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The USB ports take the place of the ADB and serial I/O ports found on earlier Macintosh computers, but they do not function the same way. The following sections describe the differences.
Apple is providing an ADB/USB shim to support processes that control ADB devices by making calls to the ADB Manager and the Cursor Device Manager. The ADB/USB shim makes it possible for processes that support an ADB keyboard to work with the USB keyboard equivalent.
For example, the ADB/USB shim allows applications to set the caps lock and num lock LEDs on the Apple USB keyboard. The ADB/USB shim also allows the Cursor Device Manager to support a USB mouse.
Keyboards other than the Apple USB keyboard can be used with the iMac computer, but they will be treated as having an ADB device ID of 2.
The ADB/USB shim does not support other types of ADB devices.
The ADB/USB shim is built into the Mac OS ROM image on the iMac computer and will be included in the Mac OS ROM image on future Power Macintosh systems that have USB ports. For more information about the Mac OS ROM image, refer to Software
The first release of the MacOS USB DDK does not include a universal Serial/USB shim that would allow processes that use the Communications Toolbox CRM to use a USB serial device. At the introduction of the iMac computer, any USB Serial Class Driver will need to implement its own Serial/USB shim.
The first release of the Mac OS USB DDK does not include a universal Serial/USB shim that would allow processes that use the Communications Toolbox CRM to find and use a USB modem device. At the introduction of the iMac computer, any USB modem device would need to implement its own communication shim to interface between the CRM and its USB Communication class driver. For more information, please refer to the Mac OS USB DDK, which is available from the Apple Developer DDK page on the World Wide Web, at
USB is a serial communications channel, but it does not replace LocalTalk functionality on Macintosh computers; you cannot connect two Macintosh computers together using the USB. The best method for networking iMac computers is through the built-in Ethernet port.
The Macintosh USB software does not support booting from an external USB storage device.
The iMac computer 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 will not be supported by the Macintosh USB system software.