Why does the mouse cursor randomly jump around when I connect a GPS to a Sealevel serial adapter?

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You are describing the “crazy mouse” syndrome, a common condition when Microsoft Windows receives NMEA 0183 data from a serial GPS device. The condition occurs when the GPS data is misinterpreted as mouse data by the Windows serial enumerator and will cause unwanted mouse cursor activity. It is most commonly encountered when connecting a serial GPS device to any USB to serial adapter. It is easily remedied in Windows Device Manager by disabling the Windows serial enumerator for the COM port connected to the GPS.

For more information, read the How to fix “crazy mouse” syndrome with USB serial adapters article.