9

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 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

12k

Ω minimum.

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.

ISL8483/85/89/91 receiver outputs are three-statable via the

active low RE input.

Driver Features

The RS-485/422 driver is a differential output device that

delivers at least 1.5Vacross 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.

Drivers of the ISL8483/85/89/91 are three-statable via the

active high DE input.

The ISL8483/88/89 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. Outputs of

ISL8485/90/91 drivers are not limited, so faster output

transition times allow data rates of at least 5Mbps.

Data Rate, Cables, and Terminations

RS-485/422 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 100’, while the 250kbps

versions can operate at full data rates with lengths in excess

of 1000’.

Twisted pair is the cable of choice for RS-485/422 networks.

Twisted pair cables tend to pick up noise and other

electromagnetically induced voltages as common mode

signals, which are effectively rejected by the differential

receivers in these ICs.

Proper termination is imperative, when using the 5Mbps

devices, to minimize reflections. Short networks using the

250kbps versions need not be terminated, but, terminations

are recommended unless power dissipation is an overriding

concern.

FIGURE 5A. TEST CIRCUIT

FIGURE 5B. MEASUREMENT POINTS

FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES (EXCLUDING ISL8488, ISL8490)

1k

Ω

GND

SW

(SHDN) for ISL8483 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

ISL8483, ISL8485, ISL8488, ISL8489, ISL8490, ISL8491