because it does not need an additional PCB layer, as a stripline does. Because each line causes a matching image current in the ground plane, which is required anyway for supplying power, the pair looks like four lines and therefore has a shorter crosstalk distance than a simple isolated pair. In fact, it behaves as well as a twisted pair. Low crosstalk is important when many lines are packed into a small space, as on a typical PCB. High-voltage differential signalling High-voltage differential (HVD) signalling uses high-voltage signals. In computer electronics, high voltage normally means 5 volts or more. SCSI-1 variations included a high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version. SCSI equipment, for example, allows a maximum total cable length of 25 meters using HVD, while single-ended SCSI allows a maximum cable length of 1.5 to 6 meters, depending on bus speed. LVD versions of SCSI allow less than 25 m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than the older HVD SCSI. The generic term high-voltage differential signalling describes a variety of systems. Low-voltage differential signalling (LVDS), on the other hand, is a specific system defined by a TIA/EIA standard. Polarity switching Some integrated circuits dealing with differential signals provide a hardware option (via strapping options, under firmware control, or even automatic) to swap the polarity of the two differential signals, called differential pair swapping, polarity reversion, differential pair inversion, polarity inversion, or lane inversion. This can be utilized to simplify or improve the routing of high-speed differential pairs of traces on printed circuit boards in hardware development, to help to cope with common cabling errors through swapped wires, or easily fix common design errors under firmware control. Many Ethernet PHY transceivers support this as auto polarity detection and correction (not to be confused with a similar auto crossover feature). PCIe and USB SuperSpeed also support lane polarity inversion. Another way to deal with polarity errors is to use polarity-insensitive line codes. See also Backplanes Current loop signalling Current mode logic (CML) DDR SDRAM Differential amplifier Differential TTL DisplayPort Humbucker Longitudinal voltage Signal integrity Single-ended signalling Transition Minimized Differential Signalling (TMDS) References Computer buses Communication circuits Telecommunications engineering ja:差動信号