9

Receivers easily meet the data rates supported by the

corresponding driver.

ISL8483E/85E 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.

Drivers of the ISL8483E/85E are three-statable via the active

high DE input.

The ISL8483E 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

the ISL8485E driver 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.

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

ISL848XE 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, ISL848XE

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.

Low Power Shutdown Mode (ISL8483E Only)

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but the ISL8483E

includes a shutdown feature that reduces the already low

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 the ISL8483E

will not enter shutdown.

Note that receiver and driver enable times increase when

the ISL8483E enables from shutdown. Refer to Notes 5-8, 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

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.

Human Body Model Testing

As the name implies, this test method emulates the ESD

event delivered to an IC during human handling. The tester

delivers the charge stored on a 100pF capacitor through a

1.5k

Ω current limiting resistor into the pin under test. The

HBM method determines an ICs ability to withstand the ESD

events typically present during handling and manufacturing.

The RS-485 pin survivability on this high ESD family has

to GND.

ISL8483E, ISL8485E