Complete, Isolated RS-485/RS-422

Data Interface

18

______________________________________________________________________________________

Driver Output Protection

There are two mechanisms to prevent excessive output

current and power dissipation caused by faults or by

bus contention. A foldback current limit on the output

stage provides immediate protection against short cir-

cuits over the entire common-mode range (see the

Typical Operating Characteristics). In addition, a ther-

mal shutdown circuit forces the driver outputs into a

high-impedance state if the die temperature rises

excessively.

Propagation Delay Skew

Typical propagation delays are shown in the Typical

Operating Characteristics using the test circuit of

Figure 5. Propagation delay skew is simply the differ-

ence between the low-to-high and high-to-low propaga-

tion delay. Small driver/receiver skew times help

reduce EMI and reflections by maintaining balanced

differential signals.

The receiver skew time,

100ns for the MAX1480A/MAX1490A and under 1µs for

the MAX1480B/MAX1480C/MAX1490B.

The driver skew time is typically 25ns for the

MAX1480A/MAX1490A and 100ns for the MAX1480B/

MAX1480C/MAX1490B.

___________Applications Information

DI and DE are intended to be driven through a

series current-limiting resistor. Directly grounding

these pins destroys the device.

The MAX1480A/MAX1480B/MAX1480C are designed

for bidirectional data communications on multipoint

bus-transmission lines. The MAX1490A/MAX1490B are

designed for full-duplex bidirectional communications

that are primarily point-to-point. Figures 13 and 14

show half-duplex and full-duplex typical network appli-

cation circuits, respectively. To minimize reflections,

terminate the line at both ends with its characteristic

impedance, and keep stub lengths off the main line as

short as possible. The slew-rate-limited MAX1480B/

MAX1480C/MAX1490B are more tolerant of imperfect

termination and stubs off the main line.

Layout Considerations

The MAX1480A/MAX1480B/MAX1480C/MAX1490A/

MAX1490B pinouts enable optimal PC board layout by

minimizing interconnect lengths and crossovers.

• For maximum isolation, the “isolation barrier” should

not be breached except by the MAX1480A/

MAX1480B/MAX1480C/MAX1490A/MAX1490B.

SHIELD (OPTIONAL)

NOTE: RESISTORS R5 AND R6 PROTECT

THE MAX1490A/B FROM TRANSIENT

CURRENTS BETWEEN SHIELD AND

TRANSMISSION LINES.

120Ω

120Ω

120Ω

A

B

Z

Y

Y

Z

R3

R4

R5, 100Ω

SH1

SH2

R1

R2

R6, 100Ω

DI

´

5V

22µF

0.1µF

C1

C2

MAX1490A/B

LOGIC GROUND

ISOLATION BARRIER

ISOLATED COMMON

DRIVER INPUT

RECEIVER OUTPUT

DI

RO

74HC86

OR EQUIVALENT

SHIELD (OPTIONAL)

24

23

22

21

20

19

18

17

1

2

3

4

5

6

7

8

AC1 (MAKE NO CONNECTION)

AC2 (MAKE NO CONNECTION)

ISO RO DRV

D2

D1

A

B

Z

Y

SD

FS

GND1

16

15

14

13

9

10

11

12

ISO COM1

ISO DI DRV

ISO RO LED

GND2

RO

DI

D

DI

RO

120Ω

B

A

R

Figure 14. Typical Full-Duplex RS-485/RS-422 Network