9

LTC485

APPLICATIO S I FOR ATIO

LTC485 Line Length vs Data Rate

The maximum line length allowable for the RS422/RS485

standard is 4000 feet.

Figures 17 and 18 show that the LTC485 is able to

comfortably drive 4000 feet of wire at 110kHz.

Figure 14. Line Length Test Circuit

TTL

OUT

LTC485

LTC485

NOISE

GENERATOR

100

Ω

C

D

4000 FT 26AWG

TWISTED PAIR

A

B

TTL

IN

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.

RO

DIFFERENTIAL

VOLTAGE A – B

DI

LTC485 • F16

Figure 16. System Differential Voltage at 19.2kHz

Figure 15. System Common-Mode Voltage at 19.2kHz

COMMON-MODE

VOLTAGE (A + B)/2

RO

DI

LTC485 • F15

Figure 17. System Common-Mode Voltage at 110kHz

RO

COMMON-MODE

VOLTAGE (A + B)/2

DI

LTC485 • F17

RO

COMMON-MODE

VOLTAGE (A – B)

DI

LTC485 • F18

Figure 18. System Differential Voltage at 110kHz

When specifying line length vs maximum data rate the

curve in Figure 19 should be used:

Figure 19. Cable Length vs Maximum Data Rate

MAXIMUM DATA RATE

10k

10

100

1k

10k

100k

1M

10M

LTC485 • F19

2.5M