Complete, Isolated, 3.3V

RS-485/RS-422 Data Interface

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9

_______________Detailed Description

The MAX3480A/MAX3480B are electrically isolated,

RS-485/RS-422 data-communications interface solu-

tions. Transceivers, optocouplers, a power driver, and a

transformer are in one standard 28-pin DIP package.

Signals and power are internally transported across the

isolation barrier (Figure 1). Power is transferred from the

logic side (non-isolated side) to the isolated side of the

barrier through a center-tapped transformer. Signals

cross the barrier through high-speed optocouplers. A

single +3.3V supply on the logic side powers both

sides of the interface.

The MAX3480B features reduced-slew-rate drivers that

minimize EMI and reduce reflections caused by

improperly terminated cables, allowing error-free trans-

mission at data rates up to 250kbps. The MAX3480A’s

driver slew rates are not limited, allowing transmission

rates up to 2.5Mbps.

The frequency-select FS is connected to GND_ in normal

operation, which selects a switching frequency of

approximately 600kHz. Connect to high for a higher

900kHz switching frequency.

Drivers are short-circuit current limited and are protect-

ed against excessive power dissipation by thermal

Figure 1. Detailed Block Diagram

AC1 (MAKE NO CONNECTION)

AC2 (MAKE NO CONNECTION)

SHIELD (OPTIONAL)

EXTERNAL RS-485/RS-422 WIRING

B

D2

D1

D1, D2

1N914

ISO RO DRV

A

B

A

SH

ISO DI IN

ISO DE IN

SD

FS

GND1

ISO COM1

ISO DI DRV

ISO DE DRV

GND2

DE

DI

ISO COM2

ISO RO LED

RO

R4

R5

R8

100

Ω

BOOSTED V+

C3

0.01uF

R6

200

Ω

R7

R1

R2

DI

´

DE

´

R3

RO

LOGIC

I/O

3.0V TO 3.6V

22µF

0.1µF

C1

C2

MAX3480A/B

LOGIC GROUND

ISOLATION BARRIER

ISOLATION COMMON

MAX485

MAX487

MAX845

1

2

28

27

3

4

5

6

7

8

9

10

11

12

13

14

26

25

24

23

22

21

20

19

18

17

16

15

MAX845

N

MAX3480A: MAX485

MAX3480B: MAX487

RE

ISO RO DRV

ISO DE IN

ISO DI IN

ISO COM1

A

B

R

D

N

Q

Q

TF/F

FS

OSC

1.1MHz/

1.6MHz

SD

GND1

D2

D1

SHIELD (OPTIONAL)

NOTE: RESISTOR R8 PROTECTS THE

MAX3480 FROM TRANSIENT

CURRENTS BETWEEN SHIELD AND

“A” & “B”.

TWISTED PAIR

TO OTHER TRANSCEIVERS

TERMINATING RESISTOR

(ONE RESISTOR ON EACH END)

TWISTED PAIR

TO OTHER TRANSCEIVERS