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Transfer Types Supported

The USB specification defines four data transfer types:

• Control transfers, used for device configuration and initialization.
• Bulk transfers, used for printers, scanners, modems, and other devices that require accurate delivery of data with relaxed timing constraints.
• 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 1.1 of the Macintosh USB system software (the version that comes with the Power Macintosh G3) provides functions that support all four transfer types. The earliest version did not support isochronous transfers; if your software requires them, it should make a Gestalt call to determine whether they are supported.