7

FN6071.1

March 15, 2005

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. To allow

for multipoint operation, the RS-485 spec requires that

drivers must handle bus contention without sustaining any

damage.

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

This device utilizes a differential input receiver for maximum

noise immunity and common mode rejection. Input

sensitivity is ±200mV, as required by the RS422 and RS-485

specifications.

Receiver input impedance surpasses the RS-422 spec of

4k

Ω, and meets the RS-485 “Unit Load” requirement of 12kΩ

minimum.

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 are a realistic concern.

All the receivers include a “fail-safe if open” function that

guarantees a high level receiver output if the receiver inputs

are unconnected (floating).

The receiver easily meets the data rate supported by the

driver, and the receiver output is tri-statable via the active

low RE input.

FIGURE 4A. TEST CIRCUIT

FIGURE 4B. MEASUREMENT POINTS

FIGURE 4. RECEIVER DATA RATE AND PROPAGATION DELAY

FIGURE 5A. TEST CIRCUIT

FIGURE 5B. MEASUREMENT POINTS

FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES

SIGNAL

GENERATOR

R

RO

RE

A

B

GND

+1.5V

15pF

RO

3V

0V

1.5V

1.5V

0V

50%

50%

A

1k

Ω

GND

SW

PARAMETER

DE

A

SW

0

+1.5V

GND

0

-1.5V

0

+1.5V

GND

0

-1.5V

0

+1.5V

GND

0

-1.5V

SIGNAL

GENERATOR

R

RO

RE

A

B

GND

15pF

RO

3V

0V

1.5V

1.5V

0V

1.5V

RO

1.5V

RE

OUTPUT HIGH

OUTPUT LOW

NOTE 7

NOTE 7

NOTE 7

ISL43485