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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (510)668-7017 • www.exar.com SP490,491_101_062711
DESCRIPTION
The SP490 and SP491 are full-duplex differen-
tial 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) 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 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 sensitiv-
ity as low as ±200mV. Input impedance of the
receivers is typically 15kΩ (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 re-
ceiver.
