LTC485I LINER [Linear Technology], LTC485I Datasheet - Page 9

• Manufacturer Part Number: LTC485I
• Description: Low Power RS485 Interface Transceiver
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC485 Line Length vs Data Rate
The maximum line length allowable for the RS422/RS485
standard is 4000 feet.
Using the test circuit in Figure 14, Figures 15 and 16 show
that with ~ 20V
line, The LTC485 is able to reconstruct the data stream at
the end of 4000 feet of twisted pair wire.
TTL
IN
Figure 15. System Common-Mode Voltage at 19.2kHz
VOLTAGE (A + B)/2
LTC485
COMMON-MODE
Figure 16. System Differential Voltage at 19.2kHz
VOLTAGE A – B
DIFFERENTIAL
GENERATOR
A
B
Figure 14. Line Length Test Circuit
NOISE
RO
RO
P-P
DI
DI
common-mode noise injected on the
U U
4000 FT 26AWG
TWISTED PAIR
W
C
100
D
LTC485
LTC485 • F16
LTC485 • F15
U
TTL
OUT
Figures 17 and 18 show that the LTC485 is able to
comfortably drive 4000 feet of wire at 110kHz.
When specifying line length vs maximum data rate the
curve in Figure 19 should be used:
VOLTAGE (A + B)/2
Figure 17. System Common-Mode Voltage at 110kHz
VOLTAGE (A – B)
COMMON-MODE
COMMON-MODE
Figure 18. System Differential Voltage at 110kHz
Figure 19. Cable Length vs Maximum Data Rate
100
10k
1k
10
10k
RO
RO
DI
DI
MAXIMUM DATA RATE
100k
1M
2.5M
LTC485 • F19
10M
LTC485
LTC485 • F17
LTC485 • F18
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