling

Overview of FileReadC and FileWriteC in TIA Portal V15

Starting with TIA Portal V15 (STEP 7 V15), Siemens introduced the FileReadC and FileWriteC extended instructions for the S7-1200/S7-1500 families. These blocks allow ASCII file I/O directly to and from a SIMATIC memory card (SMC) inserted in the CPU, eliminating the need for separate recipe or DataLog functions when only a flat byte stream needs to be persisted.

Typical use cases include:

• Storing machine recipes as raw byte streams
• Capturing periodic log or report snapshots
• Archiving trace buffers, alarm histories, or production counters
• Exchanging configuration data with an external tool that reads the SMC offline

The blocks are asynchronous; they are triggered with a positive edge on REQ and report completion on DONE, BUSY, and ERROR/STATUS. Refer to the official FileReadC documentation (TIA Portal cloud manual collection) and FileWriteC documentation (TIA Portal cloud manual collection) for the canonical block description.

CPU and Firmware Prerequisites

Block Interface Reference

Both FileReadC and FileWriteC share a similar parameter signature; the differences are listed below.

Step-by-Step: Implementing FileWriteC and FileReadC

1. Create the byte buffer. In a global data block (e.g. DataBlock1), declare the source/target array:

   DATA_BLOCK "DataBlock1"
   VAR
       MyWriteData : ARRAY[0..255] OF BYTE;
       MyReadData  : ARRAY[0..255] OF BYTE;
   END_VAR
   END_DATA_BLOCK

2. Populate the buffer. Move the payload into MyWriteData using MOVE_BLK, FILL_BLK, or program logic. Remember that the only legal DATA VARIANT is an Array of Byte.
3. Insert FileWriteC. Drag it from Extended instructions → File handling. Wire REQ := writeTrigger, NAME := 'UserFiles\myFile.txt', DATA := "DataBlock1".MyWriteData, and the length.
4. Trigger once per significant event. Drive REQ with a rising edge when a write is genuinely required. Continuous toggling on the SMC will shorten card lifetime.
5. Read back with FileReadC. Insert the block, supply the same path, and point DATA to a byte array of equal or larger size. The block will return the number of bytes actually read.
6. Reconstruct native types. If the payload originated as INT, REAL, or a STRUCT, serialize it into the byte buffer before writing and deserialize after reading.
7. Monitor the status. Evaluate BUSY, DONE, ERROR, and STATUS in a watch table or HMI to confirm end of operation.

Path Conventions: The UserFiles Folder

The NAME parameter must always be a relative path inside the CPU file system. The folder UserFiles is the only writable area exposed to user-level instructions. Examples:

'UserFiles\recipe.bin'
'UserFiles\logs\2024_05.csv'
'UserFiles\backup\cfg_001.bin'

Subdirectories must exist before writing; the block will not create them. They can be created offline by inserting the SMC into a card reader and creating the folders on the FAT partition, or online with third-party Web-API tools.

Data Type Limitation and Serialization Strategy

The single most common field issue is error 16#8A51: Invalid data type of the "DATA" parameter. It is raised whenever DATA is bound to anything other than an Array of Byte (or a single BYTE tag in some firmware revisions). The following types all trigger 16#8A51:

• Array of INT, Array of REAL, Array of BOOL
• STRING, WSTRING
• UDT / STRUCT

To persist richer data, serialize the value to a byte buffer before writing. Siemens provides several mechanisms:

A minimal STL/SCL example for an INT:

// Write side
"MyLib".Serialize(IN  := recipeWord,
                   OUT := %DB20.DBX0.0 BYTE 2); // 2 bytes little-endian
FileWriteC(REQ := writeTrig,
           NAME := 'UserFiles\recipe.bin',
           DATA := "DataBlock1".MyWriteData,
           LEN  := 2);

// Read side
FileReadC(REQ := readTrig,
          NAME := 'UserFiles\recipe.bin',
          DATA := "DataBlock1".MyReadData,
          LEN  => actualLen);
"MyLib".Deserialize(IN  := %DB20.DBX0.0 BYTE 2,
                     OUT => recipeWord);

Memory Card Write-Cycle Considerations

SMC media is NAND flash and has a finite program/erase budget. Siemens specifies a typical endurance of 100,000 to 1,000,000 write cycles per sector depending on card generation. For continuous logging, the wear leveling is sufficient for years, but bursty or mis-triggered writes will exhaust a sector prematurely.

Mitigation checklist:

• Buffer records in a RAM DB and flush in groups of N to amortize writes.
• Use DataLogCreate / DataLogWrite instead when high-frequency appends are needed (these blocks manage rotation and wear).
• Use SMCInfo or the online diagnostics to monitor CardLifeRemaining on newer firmware.
• Schedule the SMC for periodic replacement in plants with heavy logging.

PLCSIM Advanced Behavior and Limitations

When the blocks are exercised inside PLCSIM Advanced, the virtual CPU maps its file system to a folder on the host PC (default %USERPROFILE%\Documents\Siemens\SIMATIC_MC). Field reports confirm the following constraints:

• Folder name must be UserFiles. Any other folder name (Files, Data, etc.) silently yields STATUS = 16#7000 with no file created or read.
• Use a relative path: 'UserFiles\data.txt'. Absolute paths pointing at the host disk root are not accepted.
• Some PLCSIM Advanced versions (e.g. 2.0) do not implement the file-handling blocks at all. Upgrade to PLCSIM Advanced 4.0 or later if DONE returns immediately with STATUS = 16#7000 and no file appears.
• The DONE counter increments because the block call itself completes; this does not guarantee that data hit the host file system. Always verify by inspecting the folder.

// Correct PLCSIM Advanced usage
FileReadC(REQ := readTrig,
          NAME := 'UserFiles\data.txt',
          DATA := "DataBlock1".MyReadData,
          LEN  => actualLen);

// Wrong: will produce no error but no file activity
FileReadC(REQ := readTrig,
          NAME := 'data.txt',                 // missing UserFiles\\
          DATA := "DataBlock1".MyReadData,
          LEN  => actualLen);

STATUS and Error Code Reference

Sample Watch-Table Commissioning Sequence

1. Set REQ to FALSE; confirm BUSY = FALSE, STATUS = 16#7000.
2. Pre-load MyWriteData with a known pattern (use Fill in the watch table: 16#AA for 16 bytes).
3. Trigger REQ := TRUE; within one OB1 cycle BUSY rises to TRUE.
4. Wait until DONE = TRUE and STATUS = 16#0000. Reset REQ := FALSE before the next trigger.
5. Pull the SMC, open the file on a PC, verify the byte pattern is present.
6. For the read side, write a different pattern from the PC, reinsert the SMC, trigger FileReadC.REQ, and verify MyReadData matches.

Common Pitfalls and Field-Proven Workarounds

Alternatives and When to Use Them

• DataLog blocks (DataLogCreate, DataLogWrite, DataLogClose): purpose-built for high-frequency append, automatic rotation, CSV support, and wear leveling. Prefer these for production logging.
• Recipe functions from the "Recipes" library: keep recipes inside the PLC project and provide structured HMI access; no raw file I/O.
• Web API / OPC UA file methods: read the SMC file from a remote client without pulling the card.
• ProDiag / user FBs: wrap FileReadC/FileWriteC inside a multi-instance FB to centralize path handling, error mapping, and trigger gating.

Verification Checklist

• FileWriteC produces a file under \UserFiles\ on the SMC with the expected byte count.
• FileReadC returns STATUS = 16#0000 and LEN matches the on-card size.
• Deserialized values equal the originally written ones across at least three cycles.
• SMC diagnostics show no CardLifeRemaining alarm after a stress run.
• PLCSIM Advanced round-trip succeeds with the path UserFiles\<name>.
• Watch-table sequence has been captured and signed off.

FAQ