iMac (Flat Panel): USB Features
USB Features
Features of the USB ports include power-saving modes and the ability to start up the computer using a USB mass-storage device.
Wake Up From Sleep
USB devices can provide a remote wakeup function for the computer. The USB root hub in the computer is set to have a remote wakeup feature whenever a device is attached to or disconnected from the bus. The first USB port (the one nearest the mini-VGA connector) and the internal modem share one USB host controller; the second and third USB ports share a second USB host controller. Each host controller provides a 12 Mbps data transfer rate that is shared between the devices connected to it. The keyboard that comes with the computer uses this method to wake the computer when you press a key.
Connect and Resume
The built-in USB circuitry allows the computer to wake from Sleep mode on connect, disconnect, and resume events. Compatible USB devices should work with the USB-suspend mode defined in the USB specification.
USB Storage Devices
The iMac can start up from a USB storage device that follows the USB Mass Storage Class specification.
Class drivers are software components that are able to communicate with many USB devices of a particular kind. If the appropriate class driver is present, any number of compliant devices can be plugged in and start working immediately without the need to install additional software. The Mac OS for the iMac includes USB Mass Storage Support 2.0, a class driver that is compatible with devices that meet the USB Mass Storage Class specification.
USB Controller
The iMac uses an Open Host Controller Interface (OHCI) controller for USB communication. Some early USB devices (most notably keyboards) can't interoperate with an OHCI controller. Those devices do not work with the Macintosh USB system software.