======================PROTOCOLS_USB===================================== USB Universal Serial Bus developed by Compaq, DEC, IBM, Intel, Microsoft, NEC, for low- to mid-speed peripherals. allow Plug and Play computer peripherals new peripherals configured automatically upon attachment allow up to 127 devices USB has a 12 Mbps bandwidth USB is host-centric, allow it to use host PC's resources to detect when a device is added or removed. Windows 9x based Dual speed: 1.5 & 12 Mbps 5 meter max connection 4-wire serial bus Up to 127 devices 2-wire differential signaling Supports isochronous transactions All transactions originate from host 8/16/32/64-byte max data packet sizes 4 packets: Token, Data, Handshake, Special Token packet sets up transactions 1 USB Overview, http://www.usb.org/ 2 USB Overview, http://www.usb.org/ 3 USB Overview, http://www.usb.org/ 4 "RTC Bus Directory: USB", RTC, August 1997, p. 39. FireWire IEEE 1394 may be seen as usurpers to the USB throne; provide a high speed connection to the PC and suited for digital camcorders,(DVD) players. USB? standardized, easy-to-use way to connect up to 127 devices to a computer. maximum of 6 megabits per second of bandwidth, "A" connectors head "upstream" toward computer, "B" connectors head "downstream" to individual devices. it is impossible to ever get confused typical USB 4-port hub accepts 4 "A" connections mice and digital cameras get their power from the bus (up to 500 milliamps at 5 volts) comes from the computer. USB cable has two wires for power (+5 volts and Ground) twisted pair of wires to carry the data. up to 500 milliamps of power at 5 volts. (+5 volts (red) a nd Ground (brown)) twisted pair (yellow and blue) of wires to carry data. The cable is also shielded. powers up, queries all of devices connected to bus and assigns each one an address (a process called enumeration -- devices are also enumerated when they connect to the bus). also what type of data transfers it wishes to perform: Interrupt - a device like a mouse or a keyboard, Bulk - A device like a printer, Isochronous - a streaming device (e.g. - speakers) host can also send commands or query parameters with control packets. As devices are enumerated, host is keeping track of total bandwidth of isochronous and interrupt devices are requesting. They can have up to 90% of 12 megabits per sec available. After 90% is used up, host denies access to other devices Control packets and packets for bulk transfers use any bandwidth left over (at least 10%). Universal Serial Bus divides the available bandwidth into frames, and the host controls the frames. Frames contain 1,500 bytes a new frame starts every millisecond. During a frame, isochronous and interrupt devices get a slot so they are guaranteed bandwidth they need. Bulk and control transfers use whatever space is left.