how write date time variable in a db?

[efornoni]

I wanted to know if anyone had managed to write a date time variable from python to the plc db.

[nikteliy]

Hi, @efornoni
Could you describe what exactly is not working for you?
In what format do you want to write a datetime variable? DATE_AND_TIME or DTL?
If you want to use the DATE_AND_TIME format, here is the code that writes datetime to the PLC and then reads it.

import struct
from datetime import datetime
import snap7


def datetime_to_dat(dt: datetime) -> bytes:
    """
    Args:
        dt (datetime): A datetime object to be converted.

    Returns:
        bytes: A byte string in the S7 DATE_AND_TIME format.

    The byte string has the following format:
    Byte 0: Year (BCD format)
    Byte 1: Month (BCD format)
    Byte 2: Day (BCD format)
    Byte 3: Hour (BCD format)
    Byte 4: Minute (BCD format)
    Byte 5: Second (BCD format)
    Byte 6: Two most significant digits of Millisecond (BCD format)
    Byte 7 (4 MSB): Two least significant digits of Millisecond (BCD format)
    Byte 7 (4 LSB): Day of week (BCD format)
    """

    def int_to_bcd(value: int) -> int:
        """Convert an integer to BCD format"""
        return (value // 10) << 4 | (value % 10)

    # Check if the year is in the valid range
    if dt.year < 1990 or dt.year > 2089:
        raise ValueError("Year should be between 1990 and 2089")
    # Convert each part of the datetime object to the corresponding byte format
    year = int_to_bcd(dt.year % 100)
    month = int_to_bcd(dt.month)
    day = int_to_bcd(dt.day)
    hour = int_to_bcd(dt.hour)
    minute = int_to_bcd(dt.minute)
    second = int_to_bcd(dt.second)
    millisecond_1 = int_to_bcd(dt.microsecond // 1000 // 10)
    millisecond_2 = int_to_bcd(dt.microsecond // 1000 % 10)
    weekday = int_to_bcd(dt.weekday())
    last_byte = millisecond_2 << 4 | weekday
    return struct.pack("8B", year, month, day, hour, minute, second, millisecond_1, last_byte)


client = snap7.client.Client()
client.connect("10.10.10.100", 0, 1)

dt = datetime.now()
date_and_time = datetime_to_dat(dt)
client.db_write(db_number=6, start=0, data=date_and_time)
plc_data_and_time = client.db_read(db_number=6, start=0, size=8)
plc_dt = snap7.util.get_date_time_object(plc_data_and_time, 0)
print(dt)
print(plc_dt)

Output:

>>> 2023-03-30 18:36:21.139895
>>> 2023-03-30 18:36:21.139000