ISL3170E, ISL3171E, ISL3172E, ISL3173E, ISL3174E, ISL3175E, ISL3176E, ISL3177E, ISL3178E

FN6307 Rev.6.00

Page 13 of 21

Aug 31, 2017

AIR-GAP DISCHARGE TEST METHOD

For this test method, a charged probe tip moves toward the

IC pin until the voltage arcs to it. The current waveform

delivered to the IC pin depends on approach speed,

humidity, temperature, etc., so it is difficult to obtain

repeatable results. The ISL317XE RS-485 pins withstand

±15kV air-gap discharges.

CONTACT DISCHARGE TEST METHOD

During the contact discharge test, the probe contacts the

tested pin before the probe tip is energized, thereby

eliminating the variables associated with the air-gap

discharge. The result is a more repeatable and predictable

test, but equipment limits prevent testing devices at voltages

higher than

±8kV. The ISL317XE survive ±8kV contact

discharges on the RS-485 pins.

Data Rate, Cables, and Terminations

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

but the maximum system data rate decreases as the

transmission length increases. Devices operating at 20Mbps

are limited to lengths less than 100ft, 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 do not need to 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 120Ω) at the end farthest

from the driver. In multi-receiver applications, keep stubs

connecting receivers to the main cable as short as possible.

Multipoint (multi-driver) systems require that the main cable

be terminated 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

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 use 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°. 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

600ns. Disabling both the driver and the receiver for less

than 50ns guarantees that the transceiver will not enter

shutdown.

Note that receiver and driver enable times increase when the

transceiver enables from shutdown. Refer to Notes 9 through

13 at the end of the “Electrical Specifications table” on page 9,

for more information.