With two or more USM-810's in a IQ system engaging the preset in one of the USM-810's (that is not the Interface) causes the unit to lock up and go offline. Aftera period of time the unit comes back on line.
This was a firmware issue prior to version 1.003.
Resolution: Upgrade units to v1.003 or newer firmware. The latest version of firmware for the USM-810 is available from the Crwon Web site.