ISL8485, ISL8490, ISL8491

FN6046 Rev 10.02

Page 9 of 15

Oct 2, 2023

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 receivers on each bus, assuming one unit load

devices. 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 multipoint operation, the RS-

485 specification requires that drivers must handle bus

contention without sustaining any damage.

An 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 4000ft, 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 use 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 impedance surpasses the RS-422 specification

of 4kΩ, and meets the RS-485 unit load requirement of 12kΩ

minimum.

Receiver inputs function with common-mode voltages as great

as

7V outside the power supplies (+12V and -7V), making

them ideal for long networks in which induced voltages are a

realistic concern.

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

ensures a high level receiver output if the receiver inputs are

unconnected (floating).

Receivers easily meet the data rates supported by the

corresponding driver.

The ISL8485 and ISL8491 receiver outputs are three-statable

using the active low RE input.

Driver Features

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

delivers at least 1.5V across a 54Ω load (RS-485), and at least

2V across a 100Ω load (RS-422). The drivers feature low

propagation delay skew to maximize bit width and to minimize

EMI.

The ISL8485 and ISL8491 drivers are three-statable using the

active high DE input.

The ISL8485 and ISL8491 driver outputs are not limited, so

faster output transition times allow data rates of at least

5Mbps.

Data Rate, Cables, and Terminations

RS-485/RS-422 are intended for network lengths up to 4000ft,

but the maximum system data rate decreases as the

transmission length increases. Devices operating at 5Mbps are

limited to lengths less than 100ft.

Twisted pair cable 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 to minimize reflections when

using the 5Mbps devices.

In point-to-point networks or point-to-multipoint (single driver

on bus) networks, terminate the main cable in its characteristic

impedance (typically 120Ω) at the end farthest from the driver.

In multi-receiver applications, stubs connecting receivers to the

main cable should be kept as short as possible. In multipoint

(multi-driver) systems, terminate the main cable in its

characteristic impedance at both ends. Keep stubs connecting

a transceiver to the main cable as short as possible.

Built-In Driver Overload Protection

The RS-485 specification requires that drivers survive worst

case bus contentions undamaged. The ISL84XX devices meet

this requirement using driver output short circuit current limits

and on-chip thermal shutdown circuitry.

The driver output stages incorporate short-circuit current

limiting circuitry that ensures the output current never exceeds

the RS-485 specification, even at the common-mode voltage

range extremes. These devices also use a foldback circuit that

reduces the short-circuit current, and thus the power

dissipation, whenever the contending voltage exceeds either

supply.

In the event of a major short-circuit condition, the ISL84XX

devices’ thermal shutdown feature disables the drivers

whenever the die temperature becomes excessive. This

eliminates the power dissipation, allowing the die to cool. The

drivers automatically re-enable after the die temperature drops

about 15°C. If the condition persists, the thermal shutdown/re-

enable cycle repeats until the fault is cleared. Receivers stay

operational during thermal shutdown.