LTC485C(RevE) 데이터 시트보기 (PDF) - Linear Technology

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LTC485C
(Rev.:RevE)

Low Power RS485 Interface Transceiver

Linear Technology 

UU W U

APPLICATIO S I FOR ATIO

LTC485 Line Length vs Data Rate

The maximum line length allowable for the RS422/RS485

standard is 4000 feet.

LTC485

Figures 17 and 18 show that the LTC485 is able to

comfortably drive 4000 feet of wire at 110kHz.

A

LTC485

B

NOISE

TTL GENERATOR

IN

100Ω

C

D

LTC485

TTL

OUT

4000 FT 26AWG

TWISTED PAIR

RO

COMMON-MODE

VOLTAGE (A + B)/2

DI

Figure 14. Line Length Test Circuit

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

that with ~ 20VP-P common-mode noise injected on the

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

the end of 4000 feet of twisted pair wire.

RO

LTC485 • F17

Figure 17. System Common-Mode Voltage at 110kHz

RO

COMMON-MODE

VOLTAGE (A – B)

DI

COMMON-MODE

VOLTAGE (A + B)/2

DI

LTC485 • F15

Figure 15. System Common-Mode Voltage at 19.2kHz

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:

10k

RO

DIFFERENTIAL

VOLTAGE A – B

DI

LTC485 • F16

Figure 16. System Differential Voltage at 19.2kHz

1k

100

10

10k

100k

1M 2.5M 10M

MAXIMUM DATA RATE

LTC485 • F19

Figure 19. Cable Length vs Maximum Data Rate

9

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