SP334CT-L-TR Datasheet, PDF (10/14 Pages) Exar Corporation – Programmable RS-232/RS-485 Transceiver

Drivers
The SP334 has three independent RS-232
single-ended drivers and two differential RS-
485 drivers. Control for the mode selection
is done by the RS232/RS485 select pin. The
drivers are pre-arranged such that for each
mode of operation the relative position and
functionality of the drivers are set up to ac-
commodate the selected interface mode. As
the mode of the drivers is changed, the elec-
trical characteristics will change to support
the requirements of clock, data, and control
line signal levels. Unused driver inputs can
be left floating; however, to ensure a desired
state with no input signal, pull–up resistors
to +5V or pull–down resistors to ground are
suggested. Since the driver inputs are both
TTL or CMOS compatible, any value resistor
less than 100kΩ will suffice.
When in RS-232 mode, the single-ended
RS-232 drivers produce compliant RS-232E
and ITU V.28 signals. Each of the three
drivers output single-ended bipolar signals
in access of ±5V with a full load of 3kΩ and
2500pF applied as specified. These drivers
can also operate at least 120kbps.
When programmed to RS-485 mode, the
differential RS-485 drivers produce complaint
RS-485 signals. Each RS-485 driver outputs
a unipolar signal on each output pin with a
magnitude of at least 1.5V while loaded with
a worst case of 54Ω between the driver's
two output pins. The signal levels and drive
capability of the RS-485 drivers allow the
drivers to also comply with RS-422 levels.
The transmission rate for the differential
drivers is 10Mbps.
Receivers
The SP334 has five single-ended receivers
when programmed for RS-232 mode and two
differential receivers when programmed for
RS-485 mode.
Control for the mode selection is done by
the same select pin as the drivers. As the
operating mode of the receivers is changed,
the electrical characteristics will change to
support the requirements of the appropriate
serial standard. Unused receiver inputs can
be left floating without causing oscillation. To
ensure a desired state of the receiver
output, a pull–up resistor of 100kΩ to +5V
should be connected to the inverting input
for a logic low, or the non–inverting input
for a logic high. For single-ended receiv-
ers, a pull–down resistor to ground of 5kΩ
is internally connected, which will ensure a
logic high output.
The RS-232 receiver has a single–ended
input with a threshold of 0.8V to 2.4V. The
RS-232 receiver has an operating voltage
range of ±15V and can receive signals up
to 120kbps. RS-232 receivers are used in
RS-232 mode for all signal types include
data, clock, and control lines of the RS-232
serial port.
The differential RS-485 receiver has an input
impedance of 15kΩ and a differential thresh-
old of ±200mV. Since the characteristics of
an RS-422 receiver are actually subsets of
RS-485, the receivers for RS-422 require-
ments are identical to the RS-485 receivers.
All of the differential receivers can receive
data up to 10Mbps.
Enable Pins
The SP334 drivers can be enabled by use
of the TXEN pin. A logic HIGH will enable
the driver outputs and a logic LOW will tri-
state the outputs. The drivers can only be
tri-stated in RS-485 mode. The drivers are
always active in RS-232 mode.
The Receiver outputs can also be tri-stated
by the use of the RXEN pin. A logic LOW
will enable the receiver outputs and a logic
HIGH will tri-state the outputs. . The receiver
tri-state capability is offered for both RS-232
and RS-485 modes. The input impedance
of the receivers during tri-state is at least
12kΩ.
Applications
The SP334 allows the user flexibility in hav-
ing a RS-232 or RS-485 serial port without
using two different discrete active IC's. Figure
18 shows a connection to a standard DB-9
RS-232 connector. In RS-485 mode, the
SP334 is a full duplex transceiver, however,
a half duplex configuration can be made by
connecting the driver outputs to receiver
inputs.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
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