9

FN6051.6

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-485/422 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 ISL8487E and ISL81487L 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. ISL81487E 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 a few hundred feet, 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.

To minimize reflections, proper termination is imperative

when using the 5Mbps device. 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. These

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, these 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 re-enable after the die

temperature drops about 15 degrees. If the contention

persists, the thermal shutdown/re-enable cycle repeats until

the fault is cleared. Receivers stay operational during

thermal shutdown.

Low Power Shutdown Mode (Excluding

ISL81487E)

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but the ISL8487E

and ISL81487L include a shutdown feature that reduces

enter shutdown whenever the receiver and driver are

period of at least 600ns. Disabling both the driver and the

receiver for less than 50ns guarantees that shutdown is not

entered.

Note that receiver and driver enable times increase when

enabling from shutdown. Refer to Notes 5-9, at the end of

the Electrical Specification table, for more information.

ESD Protection

All pins on these interface devices include class 3 Human

Body Model (HBM) ESD protection structures, but the

RS-485 pins (driver outputs and receiver inputs)

incorporate advanced structures allowing them to survive

ESD events in excess of

±15kV HBM. The RS-485 pins are

particularly vulnerable to ESD damage because they

typically connect to an exposed port on the exterior of the

finished product. Simply touching the port pins, or

connecting a cable, can cause an ESD event that might

destroy unprotected ICs. These new ESD structures

protect the device whether or not it is powered up, protect

without allowing any latchup mechanism to activate, and

without degrading the RS-485 common mode range of -7V

to +12V. This built-in ESD protection eliminates the need

for board level protection structures (e.g., transient

suppression diodes), and the associated, undesirable

capacitive load they present.

ISL8487E, ISL81487L, ISL81487E