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Low voltage differential signaling

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Low voltage differential signaling, or LVDS, is an electrical signalling system that can be run at very high speeds. It has recently become very popular in the computer arena, where it is being used to build very high speed networks and computer buses.

In the past most signalling schemes relied on detecting the presence of a signal in a single wire, compared to ground. For instance, the widely used RS-232 system uses +/- 12V to represent a signal, and anything less than +/- 5V to represent the lack of a signal. The high voltage levels are used to make them largely immune to noise, as there are few naturally occurring signals that can create that sort of voltage. They also have the advantage of requiring only one wire per signal.

However they also have a serious disadvantage, they cannot be run at high speeds. This is due to the effects of capacitance and inductance, which filter out high-frequency signals. You can reduce this problem by using smaller voltages. However when you use smaller voltages, the chance of random environmental noise being mistaken for signal becomes much more of a problem.

LVDS relies on detecting not the voltage with respect to ground, but the difference in voltage between two wires. A small current, about 3 milliamps, is injected into one wire or the other and returns in the opposite direction along the other wire. The voltage difference between the two wires is about 300 millivolts. The small amplitude of the signal reduces the effects of capacitance and inductance. The system also reduces problems with noise, because noise sources tend to add the same amount of voltage (called common-mode noise) to both wires, so the difference between the voltages remains the same. Noise is further reduced by twisting the two wires of a pair together, so that noise induced in one half-twist tends to cancel the noise induced in the neighbouring half-twist.

LVDS only became popular in the latter half of the 1990s. Prior to that point it could signal faster than the computers it was running in, and the need to run twice as many wires for the same amount of data was too high a price to pay. Yet there was widespread interest in it for multimedia and supercomputer users, both of whom needed to move large amounts of data over links several meters long, from a disk drive to a workstation for instance.

LVDS is defined by the ANSI/TIA/EIA[?]-644-A standard.

Applications Two examples of LVDS use in computer buses are HyperTransport and FireWire, both of which trace their ancestry back to the post-Futurebus work which also led to SCI.

LVDS is also used to transport video data from graphics adapters[?] to computer monitors, particularly flat panels, using the Flat Panel Display (FPD[?]) Link, LVDS Display Interface (LDI[?]) and OpenLDI[?] standards. These standards allow a maximum pixel clock of 112 MHz, which is sufficient for a display resolution of 1400 x 1050 (SXGA+) at 60 Hz refresh. A dual link can also be used to boost the maximum display resolution to 2048 x 1536 (QXGA) at 60 Hz. FPD-Link works with cable lengths up to about 5 m, and LDI extends this to about 10 m.

External links

Introduction to LVDS (http://www.national.com/appinfo/lvds/files/ch1.pdf)



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