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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-707 • www.exar.com

SP490,49_00_07408

DESCRIPTION

The

SP490andSP491arefull-duplexdifferen-

tialtransceiversthatmeettherequirementsof

RS-485 and RS-422. Fabricated with a

Sipex proprietary BiCMOS process, both

products require a fraction of the power of

older bipolar designs.

The RS-485 standard is ideal for multi-drop

applications or for long-distance interfaces.

RS-485 allows up to 32 drivers and 32

receivers to be connected to a data bus,

making it an ideal choice for multi-drop

applications. Since the cabling can be as

long as 4,000 feet, RS-485 transceivers are

equipped with a wide (-7V to +2V) com-

mon mode range to accommodate ground

potential differences. Because RS-485

is a differential interface, data is virtually

immune to noise in the transmission line.

Driver...

The drivers for both the

SP490 and SP491

have differential outputs. The typical voltage

output swing with no load will be 0 volts to

+5 volts. With worst case loading of 54Ω

across the differential outputs, the driver can

maintain greater than .5V voltage levels.

The driver of the

SP491 has a driver enable

control line which is active high. A logic high

on DE (pin 4) of the

SP491 will enable

the differential driver outputs. A logic low

on DE (pin 4) of the

SP491 will tri-state the

driver outputs. The

SP490 does not have a

driver enable.

Receiver...

The receivers for both the

SP490andSP491

havedifferentialinputswithaninputsensitiv-

ityaslowas±200mV.Inputimpedanceofthe

receivers is typically 15kΩ (12kΩ minimum).

A wide common mode range of -7V to +2V

allows for large ground potential differences

between systems.The receivers for both the

SP490 and SP491 are equipped with the

fail-safe feature. Fail-safe guarantees that

the receiver output will be in a high state

when the input is left unconnected.

The receiver of the

SP491 has a receiver

enable control line which is active low.Alogic

low on REB (pin 3) of the

SP491 will enable

the differential receiver. A logic high on REB

(pin 3) of the

SP491 will tri-state the re-

ceiver.