MAX3080E–MAX3089E
Applications Information
256 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 MAX3080E family of transceivers
have a 1/8-unit-load receiver input impedance (96kΩ),
allowing up to 256 transceivers to be connected in par-
allel on one communication line. Any combination of
these devices and/or other RS-485 transceivers with a
total of 32 unit loads or less can be connected to the
line.
Reduced EMI and Reflections
The MAX3080E–MAX3085E, and MAX3089E with SRL =
V
CC
or unconnected, are slew-rate limited, minimizing
EMI and reducing reflections caused by improperly ter-
minated cables. Figure 15 shows the driver output
waveform and its Fourier analysis of a 20kHz signal
transmitted by a MAX3086E/MAX3087E/MAX3088E,
and MAX3089E with SRL = GND. High-frequency har-
monic components with large amplitudes are evident.
Figure 16 shows the same signal displayed for a
MAX3083E/MAX3084E/MAX3085E, and MAX3089E with
SRL = V
CC, transmitting under the same conditions.
Figure 16’s high-frequency harmonic components are
much lower in amplitude, compared with Figure 15’s,
and the potential for EMI is significantly reduced. Figure
17 shows the same signal displayed for a MAX3080E/
MAX3081E/MAX3082E, and MAX3089E with SRL =
unconnected, transmitting under the same conditions.
Figure 17’s high-frequency harmonic components are
even lower.
In general, a transmitter’s rise time relates directly to the
length of an unterminated stub, which can be driven with
only minor waveform reflections. The following equation
expresses this relationship conservatively:
Length = t
RISE
/ (10 x 1.5ns/ft)
where t
RISE
is the transmitter’s rise time.
For example, the MAX3080E’s rise time is typically
1320ns, which results in excellent waveforms with a stub
length up to 90 feet. A system can work well with longer
unterminated stubs, even with severe reflections, if the
waveform settles out before the UART samples them.
Low-Power Shutdown Mode
(Except MAX3082E/MAX3085E/MAX3088E)
Low-power shutdown mode is initiated by bringing both
RE high and DE low. In shutdown, the devices typically
draw only 1nA of supply current.
RE and DE may be driven simultaneously; the parts are
guaranteed not to enter shutdown if RE is high and DE
is low for less than 50ns. If the inputs are in this state
for at least 600ns, the parts are guaranteed to enter
shutdown.
Enable times t
ZH
and t
ZL
in the
Switching Char-
acteristics
tables assume the part was not in a low-
power shutdown state. Enable times t
ZH(SHDN)
and
t
ZL(SHDN)
assume the parts were shut down. It takes
drivers and receivers longer to become enabled from
low-power shutdown mode (t
ZH(SHDN)
, t
ZL(SHDN)
) than
from driver/receiver-disable mode (t
ZH
, t
ZL
).
±15kV ESD-Protected, Fail-Safe, High-Speed (10Mbps),
Slew-Rate-Limited RS-485/RS-422 Transceivers
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