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Catching bootup on diagnostics
Open a standard-size windows command prompt and ping your NetLinx constantly:
ping /t 192.168.1.10
Immediately after programming several pings will fail. Then it will start replying:
Reply from 192.168.1.10: bytes=32 time=45ms TTL=64
The moment that these reply messages reach the top of the command window is almost exactly the same time that the controller will start accepting Diagnostics connections.
ping /t 192.168.1.10
Immediately after programming several pings will fail. Then it will start replying:
Reply from 192.168.1.10: bytes=32 time=45ms TTL=64
The moment that these reply messages reach the top of the command window is almost exactly the same time that the controller will start accepting Diagnostics connections.

Comments
Of course if you want to see the Device Notifications, pinging is the best method.