Date: 03-04-05

Programmable Dual RS-232/RS-485 Transceiver

© Copyright 2005 Sipex Corporation

10

Drivers

The SP331 has four independent RS-232 single-

ended drivers and two differential RS-485 driv-

ers. Control for the mode selection 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 accommodate the selected

interface mode. As the mode of the drivers is

changed, the electrical 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 sug-

gested. Since the driver inputs are both TTL or

CMOS compatible, any value resistor less than

100k

Ω 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 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 differ-

ential RS-485 drivers produce complaint RS-

485 signals. Each RS-485 driver outputs a uni-

polar signal on each output pin with a magnitude

of at least 1.5V while loaded with a worst case

of 54

Ω 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 when

programmed for RS-232 mode and two differ-

ential 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 oper-

ating mode of the receivers is changed, the

electrical characteristics will change to support

the requirements of the appropriate serial stan-

dard. Unused receiver inputs can be left floating

without causing oscillation. To ensure a desired

state of the receiver output, a pull–up resistor of

100k

Ω to +5V should be connected to the in-

verting input for a logic low, or the non–invert-

ing input for a logic high. For single-ended

receivers, a pull–down resistor to ground of 5k

Ω

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 15k

Ω and a differential threshold

of

±200mV. Since the characteristics of an RS-

422 receiver are actually subsets of RS-485, the

receivers for RS-422 requirements 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 receivers via a

4–bit switch. Referring to Table 1; RS-232

mode, RS-485 mode, or two different combina-

tions of RS-232/RS-485 can be configured us-

ing the SEL_A and 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 driv-

ers.

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 out-

puts 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 discon-

nected 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 15k

Ω to

ground.