MAX3465–MAX3469
+5V, Fail-Safe, 40Mbps, Profibus RS-485/
RS-422 Transceivers
11
Applications Information
128 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12kΩ
(one unit load), and the standard driver can drive up to
32 unit loads. The MAX3465–MAX3469 family of trans-
ceivers has a 1/4-unit-load receiver input impedance
(48kΩ), allowing up to 128 transceivers to be connect-
ed in parallel on one communication line. Any combina-
tion of these devices and/or other RS-485 transceivers
with a total of 32 unit loads or less can be connected to
the line.
Low-Power Shutdown Mode
(Except MAX3467)
Low-power shutdown mode is initiated by bringing
SHDN high (MAX3465/MAX3466), or both RE high and
DE low. In shutdown, the devices typically draw only
1µA of supply current. RE and DE can be driven simul-
taneously; the devices are guaranteed not to enter shut-
down if RE is high and DE is low for less than 50ns. If
the inputs are in this state for at least 800ns, the devices
are guaranteed to enter shutdown.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or by bus con-
tention. The first, a foldback current limit on the output
stage, provides immediate protection against short cir-
cuits over the whole common-mode voltage range (see
the
Typical Operating Characteristics
). The second, a
thermal-shutdown circuit, forces the driver outputs into
a high-impedance state if the die temperature exceeds
+140°C.
Propagation Delay
Many digital encoding schemes depend on the difference
between the driver and receiver propagation delay times.
Typical propagation delays are shown in the
Typical
Operating Characteristics
. The difference in receiver delay
times, |t
PLH
- t
PHL
|, is a maximum of 2ns. The driver skew
time |t
PLH
- t
PHL
| is also a maximum of 2ns.
Typical Applications
The MAX3465–MAX3469 transceivers are designed for
bidirectional data communications on multipoint bus
transmission lines. Figures 13 and 14 show typical net-
work applications circuits. To minimize reflections, the
line should be terminated at both ends in its character-
istic impedance, and stub lengths off the main line
should be kept as short as possible.
Profibus Termination
The MAX3465–MAX3469 are designed for driving
Profibus termination networks. With a worst-case load-
ing of two termination networks with 220Ω termination
impedance and 390Ω pullups and pulldowns, the dri-
vers can drive V
A-B
> 2.1V output.
Chip Information
PROCESS: BiCMOS
