11

FN6085.9

April 22, 2008

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

is ±200mV, as required by the RS-422 and RS-485

specifications.

Receiver input resistance of 96k

Ω surpasses the RS-422

specification of 4k

Ω, and is eight times the RS-485 “Unit

Load (UL)” requirement of 12k

Ω minimum. Thus, these

products are known as “one-eighth UL” transceivers, and

there can be up to 256 of these devices on a network while

still complying with the RS-485 loading specification.

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 “full fail-safe” function that

guarantees a high level receiver output if the receiver inputs

are unconnected (floating) or shorted.

Receivers easily meet the data rates supported by the

corresponding driver, and all receiver outputs are

three-statable via the active low RE input (except for the

ISL83084E).

Driver Features

The RS-485/RS-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 three-statable via the active high DE input

(except for the ISL83084E).

The 115kbps and 500kbps driver outputs are slew rate

limited to minimize EMI, and to minimize reflections in

unterminated or improperly terminated networks. Outputs of

the ISL83086E, ISL83088E drivers are not limited, so faster

output transition times allow data rates of at least 10Mbps.

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 ISL83080,

ISL83082, ISL83083, ISL83085 versions incorporate a “Hot

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

than ~3.4V. 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 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/RS-422 are intended for network lengths up to

4000’, but the maximum system data rate decreases as the

transmission length increases. Devices operating at 10Mbps

are limited to lengths less than 100’, while the 115kbps

versions can operate at full data rates with lengths of several

1000’.

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 receivers in these ICs.

Proper termination is imperative (when using the 10Mbps

devices) to minimize reflections. Short networks using the

FIGURE 7.

HOT PLUG PERFORMANCE (ISL83080E) vs

DEVICE WITHOUT HOT PLUG CIRCUITRY

(ISL83086E)

TIME (40µs/DIV)

2.5

5.0

2.5

5.0

RO

0

2.5

5.0

0

0

A

/Y

ISL83080E

ISL83080E

3.2V

3.4V

ISL83080E, ISL83082E, ISL83083E, ISL83084E, ISL83085E, ISL83086E, ISL83088E