How can I ensure I can get an initial value when using add_device_notification?

[liwen-tj]

Hello everyone!

I met a problem when trying to use NotificationAttrib
What I want is I can get an initial value when startup and whenever data changes, I can immediately get the changed value.
How do I set trans_mode and max_delay and cycle_time param?

I have read https://infosys.beckhoff.com/english.php?content=../content/1033/tc3_adsdll2/117553803.html&id=1821528703544766287 but it seems that we can't always get initial value when plc value is not changed.
What is weird is that sometimes I can get the initial value while other time I can not get the initial value when the value is always constant.

import ctypes
import time

import pyads
from pyads.structs import NotificationAttrib, SAdsNotificationHeader
from pyads.constants import ADSTRANS_SERVERONCHA, ADSTRANS_SERVERCYCLE

from covariant_libs import logger


def register_callback(client) -> None:
    def callback_func(notification: SAdsNotificationHeader, key: str):
        _, _, current_status = client.parse_notification(notification, pyads.PLCTYPE_UINT)
        logger.info(f"--> {key=}, {current_status=}")

    client.add_device_notification(
        data="MAIN.safety_2.safety_status",
        attr=NotificationAttrib(length=ctypes.sizeof(pyads.PLCTYPE_UINT), trans_mode=ADSTRANS_SERVERONCHA, max_delay=1, cycle_time=0),
        callback=callback_func,
    )
    
    client.add_device_notification(
        data="MAIN.machine_control_2.machine_status",
        attr=NotificationAttrib(length=ctypes.sizeof(pyads.PLCTYPE_UINT), trans_mode=ADSTRANS_SERVERONCHA, max_delay=1, cycle_time=0),
        callback=callback_func,
    )

if __name__ == "__main__":
    ams_net_port = 851
    plc_ams_net_id = "192.168.**.**.1.1"
    plc_ip_address = "192.168.**.**"
    plc_password = "**"
    plc_user_name = "**"
    route_name = "***_PLC_2"
    sender_ams_address = "192.168.**.**.1.1"
    sender_ip_address = "192.168.**.**"
    pyads.open_port()
    pyads.set_local_address(sender_ams_address)
    pyads.add_route_to_plc(
        sender_ams_address,
        sender_ip_address,
        plc_ip_address,
        plc_user_name,
        plc_password,
        route_name=route_name,
    )
    pyads.close_port()

    plc = pyads.Connection(
        ams_net_id=plc_ams_net_id,
        ams_net_port=ams_net_port,
        ip_address=plc_ip_address,
    )

    plc.open()
    register_callback(plc)
    
    print(f"{time.time()} plc local address:", plc.get_local_address())
    print(f"{time.time()} Plc symbols length: ", len(plc.get_all_symbols()))

    time.sleep(10000)
    plc.close()

[chrisbeardy]

I believe this is the correct use of the notification, my understanding of the notification system is that, when you create the notification, if there was a change previously in the buffer (changed since PLC startup), you may get the "initial" value. If the value hasn't changed at all since PLC startup when registering the notification, you may not get a notification, although I don't think this is 100% true, someone else may have more knowledge. I think there is not much you can do to guarantee you get the initial value, unfortunately. Sending the notification or not is in the ADS layer, pyads is just choosing to subscribe to it.