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Unity fails to connect to the M340 or Quantum after swapping the USB cable from an M580.

Goals and Symptoms
Unity fails to connect to the M340 or Quantum after swapping the USB cable from an M580.

Facts and Change
Unity

Causes and Fixes
After disconnecting from the Unity PLC, opening another application and swapping the USB cable from an M580 to an M340, 
attempts to connect to the M340 might fail and return the error 'This isn't a valid PLC Address, or PLC is busy, or media is off'.





The reason why this occurs is due to the Windows operating system behavior of taking up to 10-15 seconds for the  detection and establishment of a
new network.   A USB connection is also considered as a type network connection.

If Unity tries to connect the PLC during this time, it will fail to find the PLC and goes through a process of waiting for a timeout until the message appears.

Any attempts to connect to the PLC afterwards will be successful.

The issue could also occur if there are multiple active PLC USB connections.  The reason is due to the fact that the same IP address (90.0.0.1/90.0.0.2) is used for the
USB connection.  If there are multiple PLC USB connections, only one can become will become active and the other PLC USB connections will be in a passive mode.
In this scenario, if the active connection is unplugged, the other passive connection might not automatically become active.  To activate a passive connection its
USB cable must be unplugged and plugged back in.
 
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