Line Length vs. Data Rate

The RS-485/RS-422 standard covers line lengths up to

4000 feet. For line lengths greater than 4000 feet, see

Figure 23.

Figures 19 and 20 show the system differential voltage

for the parts driving 4000 feet of 26AWG twisted-pair

wire at 110kHz into 120

Ω

loads.

Typical Applications

The MAX481, MAX483, MAX485, MAX487–MAX491, and

MAX1487 transceivers are designed for bidirectional data

communications on multipoint bus transmission lines.

Figures 21 and 22 show typical network applications

circuits. These parts can also be used as line

repeaters, with cable lengths longer than 4000 feet, as

shown in Figure 23.

To minimize reflections, the line should be terminated at

both ends in its characteristic impedance, and stub

lengths off the main line should be kept as short as possi-

ble. The slew-rate-limited MAX483 and MAX487–MAX489

are more tolerant of imperfect termination.

Low-Power, Slew-Rate-Limited

RS-485/RS-422 Transceivers

______________________________________________________________________________________ 13

DI

RO

5V

0V

1V

0V

-1V

5V

0V

2µs/div

DI

RO

5V

0V

1V

0V

-1V

5V

0V

2µs/div

DI

RO

DE

RE

A

B

RE

RE

RE

RO

RO

RO

DI

DI

DI

DE

DE

DE

D

D

D

R

R

R

B

B

B

A

A

A

120

Ω

120

Ω

D

R

MAX481

MAX483

MAX485

MAX487

MAX1487

Figure 19. MAX481/MAX485/MAX490/MAX491/MAX1487

System Differential Voltage at 110kHz Driving 4000ft of Cable

Figure 20. MAX483, MAX487–MAX489 System Differential

Voltage at 110kHz Driving 4000ft of Cable

Figure 21. MAX481/MAX483/MAX485/MAX487/MAX1487 Typical Half-Duplex RS-485 Network

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