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USB, Universal Serial Bus

USB logo

The universal serial bus is a standard for connecting serial devices in a network fashioned way. The standard defines three different devices: hosts, hubs and functions. Hosts are the initiating devices, like PCs, and only 1 host may exist in a network. Functions are dumb devices, like keyboards, mice, printers. And hubs are multi-port repeaters which act like distributing devices in the serial network.

The maximum speed on an USB network is 12 Mbps, which uses an STP cable. UTP is also possible, but this limits the speed to 1.5 Mbps. The upstream ports (hub to hub or hub to host) always use 12 Mbps, only connections to functions may be 1.5 Mbps.

Quick overview

  12 Mbps 1.5 Mbps
Max. Cable length 5 meter 3 meter
Connector A-Series or B-Series
Max. amount of HUBs 5
Max. amount of units 127

There are two types of cables. The standard USB cable which is used for 12 Mbps and has an A-series connector consists of one pair 20-28 AWG wire for power and one 28 AWG twisted pair for data. The cable has a shield and an overall jacket which makes it a STP-cable. The alternative cable is used for the 1.5 Mbps version and has a B-type connector. This cable has one pair of 28 AWG wire stranded copper for data and one pair 20-28 AWG for power. This cable is only used in sub-channel applications.


A-series B-series
USB A Male USB A Female USB B Male USB B Female
Male Female Male Female



Electrical Specifications
A differential "1" is defined as (D+) - (D-) > 200 mV and a "0" is defined as (D+) - (D-) < -200 mV.
The line encoding used is always NRZI. This is independed of the low or high speed version.
The maximum end-to-end signal delay is 70 ns, which gives us a maximum configuration of 5 hubs per link between function and host. If all cables are high-speed cables, the max. distance between a function and a host is 30 meters.

USB Maximum Topology

The devices
Within the USB specifications there are a couple of different device classes specified. The difference between the devices has to do with the amount of power a unit can support to the bus and the amount of power the unit consumes from the bus. To make it easy an unit load is defined as being 100 mA. Power is always supplied downstream and received from upstream.

Devices may be attached and detached on the fly, without powering down the entire bus.

HUB- bus-powered These HUBs draw their power from the bus, and often contain one embedded function. The maximum number of downstream ports is 4 (1 unit load for the HUB and embedded function + 4 unit loads for the output ports = 5 unit loads)

HUB- self-powered The units have a maximum of 7 ports with a per port maximum current of 5 unit loads.

FUNCTION- low bus-powered A low-power function is a function that draws less the 1 unit load from the bus.

FUNCTION- high bus-powered The high-power function draws between 1 and 5 unit loads from the bus. You could receive a message from such a unit that there is not enough power, which means that the upstream device is not able to send enough unit loads to the unit to make it function correctly.

FUNCTION- self-powered For this function are no limits.

HOST Always supplies the power to the bus.