<P> To this aim, each hub has a single interrupt endpoint "1 IN" (endpoint address 1, hub - to - host direction) used to signal changes in the status of the downstream ports . When someone plugs in a device, the hub detects voltage on either D+ or D - and signals the insertion to the host via this interrupt endpoint . When the host polls this interrupt endpoint, it learns that the new device is present . It then instructs the hub (via the default control pipe) to reset the port where the new device was plugged in . This reset makes the new device assume address 0, and the host can then interact with it directly; this interaction will result in the host assigning a new (non-zero) address to the device . </P> <P> Any USB 2.0 hub that supports a higher standard than USB 1.1 (12 Mbit / s) will translate between the lower standard and the higher standard using what is called a transaction translator (TT). For example, if a USB 1.1 device is connected to a port on a USB 2.0 hub, then the TT would automatically recognize and translate the USB 1.1 signals to USB 2.0 on the uplink . However, the default design is that all lower - standard devices share the same transaction translator and thus create a bottleneck, a configuration known as the single transaction translator . Consequently, multi transaction translators (Multi-TT) were created, which provide more transaction translators such that bottlenecks are avoided . Note that USB 3.0 hubs do not currently perform transaction translation to super-speed for USB 2.0 devices . </P> <P> Most USB hubs use one or more integrated controllers (ICs), of which several designs are available from various manufacturers . Most support a four - port hub system, but hubs using 16 - port hub controllers are also available in the industry . The USB bus allows seven cascading tiers of ports . The root hub is the first tier, and the last devices are on the seventh tier, which allows 5 tiers worth of hubs between them . The maximum number of user devices is reduced by the number of hubs . With 50 hubs attached, the maximum number is 127 − 50 = 77 . </P> <P> Also available are so - called "sharing hubs", which effectively are the reverse of a USB hub, allowing several PCs to access (usually) a single peripheral . They can either be manual, effectively a simple switch - box, or automatic, incorporating a mechanism that recognises which PC wishes to use the peripheral and switches accordingly . They cannot grant both PCs access at once . Some models, however, have the ability to control multiple peripherals separately (e.g., two PCs and four peripherals, assigning access separately). Only the simpler switches tend to be automatic, and this feature generally places them at a higher price point too . Modern KVM switches can also often share USB devices between several computers . </P>

What is the maximum number of usb devices that can be attached to a single usb controller
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