SP481R Datasheet, PDF (6/9 Pages) Sipex Corporation – High-Fanout RS-485 Transceiver

GENERAL DESCRIPTION
The SP485R is a low power RS-485 differential
transceiver. Similar to the SP485, the SP485R
contains a half-duplex driver and receiver with
tri-state control. However, the SP485R is in-
tended for increased connections on a single bus
compared to the orignal RS-485 specification.
The RS-485 standard is ideal for multi-drop
applications where one bus can contain many
drivers and/or receivers. The RS-485 standard
implementation allows up to 32 transceivers to
be connected on to the data bus. RS-485 is also
specified for driving higher speeds over long
cable lengths of up to 4,000 feet. The SP485R
and SP481R exceed the standard b allowing up
to 400 receivers to share a bus.
DRIVERS
The driver output complies with the RS-485
electrical characteristics as specified by the
standard. The output swings from 0V to V_CC and
maintains greater than +1.5V with a 54Ω load
attached between the two outputs. In adhering
to the RS-485 specification, the driver outputs
inherently comply with the RS-422 standard.
With a load of 100Ω between the two outputs,
the driver can sustain at least +2.0V.
The driver contains an enable pin (DE) which
tri-states the output when DE is a logic LOW.
The outputs during the tri-stated condition are
at a high impedance (>100kΩ). A logic HIGH
enables the driver for normal operation. The
driver can operate to at least 5Mbps.
RECEIVERS
The SP485R receiver has differential inputs
with an input sensitivity of lower than ±200mV.
As mentioned above, the RS-485 specification
allows up to 32 transceivers on a the same bus.
The SP485R allows over 400 transceivers on
the same bus due to the high input impedance of
at least 120kΩ. This higher capacity allows
more components to be attached to the same
bus without degrading the signal quality. The
drivers are still able to drive an equivalent 54Ω
from the 320 transceivers with an input
impedance of at least 120kΩ in parallel along
with the two 125Ω cable termination resistors
on each end.
The receiver contains a enable pin (RE) which
enables the receiver when a logic LOW is
asserted. A logic HIGH will tri-state the
receiver output and the inputs will maintain
at least 120kΩ impedance. The receiver can
operate to at least 1Mbps.
The receiver also contains a fail-safe feature
which outputs a logic HIGH when the inputs are
open as in a disconnected cable.
SHUTDOWN MODE
The SP481R includes a shutdown function to
reduce power consumption. The shutdown is
activated by simultaneously applying a logic
LOW to DE and a logic HIGH to RE. While in
the shutdown mode, the power supply current is
typically less than 1µA. The driver outputs are
disabled and are at a high impedance state
determined by the receiver input impedance
which should be at least 120kΩ. The receiver
output is at also at high impedance during
shutdow. Output leakage current when the
receiver is disabled is under 1µA.
Date: 6/21/04
SP485R RS-485 Serial Transceiver
6
© Copyright 2004 Sipex Corporation