Complete, Isolated RS-485/RS-422

Data Interface

______________________________________________________________________________________

13

Under these conditions, the MAX1480B/MAX1480C/

MAX1490B supply current is reduced to as low as 0.2µA.

The high-speed optocouplers on the MAX1480A/

MAX1480C/MAX1490A consume an additional 10mA

completely shut down these devices, use an external P-

channel MOSFET as shown in Figure 3. In normal opera-

tion, SD is low, turning the MOSFET on and thereby pro-

the power oscillator is disabled and the switch is turned

off, disconnecting power from the DI and DE optocou-

plers. In normal operating mode, the switch carries only

the optocoupler currents, so an on-resistance of several

ohms will not significantly degrade efficiency.

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

22

µF

0.1

µF

C1

C2

MAX1490A/B

LOGIC GROUND

ISOLATION BARRIER

ISOLATED COMMON

DRIVER INPUT

RECEIVER OUTPUT

DI

RO

74HC86

OR EQUIVALENT

MAX845

N

MAX1490A: MAX490

MAX1490B: MAX488

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 MAX1490A/B 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

Figure 2. MAX1490A/MAX1490B Detailed Block Diagram and Typical Application Circuit

Table 2. Pull-Up and LED Drive Resistors

1000

3000

R2 (

Ω)

1000

3000

R4 (

Ω)

330

330

R3 (

Ω)

200

MAX1490A

200

MAX1490B

R1 (

Ω)

PART