±15kV ESD-Protected, Isolated RS-485/RS-422

Data Interfaces

______________________________________________________________________________________

11

MAX1480EC/MAX1490EB:

Reduced EMI and Reflections

The MAX1480EC/MAX1490EB are slew-rate-limited,

minimizing EMI and reducing reflections caused by

improperly terminated cables. Figure 11 shows both

the driver output waveform of a MAX1480EA/

MAX1490EA transmitting a 150kHz signal and the

Fourier analysis of that waveform. High-frequency har-

monics with large amplitudes are evident. Figure 12

shows the same information for the slew-rate-limited

MAX1480EC/MAX1490EB transmitting the same signal.

The high-frequency harmonics have much lower ampli-

tudes, and therefore the potential for EMI is significantly

reduced.

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-

SHIELD (OPTIONAL)

EXTERNAL RS-485/RS-422 WIRING

A

B

Z

Y

R3*

R4*

R5, 100

Ω

SH1

SH2

R1*

R2*

R6, 100

Ω

DI

´

+5V

C1

22

µF

C2

0.1

µF

LOGIC GROUND

ISOLATION BARRIER

ISOLATED COMMON

DRIVER INPUT

RECEIVER OUTPUT

DI

RO

74HC86

OR EQUIVALENT

MAX845

N

MAX1490EA: MAX490E

MAX1490EB: MAX488E

ISO DI DRV

ISO RO DRV

A

B

Z

Y

D

R

N

Q

Q

T F/F

FS

OSC

1.07MHz/

1.45MHz

SD

GND1

D2

D1

SHIELD (OPTIONAL)

NOTE: RESISTORS R5 AND R6 PROTECT

THE MAX1490EA/MAX1490EB FROM

TRANSIENT CURRENTS BETWEEN SHIELD AND

TRANSMISSION LINES.

TWISTED PAIR

TO OTHER TRANSCEIVERS

TERMINATING RESISTOR

(ONE RESISTOR ON EACH END)

TWISTED PAIR

TO OTHER TRANSCEIVERS

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

R0

DI

MAX1490EA/

MAX1490EB

*SEE TABLE 2.

270 pF

4kV

Figure 2. MAX1490EA/MAX1490EB Detailed Block Diagram and Typical Application Circuit

Table 2. Pullup and LED Drive Resistors for Figure 2

MAX1490EB

200

3000

330

3000

1000

330

1000

200

MAX1490EA

R4 (

Ω)

R3 (

Ω)

R2 (

Ω)

R1 (

Ω)

PART