SP331 Datasheet, PDF (10/13 Pages) Sipex Corporation – Programmable RS-232/RS-485 Transceiver

Drivers
The SP331 has four independent RS-232
single-ended drivers and two differential
RS-485 drivers. Control for the mode selec-
tion is done via a four–bit control word. 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 four driv-
ers 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.
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.
Select Mode Pins
Similar to our SP500 family of multiprotocol
products, the SP331 has the ability to change
the configuration of the drivers and receiv-
ers via a 4–bit switch. Referring to Table 1;
RS-232 mode, RS-485 mode, or two differ-
ent combinations of RS-232/RS-485 can be
configured using the SEL_Aand SEL_B pins.
The drivers can be put into tri-state mode
by using the SEL_C and SEL_D pins. All
receivers remain active during any tri-state
condition of the drivers.
Receivers
The SP331 has four 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 via a
4–bit control word, as in 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
Loopback Mode
Loopback is invoked by asserting "xx11"
into the select pins. In RS-232/RS-485 or
RS-485/RS-232 loopback mode, the RS-232
driver outputs loop back into the RS-232
receiver inputs and the RS-485 differential
driver loops back into the RS-485 receiver.
During loopback, the driver outputs and
receiver inputs are disconnected from the
outside world. The driver outputs are in tri-
state and the receiver inputs are disabled.
The input impedance of the receivers
during loopback is approximately 15kΩ to
ground.
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