Line drivers and receivers: innovating the interface

Ever more demanding transmission line data rates and distances continue to push line driver and receiver innovations. These chips interface between the digital ones and zeros and the analog transmission line where noise (both generated and tolerated), impedance discontinuities, ground-voltage differences, and decreasing power-supply voltages work against reliable data transmission. If microprocessors are the brains and clock chips are the heartbeat of electronic systems, then interface is the electronic circulatory system. Robust circulation is essential to healthy system operation, and therefore innovative interface is crucial to next-generation systems.

The EIA RS232 standard has been around since 1962. It is a single-ended technology that has been quite successful; the 20-kbit/second data rates and 50-foot distances were considered fast back in the '60s. It's a testament to RS232's durability that even today, designers choose it for new products.

Near the end of the Cold War, the increasing data-transfer rates set the stage for new interface innovations. The EIA created RS422/423 and then RS485, which abandoned single-ended signals for revolutionary differential signals to achieve faster data rates over longer distances. Differential signals use both the true signal and its complement. The differential signal employs the complement as the true signal's reference, which sets it free from referencing unstable grounds. Also, differential signals tolerate induced noise because it equally affects both the true and complement signal. Therefore the differential voltage remains unaffected.

Soon systems were requiring much faster data rates at lower power than RS485's 10 Mbits/s. The EIA/TIA-644-A standard took on the challenge by standardizing the low-voltage differential-signaling (LVDS) technology. LVDS has become mainstream data-transfer technology today because of its kbit/s to Gbit/s capability, low noise and low power. System designers use it in many applications including computer displays, telecom and multiple industrial application segments because LVDS solves so many of the interface problems.

As successful as LVDS is, its point-to-point limitations hindered its use in multipoint applications such as RS485. This led to the invention of bus LVDS. Bus LVDS' innovation was to increase the drive current so that it would drive doubly terminated transmission lines. This innovation was the first of the many flavors of application-specific LVDS that are available today.

Further innovations to bus LVDS addressed the receiver's low immunity to the ground potential difference problems. LVDS and bus LVDS both use ac coupling in order to mitigate the harmful effects of signals outside their power supply rails. To address this, the TIA/EIA-899 standard created multipoint LVDS, further improving bus LVDS by providing the -1.4-volt to +3.8-V common-mode receiver tolerance. Additionally, there are receiver threshold innovations to provide built-in failsafe operation, and drivers that must drive 32 bused loads.

LVDS innovations continue to deliver valuable features that position it as the line driver and receiver technology of the future. For example, the LVDS multigigabit data rates enable using it in serializer and deserializer products that reduce the size and cost of connectors, cables and backplanes. Those products also add innovative features like dc balancing, which is necessary for ac-coupling techniques to extend cable transmission lengths.

Features like receiver adaptive equalization, driver pre-emphasis and de-emphasis, and back-channel signaling are finding their way into line drivers and receivers. These signal-conditioning techniques will reliably carry the ones and zeros through the signal path transmission lines for many years to come.

About the Author
As Marketing Director for the Interface Division at National Semiconductor Corp., Stephen Kempainen is responsible for high-speed interface, industrial Ethernet connectivity, and clock-conditioning products. He also directs technical marketing and system architecture for new products.

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