8

FN6543.2

October 18, 2007

Application Information

RS-485 and RS-422 are differential (balanced) data

transmission standards for use in long haul or noisy

environments. RS-422 is a subset of RS-485, so RS-485

transceivers are also RS-422 compliant. RS-422 is a

point-to-multipoint (multidrop) standard, which allows only one

driver and up to 10 (assuming one unit load devices) receivers

on each bus. RS-485 is a true multipoint standard, which allows

up to 32 one unit load devices (any combination of drivers and

receivers) on each bus.

Another important advantage of RS-485 is the extended

common mode range (CMR), which specifies that the driver

outputs and receiver inputs withstand signals that range from

+12V to -7V. RS-422 and RS-485 are intended for runs as

long as 4000’, so the wide CMR is necessary to handle

ground potential differences, as well as voltages induced in

the cable by external fields.

Receiver Features

These devices utilize a differential input receiver for maximum

noise immunity and common mode rejection. Input sensitivity

is better than ±200mV, as required by the RS-422 and RS-485

specifications.

Receiver input resistance of 96k

Ω surpasses the RS-422

specification of 4k

Ω and is eight times the RS-485 “Unit

Load (UL)” requirement of 12k

Ω minimum. Thus, these

products are known as “one-eighth UL” transceivers and

there can be up to 256 of these devices on a network while

still complying with the RS-485 loading specification.

Receiver inputs function with common mode voltages as great

as +9V/-7V outside the power supplies (i.e., +12V and -7V),

making them ideal for long networks where induced voltages,

and ground potential differences are realistic concerns.

Test Circuits and Waveforms

FIGURE 1A. TEST CIRCUIT

FIGURE 1B. MEASUREMENT POINTS

FIGURE 1. RECEIVER PROPAGATION DELAY AND DATA RATE

FIGURE 2A. TEST CIRCUIT

FIGURE 2B. MEASUREMENT POINTS

FIGURE 2. RECEIVER ENABLE AND DISABLE TIMES (EXCEPT ISL3280E AND ISL3284E)

SIGNAL

GENERATORS

R

RO

RE

A

B

GND

15pF

RE

RO

+1V

-1V

0V

0V

0V

50%

50%

A

B

1k

Ω

GND

SW

PARAMETER

A

SW

+1.5V

GND

-1.5V

+1.5V

GND

-1.5V

SIGNAL

GENERATOR

R

RO

RE OR RE

A

B

GND

15pF

RO

3V

0V

1.5V

1.5V

0V

50%

RO

50%

RE

OUTPUT HIGH

OUTPUT LOW

(INVERT FOR RE)

ISL3280E, ISL3281E, ISL3282E, ISL3283E, ISL3284E, ISL3285E