What is USB? Prototyping Unit

What is USB? USB stands for ‘Universal Serial Bus’ and is the next step in creating a computer interface that really works universally. It boasts cross-platform compatibility for Macintosh, Linux/UNIX operating systems, and all versions of Windows since 98SE. The USB connector ports are available standard on virtually every computing machine manufactured in the past decade.

Why use USB? USB is the first cross-platform “hot-swappable” interface- no more operating system incompatibility, no more restarting before unplugging or plugging in.

What are the types of connectors? There are two types of connectors on USB cables: a long, flat rectangle for plugging into computers and hubs called 'USB-A', and 'USB-B‘, a smaller, square connector for plugging into devices like external CD and Hard drives, scanners, and printers.

How USB Works: an Overview USB uses a four-wire cable interface. Two of the wires are used in a differential mode for both transmitting and receiving data, and the remaining two wires are power and ground. The source of the power to a USB device can come from the host, a hub, or the device can be "self powered.“ There are two different connector types on each end of a USB cable. One of these connectors is for upstream communications, and the other for downstream. Each cable length (maximum) is limited to about 5 meters.

How USB Works: an Overview USB has four types of communication transfer modes: control, interrupt, bulk, and isochronous. Control mode is initiated by the host. In this mode, every data transfer must send data in both directions, but only in one direction at a time. The control mode is used mainly for initialization of devices, but it can also be used to transfer small amounts of data. In interrupt mode, as in control mode, the host has to initiate the transfer of data. Interrupt mode works by the host querying devices to see if they need to be serviced. Bulk mode and isochronous mode complement each other in a sense. Bulk mode is used when data accuracy is of prime importance, but the rate of data transfer is not guaranteed. An example of this would be disk drive storage. Isochronous mode sacrifices data accuracy in favor of guaranteed timing of data delivery. An example of this would be USB audio speakers.

USB Wiring Warning! Improperly wiring a USB port can result in permanent and fatal damage to the computer motherboard, the USB port and ANY peripheral connected to the motherboard. It is imperative that the specific USB pin out assignment is identified before any attempt is made to connect the cable to the motherboard.  Simply knowing which pin is #1 is not enough information to properly wire a USB port.  Trial and error will surely result in the destruction of the computer motherboard, the computer system and/or any attached peripheral. 

USB “A” Pinouts Pin Name Color Notes 1 VBUS Red Power 2 D- White Data - 3 D+ Green Data + 4 GND Black Ground

USB “B” Pinouts Pin Name Color Notes 1 VBUS Red Power 2 D- White Data - 3 D+ Green Data + 4 GND Black Ground

USB Voltage Supplied voltage by a host computer, or a powered hub port is between 4.75 and 5.25 Volts of Direct Current (DC). Maximum voltage drop for bus-powered hubs is 0.35 V from it's computer host, or hub, to the hubs output port. Normal operational voltage for USB circuits is a minimum of 4.75 Volts DC.