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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com

SP33_00_0260

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

TTLor 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 20kbps.

When programmed to RS-485 mode, the

differentialRS-485driversproducecomplaint

RS-485 signals. Each RS-485 driver outputs

a unipolar signal on each output pin with a

magnitude of at least .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

SP331 has four single-ended receivers

whenprogrammedforRS-232modeandtwo

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

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 ±5V and can receive signals up

to 20kbps. 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.

ThedifferentialRS-485receiverhasaninput

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

SP331hastheabilitytochange

the configuration of the drivers and receiv-

ers via a 4–bit switch. Referring to Table ;

RS-232 mode, RS-485 mode, or two differ-

ent combinations of RS-232/RS-485 can be

configuredusingtheSEL_AandSEL_Bpins.

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.

Loopback Mode

Loopback is invoked by asserting "xx11"

into the select pins. In RS-232/RS-485 or

RS-485/RS-232loopbackmode,theRS-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.