USB Compatibility Issues

The USB ports augment the ADB ports and take the place of the 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

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 Power Macintosh G3 computer, but they will be treated as having an ADB device ID of 2.

Serial Port Compatibility

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's Communications Resource Manager (CRM) to find and use a USB modem device. Any USB modem device would need to implement its own communication shim to provide an 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

Macintosh-To-Macintosh Connections

USB is a serial communications channel, but it does not replace LocalTalk networking on Macintosh computers; you cannot connect two Macintosh computers using the USB. The best method for networking Power Macintosh G3 computers is through the built-in Ethernet port.

USB Storage Devices

The Macintosh USB software does not support booting from an external USB storage device.

USB Controller

The Power Macintosh G3 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 are not supported by the Macintosh USB system software.