ISL32470E, ISL32472E, ISL32475E, ISL32478E

11

FN7784.0

January 21, 2011

Application Information

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

standards used for 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 feet; thus,

the wide CMR is necessary to handle ground potential differences, as

well as voltages induced in the cable by external fields.

The ISL32470E, ISL32472E, ISL32475E, ISL32478E is a family of

ruggedized RS-485 transceivers that improves on the RS-485 basic

requirements and therefore increases system reliability. The CMR

increases to ±15V, while the RS-485 bus pins (receiver inputs and

driver outputs) include fault protection against voltages and

transients up to ±60V. Additionally, larger-than-required differential

built-in ESD protection complements the fault protection.

Receiver (Rx) Features

These devices utilize a differential input receiver for maximum noise

immunity and common mode rejection. Input sensitivity is better

than ±200mV, as required by the RS-422 and RS-485 specifications.

Receiver input (load) current surpasses the RS-422 specification of

3mA and is four times lower than the RS-485 “Unit Load (UL)”

requirement of 1mA maximum. Thus, these products are known as

“one-quarter UL” transceivers, and there can be up to 128 of these

devices on a network while still complying with the RS-485 loading

specification.

The Rx functions with common mode voltages as great as ±15V,

making them ideal for industrial or 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), shorted together, or connected to a terminated bus with all

the transmitters disabled (i.e., an idle bus).

Rx outputs feature high drive levels (typically 22mA @ VOL = 1V) to

ease the design of optically coupled isolated interfaces.

FIGURE 7A. TEST CIRCUIT

FIGURE 7B. MEASUREMENT POINTS

FIGURE 7. RECEIVER PROPAGATION DELAY AND DATA RATE

FIGURE 8A. TEST CIRCUIT

FIGURE 8B. MEASUREMENT POINTS

FIGURE 8. RECEIVER ENABLE AND DISABLE TIMES

SIGNAL

GENERATOR

R

RO

RE

A

B

15pF

SIGNAL

GENERATOR

RO

750mV

-750mV

0V

0V

0V

50%

50%

A

B

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

1kΩ

GND

SW

SIGNAL

GENERATOR

R

RO

RE

A

B

15pF

RO

0V

1.5V

1.5V

0V

1.5V

RO

1.5V

RE

OUTPUT LOW

3V

(Note 11)

(Note 11)

(Note 11)

OUTPUT HIGH