USB hub

A USB hub is a accessory that expands a individual USB anchorage into several so that there are added ports accessible to affix accessories to a host system.

USB hubs are generally congenital into accessories such as computers, keyboards, monitors, or printers. When such a accessory has abounding USB ports they all usually axis from one or two centralized USB hubs rather than anniversary anchorage accepting absolute USB circuitry.

Physically abstracted USB hubs appear in a advanced array of anatomy factors: from alien boxes (looking agnate to a arrangement hub) connectible with a continued cable, to baby designs that can be anon acquainted into a USB anchorage (see the 'compact design' picture). In the average case, there are "short cable" hubs which about use an basic 6 inch cable to hardly ambit a baby hub abroad from concrete anchorage bottleneck and of advance access the amount of accessible ports.

Laptop computers may be able with abounding USB ports, but an alien USB hub can consolidate several accustomed accessories (like a abrasion and a printer) into a individual hub to accredit one-step adapter and abatement of all the devices.

Inverse or Sharing Hubs

Also accessible are alleged "sharing hubs", which finer are the about-face of a USB hub, acceptance several PCs to admission (usually) a individual peripheral. They can either be manual, finer a simple switch-box, or automatic, accumulation a apparatus that recognises which PC wishes to use the borderline and switches accordingly. They cannot admission both PCs admission at once. Some models, however, accept the adeptness to ascendancy assorted peripherals alone (e.g., 2 PCs and 4 peripherals, allotment admission separately). Only the simpler switches tend to be automatic, and this affection about places them at a college amount point too. Modern KVM switches can aswell generally allotment USB accessories amid several computers.

Physical layout

A USB arrangement is congenital from USB hubs affiliated after to USB ports, which themselves may axis from USB hubs. USB hubs can extend a USB arrangement to a best of 127 ports. The USB blueprint requires that bus-powered hubs may not be affiliated in alternation to added bus-powered hubs.

USB ports are about carefully spaced. Consequentially active a accessory into one anchorage may physically block an adjoining port, decidedly if the bung is not allotment of a cable but is basic to a accessory such as a USB beam drive. A accumbent arrangement of accumbent sockets may be simple to fabricate, but may could cause alone two out of four ports to be accessible (depending on bung width).

Port arrays in which the anchorage acclimatization is erect to the arrangement acclimatization about accept beneath blockage problems. External "Octopus" or "Squid" hubs (with anniversary atrium at the end of a actual abbreviate cable maybe 2 inches long), or "star" hubs (with anniversary anchorage adverse in a altered direction, as pictured) abstain this botheration completely.

Length limitations

USB cables are bound to 3m for low-speed USB 1.1 device. A hub can be acclimated as an alive USB captive to extend cable breadth for up to 5m lengths at a time. Alive Cables (specialized connector-embedded one-port hubs) accomplish the aforementioned function, but back they are carefully bus-powered, evidently powered (non-bus-powered) USB hubs would acceptable be appropriate for some of the segments.

Power

A bus-powered hub is a hub that draws all its ability from the host computer's USB interface. It does not charge a abstracted ability connection. However, abounding accessories crave added ability than this adjustment can provide, and will not plan in this blazon of hub.

USB accepted (related to power) is allocated in units of 100 mA up to a best absolute of 500 mA per port. Accordingly a adjustable bus powered hub can accept no added than four after ports and cannot action added than four 100 mA units of accepted in absolute to after accessories (since the hub needs one assemblage for itself). If a accessory requires added units of accepted than the anchorage it is acquainted into can supply, the operating arrangement usually letters this to the user.

In adverse a self-powered hub is one that takes its ability from an alien ability accumulation assemblage and can accordingly accommodate abounding ability (up to 500 mA) to every port. Abounding hubs can accomplish as either bus powered or cocky powered hubs.

However, there are abounding non-compliant hubs on the bazaar which advertise themselves to the host as self-powered admitting absolutely getting bus-powered. Equally there are affluence of non-compliant accessories that use added than 100 mA after announcement this actuality (or absolutely sometimes after anecdotic themselves as USB accessories at all). These hubs and accessories do acquiesce added adaptability in the use of ability (in accurate abounding accessories use far beneath than 100 mA and abounding USB ports can accumulation added than 500 mA afore traveling into afflict shut-off) but they are acceptable to accomplish ability problems harder to diagnose.

Some self-powered hubs do not accumulation abundant ability to drive a 500mA amount on every port. For example, abounding seven anchorage hubs accept a 1A ability supply, if in actuality seven ports could draw a best of 7 x 0.5 = 3.5A, additional ability for the hub itself. Designers accept the user will a lot of acceptable affix abounding low ability accessories and alone one or two acute a abounding 500mA. On the added duke the are some self-powered hubs area architect declares that assertive amount of ports can drive a 500 mA abounding load. For archetype if architect is appreciative of his 7 anchorage hub with capabitily of 4 ports acute a abounding 500mA amount and hub comes with 2A ability supply, the adventure can be true.

Speed

To acquiesce accelerated accessories to accomplish in their fastest approach all hubs amid the accessories and the computer accept to be top speed. Accelerated accessories should abatement aback to full-speed if acquainted in to a full-speed hub (or affiliated to an earlier full-speed computer port). While accelerated hubs can acquaint at all accessory speeds, low and full-speed cartage is accumulated and absolute from accelerated cartage through a transaction translator. Each transaction translator segregates lower acceleration cartage into its own pool, about creating a basic full-speed bus. Some designs use a individual transaction translator, while added designs accept assorted translators. Having assorted translators is a cogent account if one connects assorted high-bandwidth full-speed devices.1

It is an important application that in accepted accent (and generally artefact marketing) USB 2.0 is acclimated as alike with high-speed. However, because the USB 2.0 specification, which alien high-speed, incorporates the USB 1.1 blueprint such that any USB 1.1 accessory is aswell a USB 2.0 device, any adjustable full-speed or low-speed accessory is still a USB 2.0 device. Thus, not all USB 2.0 hubs accomplish at high-speed.

Protocol

Each hub has absolutely one upstream anchorage and a amount of after ports. The upstream anchorage connects the hub (directly or through added hubs) to the host. Added hubs or accessories can be absorbed to the after ports. During accustomed transmission, hubs are about transparent: abstracts accustomed from its upstream anchorage is advertisement to all accessories absorbed to its after ports; abstracts accustomed from a after anchorage is about forwarded to the upstream anchorage only. This way, what is beatific by the host is accustomed by all hubs and devices, and what beatific by a accessory is accustomed by the host but not by the added accessories (an barring is resume signaling).

Hubs are not cellophane if ambidextrous with changes in the cachet of after ports, such as admittance or abatement of devices. In particular, if a after anchorage of a hub changes status, this change is dealt with in an alternation amid the host and this hub; the hubs amid them act as cellophane in this case.

To this aim, anniversary hub has a individual arrest endpoint "1 IN" (endpoint abode 1, hub-to-host direction) acclimated to arresting changes in the cachet of the after ports. If anyone plugs in a device, the hub detects voltage on either D+ or D- and signals the admittance to the host via this arrest endpoint. If the host acclamation this arrest endpoint, it learns that the new accessory is present. It again instructs the hub (via the absence ascendancy pipe) to displace the anchorage area the new accessory was acquainted in. This displace makes the new accessory accept abode 0, and the host can again collaborate with it directly; this alternation will aftereffect in the host allotment a new (non-zero) abode to the device.23