USB Hub Design and Windows
On This Page
 | USB Hubs: Design Contrasts and Restrictions |
| Windows Operating Systems and USB Hub Designs |
| Avoid Using Bus-Powered Hubs in Monitors |
| Using a Pass-through Hub Is Prohibited by USB 1.1 |
USB Hubs: Design Contrasts and Restrictions
USB hubs can be based on bus-powered or self-powered designs as described in sections 7.2.1, 7.2.1.1, and 7.2.2.2 of the USB Specification, Version 1.1, which is available from http://www.usb.org 
. Self-powered hubs have significant advantages over bus-powered hubs for high-power devices. This article discusses:
| • | What advantages self-powered hubs have over bus-powered hubs for high-power devices. |
| • | Why bus-powered hubs should not be placed on devices that already have their own power source, for example, on monitors. |
| • | Why pass-through USB hubs are prohibited under the terms of the USB Specification, Version 1.1. |
USB hubs can be built that operate in either self-powered or bus-powered mode. Self-powered hubs draw their power from the electrical outlet, while bus-powered hubs draw their power from the USB bus. From the aspect of user experience, hubs operating in self-powered mode have a significant advantage over hubs operating in bus-powered mode for the following reasons:
| • | A user can plug any bus-powered USB device into any port on a self-powered hub, and the device will always have enough power to function. The power needed by a bus-powered USB device in order to function can range from a few mA up to a maximum of 500mA. | ||||
| • | A user can plug a bus-powered USB device into a port on a bus-powered hub, but the device might not have enough power to function. Specifically:
|
Even if it seems unlikely that a user would use USB for a high-power device, the problem with bus-powered USB hub designs is the user's expectation that any USB device can be plugged into any hub and it will work. This is not true with bus-powered hub designs.
Windows Operating Systems and USB Hub Designs
High-power, bus-powered devices can draw up to 500mA after they are configured by host system software, but must not draw more than 100mA until they are configured (USB Specification, Version 1.1, Section 7.2.1). The device circuitry and self-descriptive information that the host system software requires to enumerate the device are available to the host in this low-power mode.
If the user who is running Microsoft Windows 98 plugs a high-power bus-powered device into a port on a bus-powered hub, the device will not work - and little information will be available to help the user understand why. Windows 98 shows the device as disabled in Device Manager, but does not warn the user with a message, for example, at the time of the hot-plug event.
Windows 98 Second Edition, Windows Me, Windows 2000, and Windows XP give the user more help in such a situation. The device is shown as disabled in Device Manager and the user is also presented with a message that indicates why the device is not operating. However, it is still up to the user to locate an unused port on the PC platform or on a self-powered hub connected to the PC platform. Then the user must disconnect the high-power bus-powered device from the bus-powered hub and reconnect it to a port that will supply enough power.
Avoid Using Bus-Powered Hubs in Monitors
Bus-powered hubs are primarily included as a design option in USB Specification, Version 1.1 to provide mobile PC users with a limited-capability hub that meets the relatively stringent power budget constraints faced by mobile PC designers.
Many monitors are being built that include an embedded USB hub that provides two or more ports for plugging in USB devices. A few such monitors embed a bus-powered hub instead of a self-powered hub. Because the monitor takes power from the electrical outlet, the monitor designer does not face the same power budget constraints as mobile PC designers do. Based on the explanation in the previous section, it is simply a bad idea for a monitor design to use a bus-powered hub because of its limited capabilities.
Using a Pass-through Hub Is Prohibited by USB 1.1
A few USB devices provide what seems to be a port on an embedded hub, but it is nothing more than one end of a USB extender cable. USB extender cables are explicitly prohibited by USB Specification, Version 1.1; see section 6.4.4.
This type of design is often referred to as a "pass through" USB hub, even though it is not a hub at all. Although this type of design can work, the following problems and limitations can occur:
| • | A USB device that can be configured by host system software when it is plugged into a port on a specification-compliant hub may not be configured successfully when it is plugged into the end of the USB extender cable that is available in a device including a pass-through USB hub. |
| • | Extender cables have been shown to be electrically unreliable when participating in data transfers over the USB bus, which is why they are prohibited by USB Specification, Version 1.1.
|
There is no way for the operating system to determine whether a pass-through USB hub is employed by a device. To determine if a device uses this unreliable, non-compliant design, see if the device offers only a single USB connector for plugging in a USB device. If there is only a single connector available, then the device could be using a USB extender cable; most true USB hubs offer at least two ports.
Call to action for designers implementing USB hubs:
| • | Self-powered hubs have significant advantages over bus-powered hubs, particularly for end users. Designers should use self-powered hub designs unless heavily constrained by available power budget or a cable reduction requirement--bus-powered hubs do not require power cables. |
| • | An alternative for external USB hub designers is a "hybrid" hub design that supports both self-powered and bus-powered modes. However, such a hub, in its descriptors, must correctly inform the host system software that it is capable of running in both modes.
|
| • | A USB hub embedded in a monitor should be designed to use a self-powered hub. |
| • | A USB extender cable must not be used in place of an embedded hub in any USB device. |