LTC485

10

485fi

LTC485 Line Length vs Data Rate

The maximum line length allowable for the RS422/RS485

standard is 4000 feet.

applicaTions inForMaTion

Figures 17 and 18 show that the LTC485 is able to com-

fortably drive 4000 feet of wire at 110kHz.

Figure 14. Line Length Test Circuit

Figure 16. System Differential Voltage at 19.2kHz

Figure 17. System Common Mode Voltage at 110kHz

485 F15

COMMON MODE

VOLTAGE (A + B)/2

RO

DI

485 F16

DIFFERENTIAL

VOLTAGE A – B

RO

DI

485 F17

COMMON MODE

VOLTAGE (A + B)/2

RO

DI

Figure 18. System Differential Voltage at 110kHz

Figure 19. Cable Length vs Maximum Data Rate

485 F18

COMMON MODE

VOLTAGE (A – B)

RO

DI

MAXIMUM DATA RATE

10k

10

100

1k

10k

100k

1M

10M

485 F19

2.5M

Figure 15. System Common Mode Voltage at 19.2kHz

TTL

OUT

LTC485

LTC485

NOISE

GENERATOR

100

C

D

4000 FT 26AWG

TWISTED PAIR

A

B

TTL

IN

485 F14

When specifying line length vs maximum data rate the

curve in Figure 19 should be used.

Using the test circuit in Figure 14, Figures 15 and 16 show

line, The LTC485 is able to reconstruct the data stream at

the end of 4000 feet of twisted pair wire.