ISL3179E, ISL3180E

FN6365 Rev. 6.00

Page 13 of 18

Nov 9, 2017

power-up and power-down, the Tx and Rx outputs remain disabled,

gives the processor/ASIC a chance to stabilize and drive the RS-485

control lines to the proper states.

Data Rate, Cables, and Terminations

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

but the maximum system data rate decreases as the

transmission length increases. Devices operating at 40Mbps are

limited to lengths less than 100ft.

Twisted pair is the cable of choice for RS-485/RS-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 receiver in this IC.

Proper termination is imperative to minimize reflections. 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

multireceiver applications, stubs connecting receivers to the

main cable should be kept as short as possible. Multipoint

(multidriver) 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.

The ISL3179E and ISL3180E may also be used at slower data

rates over longer cables, but some limitations apply. The Rx is

optimized for high speed operation, so its output may glitch if the

Rx input differential transition times are too slow. Keeping the

transition times below 500ns, which equates to the Tx driving a

1000ft (305m) CAT 5 cable, yields excellent performance over

the full operating temperature range.

Built-in Driver Overload Protection

As stated previously, the RS-485 specification requires that

drivers survive worst case bus contentions undamaged. These

transmitters meet this requirement using 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 specification, even at the common-mode voltage range

extremes. In the event of a major short-circuit condition, the device

also includes 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°C. 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

The BiCMOS transceivers use a fraction of the power required by

their bipolar counterparts, but they also include a shutdown

trickle. The devices enter shutdown whenever the receiver and

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

receiver for less than 60ns 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 12, 13, 14,

15 and 16 at the end of “Electrical Specifications” on page 5 for

more information.

FIGURE 21. HOT PLUG PERFORMANCE (ISL3179E) vs ISL83485

WITHOUT HOT PLUG CIRCUITRY

TIME (40µs/DIV)

2

4

2

4

RO

0

2

4

0

0

A

/Y

2.3V

2.5V

ISL3179E

RE = GND

ISL3179E