introduced with the USB 3.1 specification. Under the most recent specifications, USB supports most scenarios connecting systems directly with a Type-C cable. For the capability to work, however, connected systems must support role-switching. Dual-role capabilities requires there be two controllers within the system, as well as a role controller. While this can be expected in a mobile platform such as a tablet or a phone, desktop PCs and laptops often will not support dual roles. Power Upstream USB connectors supply power at a nominal 5V DC via the V_BUS pin to downstream USB devices. Low-power and high-power devices Low-power devices may draw at most 1-unit load, and all devices must act as low-power devices when starting out as unconfigured. 1 unit load is 100 mA for USB devices up to USB 2.0, while USB 3.0 defines a unit load as 150 mA. High-power devices (such as a typical 2.5-inch USB hard disk drive) draw at least 1 unit load and at most 5-unit loads (5x100mA = 500 mA) for devices up to USB 2.0- or 6-unit loads (6x150mA= 900 mA) for SuperSpeed (USB 3.0 and up) devices. To recognize Battery Charging mode, a dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals. Shorted or near-shorted data lanes with less than 200 Ω of resistance across the "D+" and "D−" terminals signify a dedicated charging port (DCP) with indefinite charging rates. In addition to standard USB, there is a proprietary high-powered system known as PoweredUSB, developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers. Signaling USB signals are transmitted using differential signaling on a twisted-pair data wires with characteristic impedance. USB 2.0 and earlier specifications define a single pair in half-duplex (HDx). USB 3.0 and later specifications define one pair for USB 2.0 compatibility and two or four pairs for data transfer: two pairs in full-duplex (FDx) for single lane variants (requires SuperSpeed connectors); four pairs in full-duplex for dual lane (×2) variants (requires USB-C connector). Low-speed (LS) and Full-speed (FS) modes use a single data pair, labelled D+ and D−, in half-duplex. Transmitted signal levels are for logical low, and for logical high level. The signal lines are not terminated. High-speed (HS) mode uses the same wire pair, but with different electrical conventions. Lower signal voltages of for low and for logical high level, and termination of 45