ISL8485IPZ datasheet(10/18 Pages) RENESAS

10 / 18 page

ISL8483, ISL8485, ISL8488, ISL8489, ISL8490, ISL8491

FN6046 Rev 9.00

Page 10 of 18

February 16, 2016

Application Information

RS-485 and RS-422 are differential (balanced) data

transmission standards for use in long haul or noisy

environments. RS-422 is a subset of RS-485, so RS-485

transceivers are also RS-422 compliant. RS-422 is a point-

to-multipoint (multidrop) standard, which allows only one

driver and up to 10 (assuming one unit load devices)

receivers on each bus. RS-485 is a true multipoint standard,

which allows up to 32 one unit load devices (any

combination of drivers and receivers) on each bus. To allow

for multipoint operation, the RS-485 spec requires that

drivers must handle bus contention without sustaining any

damage.

Another important advantage of RS-485 is the extended

common mode range (CMR), which specifies that the driver

outputs and receiver inputs withstand signals that range

from +12V to -7V. RS-422 and RS-485 are intended for runs

as long as 4000’, so the wide CMR is necessary to handle

ground potential differences, as well as voltages induced in

the cable by external fields.

Receiver Features

These devices utilize a differential input receiver for maximum

noise immunity and common mode rejection. Input sensitivity

200mV, as required by the RS422 and RS-485

specifications.

Receiver input impedance surpasses the RS-422 spec of

, and meets the RS-485 “Unit Load” requirement of

12k

 minimum.

Receiver inputs function with common mode voltages as

great as

7V outside the power supplies (i.e., +12V and

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

voltages are a realistic concern.

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.

ISL8483/85/89/91 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 1.5Vacross a 54

 load (RS-485), and at

least 2V across a 100

 load (RS-422). The drivers feature

low propagation delay skew to maximize bit width, and to

minimize EMI.

Drivers of the ISL8483/85/89/91 are three-statable via the

active high DE input.

The ISL8483/88/89 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

ISL8485/90/91 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 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

) 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

ISL84XX 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, ISL84XX

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 (ISL8483 Only)

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but the ISL8483

includes a shutdown feature that reduces the already low

quiescent ICC to a 1nA trickle. The ISL8483 enters shutdown

whenever the receiver and driver are simultaneously

disabled (RE =VCC and DE = GND) for a period of at least

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

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