3.3V-Powered, 10Mbps and Slew-Rate-Limited
True RS-485/RS-422 Transceivers
Driver Output Protection
Excessive output current and power dissipation caused
by faults or by bus contention are prevented by two
mechanisms. A foldback current limit on the output stage
provides immediate protection against short circuits over
the whole common-mode voltage range (see
Typical
Operating Characteristics
). In addition, a thermal shut-
down circuit forces the driver outputs into a high-imped-
ance state if the die temperature rises excessively.
Propagation Delay
Figures 15–18 show the typical propagation delays. Skew
time is simply the difference between the low-to-high and
high-to-low propagation delay. Small driver/receiver
skew times help maintain a symmetrical mark-space
ratio (50% duty cycle).
The receiver skew time,
|
t
PRLH
- t
PRHL|
, is under 10ns
(20ns for the MAX3483/MAX3488). The driver skew
times are 8ns for the MAX3485/MAX3490/MAX3491,
11ns for the MAX3486, and typically under 100ns for
the MAX3483/MAX3488.
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 parts driving 4000 feet of 26AWG twisted-pair wire
at 125kHz into 120Ω loads.
Typical Applications
The MAX3483, MAX3485, MAX3486, MAX3488,
MAX3490, and MAX3491 transceivers are designed for
bidirectional data communications on multipoint bus
transmission lines. Figures 21 and 22 show typical net-
work 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
possible. The slew-rate-limited MAX3483/MAX3488
and the partially slew-rate-limited MAX3486 are more
tolerant of imperfect termination.
DI RO DE
RE
A
B
RE
RERE
RO
RO
RO
DI
DI
DI
DE
DE
DE
D
D
D
R
R
R
B
B
B
A
A
A
120Ω
120Ω
D
R
MAX3483
MAX3485
MAX3486
Figure 21. MAX3483/MAX3485/MAX3486 Typical RS-485 Network
MAX3483/MAX3485/MAX3486/
MAX3488/MAX3490/MAX3491
