8

FN6074.2

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 120k

Ω surpasses the RS-422

spec of 4k

Ω, and is more than eight times the RS-485 “Unit

Load” requirement of 12k

Ω. 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 rate supported by the

corresponding driver, and 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 1.5V 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, and driver outputs are three-statable via the

active high DE input.

The ISL4489E driver outputs are slew rate limited to further

reduce EMI, and to minimize reflections in unterminated or

improperly terminated networks. Data rates on these slew

rate limited versions are a maximum of 250kbps. Outputs of

ISL4491E drivers are not limited, so faster output transition

times allow data rates of at least 15Mbps.

Data Rate, Cables, and Terminations

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.

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 15Mbps

are limited to lengths of a few hundred feet, while the

250kbps versions can operate at full data rates with lengths

in excess of 1000’.

Proper termination is imperative, when using the 15Mbps

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. In point-to-point, or point-to-multipoint (single driver

on bus) networks, the main cable should be terminated in its

characteristic impedance (typically 120

Ω) 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. Multipoint (multi-driver) systems require

that the main cable be terminated in its characteristic

impedance at both ends. Stubs connecting a transceiver to

the main cable should be kept as short as possible.

Built-In Driver Overload Protection

As stated previously, the RS-485 spec requires that drivers

survive worst case bus contentions undamaged. The

ISL44XXE devices meet this requirement via driver output

short circuit current limits, and on-chip thermal shutdown

circuitry.

The driver output stages incorporate short circuit current

limiting circuitry which ensures that the output current never

exceeds the RS-485 spec, even at the common mode

voltage range extremes. Additionally, these devices utilize a

foldback circuit which 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, ISL44XXE

devices also include a thermal shutdown feature that

disables the drivers whenever the die temperature becomes

excessive. This eliminates the power dissipation, allowing

the die to cool. The drivers automatically reenable after the

die temperature drops about 15 degrees. If the contention

persists, the thermal shutdown/reenable cycle repeats until

the fault is cleared. Receivers stay operational during

thermal shutdown.

ISL4489E, ISL4491E