US Patent 7498687 Power supply control system for a vehicle trailer

A controller is usable with a tractor that has a power supply and a trailer that has first and second auxiliary devices. The controller is configured to selectively enable and disable each of the auxiliary devices based on a predetermined set of device disabling conditions wherein: When any of the device disabling conditions occurs, then both auxiliary devices are disabled. When none of the device disabling conditions occurs then, when a first device-selecting condition occurs then the first auxiliary device is enabled and the second auxiliary device is disabled. And when a second device-selecting condition occurs then the second auxiliary device is enabled and the first auxiliary device is disabled.