8

FN6050.7

July 31, 2006

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

These devices utilize a differential input receiver for maximum

noise immunity and common mode rejection. Input sensitivity

is

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

specifications.

Receiver input resistance of 96k

Ω surpasses the RS-422

spec 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 spec.

Receiver inputs function with common mode voltages as

great as

±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).

Receivers easily meet the data rates supported by the

corresponding driver, and receiver outputs are three-statable

via the active low RE input.

Driver Features

The RS-485 and RS-422 driver is a differential output device

that delivers at least 1.5V across a 54

Ω load (RS-485), and

at least 2V across a 100

Ω load (RS-422). The drivers feature

low propagation delay skew to maximize bit width, and to

minimize EMI.

Driver outputs are three-statable via the active high DE

input.

The ISL8487 and ISL81483 driver outputs are slew rate

limited to minimize EMI, and to minimize reflections in

unterminated or improperly terminated networks. Data rate

on these slew rate limited versions is a maximum of

250kbps. ISL81487 drivers are not limited, so faster output

transition times allow data rates of at least 5Mbps.

Data Rate, Cables, and Terminations

RS-485 and RS-422 are intended for network lengths up to

4000’, but the maximum system data rate decreases as the

transmission length increases. Devices operating at 5Mbps

are limited to lengths less than a few hundred feet, while the

FIGURE 5A. TEST CIRCUIT

FIGURE 5B. MEASUREMENT POINTS

FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES

1k

Ω

GND

SW

(SHDN) for ISL8487 and ISL81483 only.

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

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

ISL8487, ISL81483, ISL81487