Line Length vs. Data Rate
The RS-485/RS-422 standard covers line lengths up to
4000 feet. For line lengths greater than 4000 feet, see
Figure 23.
Figures 19 and 20 show the system differential voltage
for the parts driving 4000 feet of 26AWG twisted-pair
wire at 110kHz into 120Ω loads.
Typical Applications
The HWD481, HWD483, HWD485, HWD487–HWD491, and
HWD1487 transceivers are designed for bidirectional data
communications on multipoint bus transmission lines.
Figures 21 and 22 show typical network applications
circuits. These parts can also be used as line
repeaters, with cable lengths longer than 4000 feet, as
shown in Figure 23.
To minimize reflections, the line should be terminated at
both ends in its characteristic impedance, and stub
lengths off the main line should be kept as short as possi-
ble. The slew-rate-limited HWD483 and HWD487–HWD489
are more tolerant of imperfect termination.
Low-Power, Slew-Rate-Limited
RS-485/RS-422 Transceivers
DI
VY-VZ
RO
5V
0V
1V
0V
-1V
5V
0V
2µs/div
DI
VY-VZ
RO
5V
0V
1V
0V
-1V
5V
0V
2µs/div
DI
RO
DE
RE
A
B
RE
RE
RE
RO
RO
RO
DI
DI
DI
DE
DE
DE
D
D
D
R
R
R
BB
B
A
A
A
120Ω
120Ω
D
R
HWD481
HWD483
HWD485
HWD487
HWD1487
Figure 19. HWD481/HWD485/HWD490/HWD491/HWD1487 System
Differential Voltage at 110kHz Driving 4000ft of Cable
Figure 20. HWD483, HWD487–HWD489 System Differential
Voltage at 110kHz Driving 4000ft of Cable
Figure 21. HWD481/HWD483/HWD485/HWD487/HWD1487 Typical Half-Duplex RS-485 Network
13
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