what is the maximum number of usb devices that can be attached to a single usb controller
USB hub 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.[citation needed] 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 âˆ’ 50 = 77.[6]
Plug and play Expansion devices are controlled and exchange data with the host system through defined memory or I/O space port addresses, direct memory access channels, interrupt request lines and other mechanisms, which must be uniquely associated with a particular device to operate. Some computers provided unique combinations of these resources to each slot of a motherboard or backplane. Other designs provided all resources to all slots, and each peripheral device had its own address decoding for the registers or memory blocks it needed to communicate with the host system. Since fixed assignments made expansion of a system difficult, devices used several manual methods for assigning addresses and other resources, such as hard-wired jumpers, pins that could be connected with wire or removable straps, or switches that could be set for particular addresses. As microprocessors made mass-market computers affordable, software configuration of I/O devices was advantageous to allow installation by non-specialist users. Early systems for software configuration of devices included the MSX standard, NuBus, Amiga Autoconfig, and IBM Microchannel. Initially all expansion cards for the IBM PC required physical selection of I/O configuration on the board with jumper straps or DIP switches, but increasingly ISA bus devices were arranged for software configuration. By 1995, Microsoft Windows included a comprehensive method of enumerating hardware at boot time and allocating resources, which was called the "Plug and Play" standard.