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FN6115.1

August 5, 2005

-7V), making them ideal for long networks where induced

voltages, and ground potential differences, are realistic

concerns.

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

guarantees a high level receiver output if the receiver inputs

are unconnected (floating) or shorted. Fail-safe with shorted

inputs is achieved by setting the Rx upper switching point to

-50mV, thereby ensuring that the Rx sees 0V differential as a

high input level.

Receivers easily meet the data rates supported by the

corresponding driver, and all receiver outputs - except on the

ISL83071E and ISL83077E- are tri-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.

All drivers are tri-statable via the active high DE input, except

on the ISL83071E and ISL83077E.

The 250kbps and 500kbps driver outputs are slew rate

limited to minimize EMI, and to reduce reflections in

unterminated or improperly terminated networks. Outputs of

the ISL83076E, ISL83078E drivers are not limited, so faster

output transition times allow data rates of at least 20Mbps.

Hot Plug Function

When a piece of equipment powers up, there is a period of

time where the processor or ASIC driving the RS-485 control

lines (DE, RE) is unable to ensure that the RS-485 Tx and

Rx outputs are kept disabled. If the equipment is connected

to the bus, a driver activating prematurely during power up

may crash the bus. To avoid this scenario, the ISL8307XE

family incorporates a “Hot Plug” function. During power up,

remain disabled for a period of time, regardless of the state of

DE and RE. This gives the processor/ASIC a chance to

stabilize and drive the RS-485 control lines to the proper states.

ESD Protection

All pins on these devices include class 3 (>7kV) 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.

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 20Mbps

are limited to lengths less than 100’, while the 250kbps

versions can operate at full data rates with lengths of several

thousand feet.

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 20Mbps

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. 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, 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

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but some also include

a shutdown feature that reduces the already low quiescent

whenever the receiver and driver are simultaneously

ISL83070E, ISL83071E, ISL83072E, ISL83073E, ISL83075E, ISL83076E, ISL83077E, ISL83078E