USB Wiring

USB2A-BIf you need information about USB and how it is wired, you have come to the right place. Leads Direct carries a huge range of USB leads , USB Adaptors and USB accessories. You can view and purchase these leads by visiting the USB section which has literally hundreds of products each with its own image.

The Purpose of USB

USB was designed to allow many peripherals to be connected using a single standardized interface and to improve plug-and-play capabilities by allowing ‘hot swapping’ of devices, i.e. allowing them to be connected and disconnected without rebooting the computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

USB is rapidly replacing all legacy varieties of serial and parallel ports, and at the time of writing most new computers ship with only USB for peripheral connection. USB can connect many computer peripherals including as mice, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices USB has become the standard connection method. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes.

The USB Specification

The USB [Universal Serial Bus] specification defines the Mechanical, Electrical and Protocol layers of the interface. Cables and connectors. The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries.

USB defines two types of hardware, known as Hubs and Functions. USB Hubs allow connection to a USB bus, while USB Functions are the devices which perform some function. The system may only have one Host, which connects to a Hub. A USB Hub may connect to another Hub or to a USB Function. Devices are arranged in a tiered Star topology. The physical wire segments are point-to-point between a Host, Hub, or Function. Each layer transition from Hub to Hub represents another Tier.

Up to 127 devices may be connected together, the limiting factor being that the specification allows for only seven address bits. The USB interface is designed to operate down to -20 degrees C.

The USB bus is a [Differential] Bi-directional serial interface cable bus. Differential NRZI data is transmitted Isochronous or Asynchronous between devices. Data is transferred at one of three different rates over a maximum cable length of 5 meters over 4 wires, 2 of which carry data on a balanced twisted pair.

USB may operate at any speed from 10kbps to 400Mbps in one of three speed modes. A Slow-Speed mode of 10kbps to 100kbps is used for devices such as a USB keyboard or USB mouse. Full-Speed mode is used by most devices and allows a transfer rate of 500kbps to 10Mbps. High-Speed mode [defined by USB 2.0] allows rates of up to 480Mbps, with a speed range of 25Mbps to 400Mbps. Transmission at the High-Speed mode requires the addition of 45 ohm termination resistors between each data line and ground. Operation at Full-Speed mode is 2.8 volts [High] to 0.3 volts [Low]. Operation at High-Speed mode is at 400mV +/-10% [High] to 0V +/- 10mV {Low]. Cable impedance for both modes is 90 ohms +/- 15% (differential).

Four different packet protocols are used; Control, Interrupt, Isochronous and Bulk. Each exchange contains 3 packets; A token packet which holds the address, a data packet which holds the data, and a handshake packet which terminate the exchange. NRZI produces a change in the signal indicating a logic zero, no change indicates a logic one. Bit stuffing is used with NRZI to stop the signal remaining in the steady state condition; if more then 6 ones are transmitted (no change in the signal) a zero is inserted to produce a transition. NRZI, with bit stuffing is self clocking, allowing the receiver to synchronize with the transmitter.

USB 3.0 (Super-Speed USB) increases the data rate to 4.8 Gbit/s, 600 MB/s. USB 3.0 ports and cabling will be designed to enable backward compatibility as well as future-proofing for optical capabilities (USB 3.0 adds fiber). USB 3.0 connectors contain legacy pins to interface to USB 2.0 devices, and a new set of pins for USB 3.0 connectivity (both sets reside in the same connector).

USB Cable

USB cable has four conductors, two for power and two for data. The data wires are 28 AWG, the power wires are 20 to 28 AWG. The power cores are un-twisted and the data lines twisted. Longer cables will use 20 AWG for power.

Standard Cables will have an A plug on one end an a B plug on the opposite end. The maximum cable length is 5 meters but that can be increased by using Hubs and repeater extensions.

USB Connectors

There are two standard connectors, the A and the B. A third ‘mini B’ connector is used for small devices such as digital cameras, telephones, and MP3 players. The standard A and B plugs have four contacts, and the standard mini B has five.

Even though the system uses only four cores in the cable, there are also a number of ‘non-standard’ mini B connectors with up to 16 contacts which are proprietary and most often created and used by camera manufacturers such as Nikon, Olympus, etc.

USB Standard Pin out

Pin # Pin Description Colour Function
1 + 5v Red Power
2 Data – Blue Negative signal
3 Data + Yellow Positive signal
4 Ground Brown Ground

USB Mini / Micro Connector Pin out

Pin # Pin Description Colour Function
1 +5V Red Power
2 Data- White Negative Signal
3 Data+ Green Positive Signal
4 ID None Permits distinction between Micro A and Micro B connectors – A connected to ground, B not connected
5 Ground Black Signal Ground

USB Receptacle Shapes and Pin Locations

usb receptacles


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