LTC485 Ver la hoja de datos (PDF) - Linear Technology
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LTC485 Datasheet PDF : 14 Pages
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LTC485
Applications Information
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 com-
fortably 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
485 F14
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.
485 F17
Figure 17. System Common Mode Voltage at 110kHz
RO
COMMON MODE
VOLTAGE (A – B)
RO
COMMON MODE
VOLTAGE (A + B)/2
DI
485 F15
Figure 15. System Common Mode Voltage at 19.2kHz
DI
485 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
485 F16
Figure 16. System Differential Voltage at 19.2kHz
10
1k
100
10
10k
100k
1M 2.5M 10M
MAXIMUM DATA RATE
485 F19
Figure 19. Cable Length vs Maximum Data Rate
485fi
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