Complete, Isolated, 3.3V

RS-485/RS-422 Data Interface

10

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shutdown circuitry that puts the driver outputs into a

high-impedance state. The receiver input has a fail-safe

feature that guarantees a logic-high output if the input

is open circuit.

The driver outputs are enabled by bringing DE

Driver-enable times are typically 200ns for the

MAX3480A and 50µs for the MAX3480B. Allow time for

the devices to be enabled before sending data. When

enabled, driver outputs function as line drivers. Driver

outputs are high impedance when DE

outputs are high impedance, they function as line

receivers.

The MAX3480A/MAX3480B typically withstand

lated outputs of these devices meet all RS-485/RS-422

specifications. The logic inputs can be driven from any

TTL/CMOS-logic family with a series resistor, and the

received data output can directly drive any of the

TTL/CMOS-logic families with only a resistive pull-up.

Boost Voltage

The MAX3480 requires external diodes on the primary

of the transformer to develop the boost voltage for the

power oscillator. In normal operation, whenever one of

the oscillator outputs (D1 and D2) goes low, the other

Figure 2. Typical Application Circuit

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

R1

R2

16

14

12

5

7

13

15

18

3

8

2

DI

DE

RO

17

20

9

10

11

RECEIVER OUTPUT

6

4

R8

100

C3

0.01µF

R6

200

Ω

R7

R3

BOOSTED V+

74HC240

DRIVER INPUT

DRIVER ENABLE

3.0V TO 3.6V

22µF

6V

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

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

Table 1. Pull-Up and LED Drive Resistors

PART

R1 (

Ω)

R2 (

Ω)

R3 (

Ω)

R4 (

Ω)

R5 (

Ω)

R6 (

Ω)

MAX3480A

150

130

680

3000

1000

200

MAX3480B

130

330

2000

2200

3000

200

R7 (

Ω)

Open

470