Apple iMac - Manual (Page 30)

C H A P T E R 3
Devices and Ports
30 USB Ports
■ Interrupt transfers, used for human interface device (HID) class devices such
as keyboards and mice, as well as devices that report status changes, such as
serial or parallel adaptors and modems.
■ Isochronous transfers, used for on-time delivery of data. Isochronous data
transactions are best suited for audio or video data streams.
Version 1.0 of the Macintosh USB system software provides functions that
support only control, bulk, and interrupt transfer types.
USB Compatibility Issues 3
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.
ADB Compatibility 3
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.
IMPORTANT
The ADB/USB shim does not support other types of ADB
devices.
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Note
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 Chapter 5, “Software.”
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