TITLE
    USB: High Bandwidth and Isochronous Devices Must Be Separated
Article ID:
Created:
Modified: 		25243
2/1/01
2/9/01
TOPIC

    Two high-bandwidth devices or two isochronous devices cannot be connected to a single Universal Serial Bus (USB).

DISCUSSION

    Symptom

    When connected to the same bus, certain combinations of USB devices experience intermittent difficulties, or do not work at all. If one of the devices is disconnected or moved to another USB bus, everything works properly

    Products affected

    • Macintosh computers with USB (either built-in or with USB interface card)
    • high-bandwidth or isochronous (time-sensitive) USB devices

    Solution

    Connect only one high bandwidth or isochronous USB device to each USB bus.

    If your computer has a single USB bus, adding a USB interface card provides a second USB bus. Without a second USB bus, you may have to completely disconnect one high bandwidth or isochronous USB device in order to utilize another.

    In general, two high bandwidth devices cannot share the same USB bus. Each USB bus has a maximum bandwidth of 12 Mbits/s. A USB device can request up to 6 Mbits/s of bandwidth. Depending on how they operate, high bandwidth devices may compete with each other and therefore may need to be placed on separate USB busses to work properly.

    Isochronous devices cannot be shared on a single bus. USB supports isochronous devices, which can be high or low bandwidth. Examples of common isochronous devices include cameras and CD-R drives.


Document Information
Product Area: iMac
Category: USB
Sub Category: Troubleshooting
Keywords: cnfg, hts

Copyright © 2000 Apple Computer, Inc. All rights reserved.