11

FN6085.8

February 27, 2008

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 specification 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

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.

FIGURE 6A. TEST CIRCUIT

FIGURE 6B. MEASUREMENT POINTS

FIGURE 6. RECEIVER ENABLE AND DISABLE TIMES (DOES NOT APPLY TO THE ISL83084E)

1k

Ω

GND

SW

PARAMETER

DE

A

SW

0

+1.5V

GND

0

-1.5V

0

+1.5V

GND

0

-1.5V

0

+1.5V

GND

0

-1.5V

SIGNAL

GENERATOR

R

RO

RE

A

B

GND

15pF

RO

3V

0V

1.5V

1.5V

0V

1.5V

RO

1.5V

RE

OUTPUT HIGH

OUTPUT LOW

NOTE 9

NOTES 9, 11

NOTES 9, 11

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