S7DataBlock

• S7DataBlock
  • Key Features
  • Data Transfer
  • Live Monitoring PLC Values
  • Installation
  • Contributing and Issues

A Python library for parsing Siemens S7 PLC data blocks, in conjunction with Snap7.

s7datablock is a Python utility designed to parse and interpret Siemens S7 PLC data blocks using structured data definitions exported from the TIA Portal. It works seamlessly with the excellent python-snap7 library to enable high-level, dictionary-like access to binary PLC data.

Instead of manually mapping byte offsets and types, you can use .db definition files exported from TIA Portal to generate a clean, versionable Python representation of your data block structures.

Key Features

• Uses exported TIA Portal *.db files for accurate structure mapping
• Read and write entire PLC data blocks with .GET() and .SET() (or the more pythonic pull() and push())
• Structure and sub-structure access via dot notation
• Clean terminal output using Rich
• Easy integration into polling and monitoring loops

Data Transfer

This library deliberately uses manual .GET() and .SET() calls instead of automatic syncing to prioritize:

• Clarity – Developers control exactly when data is transferred
• Performance – Avoid unnecessary communication overhead
• Atomicity – Batch multiple updates into one write

Live Monitoring PLC Values

You can monitor PLC values by creating a polling loop. Here's an example using Rich for clean terminal output:

from time import sleep
from rich.live import Live
from datetime import datetime
from io import StringIO

from s7datablock.mapping import S7DataBlock

# Example in-line definition file contents. Alternatively, you can pass a PAth to a *.db file.
df_file_contents = StringIO(
    """
        TYPE "my_udt"
        VERSION : 0.1
        STRUCT
            PLC_DQ_0 : Bool;
            PLC_DQ_1 : Bool;
            WHEN : DTL;
            Value1 : Int;
            Value2 : Real;
        END_STRUCT;
        END_TYPE
        DATA_BLOCK "my_udt"
        VERSION : 0.1
        "my_udt"
        BEGIN
        END_DATA_BLOCK
    """
)

db = S7DataBlock.from_definition_file(df_file_contents, db_number=1200)
print(db) # This will print the structure of the data block as a table

print(repr(db))  # Print the raw representation of the data block
# In a real application, you would create a snap7 client and connect to the PLC

# client = snap7.client.Client()
# client.connect('192.168.0.1', 0, 1)

with Live(db.to_table(), refresh_per_second=10) as live:
    while True:
        # In a real application, you would call GET() here
        # db.GET(client)

        # For demo, we'll just modify some values
        db["WHEN"] = datetime.now()
        db["PLC_DQ_0"] = not db["PLC_DQ_0"]  # Toggle the boolean
        db["Value1"] = db["Value1"] + 1  # Increment the integer value
        db["Value2"] = db["Value2"] + 0.1  # Increment the real value

        # Update the display with current values
        live.update(db.to_table())
        sleep(0.2)

Running this code should render a live table in your terminal

                        Data Block Structure
┏━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━┓
┃ Name              ┃ Data type   ┃   Offset ┃ Value               ┃
┡━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━┩
│ PLC_DQ_0          │ Bool        │      0.0 │ True                │
│ PLC_DQ_1          │ Bool        │      0.1 │ False               │
│ WHEN              │ DTL         │      2.0 │ 2025-07-14 17:08:40 │
│   WHEN.YEAR       │ UInt        │      2.0 │ 2025                │
│   WHEN.MONTH      │ USInt       │      4.0 │ 7                   │
│   WHEN.DAY        │ USInt       │      5.0 │ 14                  │
│   WHEN.WEEKDAY    │ USInt       │      6.0 │ 1                   │
│   WHEN.HOUR       │ USInt       │      7.0 │ 17                  │
│   WHEN.MINUTE     │ USInt       │      8.0 │ 8                   │
│   WHEN.SECOND     │ USInt       │      9.0 │ 40                  │
│   WHEN.NANOSECOND │ UDInt       │     10.0 │ 568441000           │
│ Value1            │ Int         │     14.0 │ 757                 │
│ Value2            │ Real        │     16.0 │ 75.699              │
└───────────────────┴─────────────┴──────────┴─────────────────────┘

Installation

pip install s7datablock

Contributing and Issues

The library is in its early stages, and contributions are welcome. Feel free to submit a pull request or open an issue on GitHub.