PoweredUSB, also known as Retail USB, USB PlusPower, and USB +Power, is an addition to the Universal Serial Bus standard that allows for higher-power devices to obtain power through their USB host instead of requiring an independent power supply or external AC adapter. It is mostly used in point-of-sale equipment, such as receipt printers and barcode readers.
History
PoweredUSB, as a proprietary variant of USB, was developed and proposed by IBM, Berg, NCR and Microsoft between 1998 and 1999, with the last revision issued in 2004. The specification is not endorsed by the USB Implementers Forum. IBM, who owns patents to PoweredUSB, charges a licensing fee for its use. PoweredUSB was licensed by Hewlett-Packard, Dell Inc., Cyberdata, Fujitsu, Wincor and others.
Implementation
PoweredUSB uses a more complex connector than standard USB, maintaining the standard USB 1.x/2.0 interface for data communications and adding a second connector for power. Physically, it is essentially two connectors stacked such that the bottom connector accepts a standard USB plug and the top connector takes a power plug. The implementation allows a choice of three different voltages, providing power at 5 V, 12 V, 24 V as well as a custom voltage. Some implementations provide 19 V or 25 V. The connectors are able to operate at up to 6 Apeak, but according to the specification, hosts are required to provide a minimum sustainable rms current of 1.5 A at 5 V or 12 V, or 2.3 A at 24 V, only. For comparison, a standard USB 1.x and 2.0 hosts supplies 5 V at up to 0.5 A. USB 3.0 supplies 5 V at up to 0.9 A whereas hosts conformant to the USB Battery Charging Specification can deliver up to 1.5 A. As each PoweredUSB plug provides one of three voltages, the plugs come keyed in three versions, so that they will only accept connections from devices requiring that version's voltage. The connectors can be color-coded for different voltages: Gray for 5 V, blue-green for 12 V, red for 24 V/25 V and violet for 19 V; alternatively, any voltages higher than 5 V can be indicated by a black connector color. The connectors, available from various manufacturers, provide an auto-locking facility to reduce the risk of accidental power failures. Special provisions for hot-plugging are recommended, but optional. The Red 24 volt plugs should never be plugged in or removed with the unit powered on. This is likely to cause the failure of the jack.
Outlook
Extending USB's power capability is also a response to Power over Ethernet, which has more flexible dynamic power negotiation capabilities up to 48 V DC and up to about 1 A, and whose maximum bandwidth is potentially greater than even USB 3.1's 10 Gbit/s. The combination of USB 3.0 and a 6 A limit at the most common voltages would permit support of a wider variety of devices, although powered Ethernet can support them at longer cable distances; however, PoweredUSB does not incorporate USB 3.x officially. In general, since the voltage drop at high currents can be significant over even a few meters, higher voltages are desired for larger distances. In 2012, the USB-IF released the USB Power Delivery Specification as an optional part of the USB 2.0 and 3.x specifications. It defines features similar to those addressed by PoweredUSB, but without requiring the use of proprietary connectors. Instead, the connectors defined in the USB 2.0 and 3.0 standards are continued to be used; higher currents require PD-aware USB-cables, though. USB hosts compliant with this USB specification can be requested by USB devices to provide alternative voltages and higher currents up to 3 A at 5 V and optionally up to 5 A at either 12 V or 20 V.
