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Date: 05/10/04

SP490/491 Full Duplex RS-485 Transceivers

© Copyright 2004 Sipex Corporation

DESCRIPTION

The SP490 and SP491 are full-duplex differential

transceivers that meet the requirements of

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

+12V) common 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 54

Ω across the

differential outputs, the driver can maintain

greater than 1.5V 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 SP490 and SP491

have differential inputs with an input sensitivity

as low as

±200mV. Input impedance of the

receivers is typically 15K

Ω (12KΩ minimum).

A wide common mode range of -7V to +12V

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. A logic

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 receiver.