MAX3080E–MAX3089E
Detailed Description
The MAX3080E–MAX3089E high-speed transceivers for
RS-485/RS-422 communication contain one driver and
one receiver. These devices feature fail-safe circuitry,
which guarantees a logic-high receiver output when the
receiver inputs are open or shorted, or when they are
connected to a terminated transmission line with all
drivers disabled (see the
Fail-Safe
section). The
MAX3080E/MAX3081E/MAX3082E feature reduced
slew-rate drivers that minimize EMI and reduce reflec-
tions caused by improperly terminated cables, allowing
error-free data transmission up to 115kbps (see the
Reduced EMI and Reflections
section). The
MAX3083E/MAX3084E/MAX3085E offer higher driver
output slew-rate limits, allowing transmit speeds up to
500kbps. The MAX3086E/MAX3087E/MAX3088E’s dri-
ver slew rates are not limited, making transmit speeds
up to 10Mbps possible. The MAX3089E’s slew rate is
selectable between 115kbps, 500kbps, and 10Mbps
by driving a selector pin with a three-state driver.
The MAX3082E/MAX3085E/MAX3088E are half-duplex
transceivers, while the MAX3080E/MAX3081E/
MAX3083E/MAX3084E/MAX3086E/MAX3087E are full-
duplex transceivers. The MAX3089E is selectable
between half- and full-duplex communication by driving
a selector pin high or low, respectively.
All of these parts operate from a single +5V supply.
Drivers are output short-circuit current limited. Thermal
shutdown circuitry protects drivers against excessive
power dissipation. When activated, the thermal shut-
down circuitry places the driver outputs into a high-
impedance state.
Receiver Input Filtering
The receivers of the MAX3080E–MAX3085E, and the
MAX3089E when operating in 115kbps or 500kbps
mode, incorporate input filtering in addition to input
hysteresis. This filtering enhances noise immunity with
differential signals that have very slow rise and fall
times. Receiver propagation delay increases by 20%
due to this filtering.
Fail-Safe
The MAX3080E family guarantees a logic-high receiver
output when the receiver inputs are shorted or open, or
when they are connected to a terminated transmission
line with all drivers disabled. This is done by setting the
receiver threshold between -50mV and -200mV. If the
differential receiver input voltage (A - B) is greater than
or equal to -50mV, RO is logic high. If A - B is less than
or equal to -200mV, RO is logic low. In the case of a
terminated bus with all transmitters disabled, the
receiver’s differential input voltage is pulled to 0V by
the termination. With the receiver thresholds of the
MAX3080E family, this results in a logic high with a
50mV minimum noise margin. Unlike previous fail-safe
devices, the -50mV to -200mV threshold complies with
the ±200mV EIA/TIA-485 standard.
MAX3089E Programming
The MAX3089E has several programmable operating
modes. Transmitter rise and fall times are programma-
ble between 2500ns, 750ns, and 25ns, resulting in
maximum data rates of 115kbps, 500kbps, and
10Mbps, respectively. To select the desired data rate,
drive SRL to one of three possible states by using a
three-state driver, by connecting it to V
CC
or GND, or
by leaving it unconnected. For 115kbps operation, set
the three-state device in high-impedance mode or
leave SRL unconnected. For 500kbps operation, drive
SRL high or connect it to V
CC
. For 10Mbps operation,
drive SRL low or connect it to GND. SRL can be
changed during operation without interrupting data
communications.
Occasionally, twisted-pair lines are connected back-
ward from normal orientation. The MAX3089E has two
pins that invert the phase of the driver and the receiver
to correct for this problem. For normal operation, drive
TXP and RXP low, connect them to ground, or leave
them unconnected (internal pulldown). To invert the dri-
ver phase, drive TXP high or connect it to V
CC
. To
invert the receiver phase, drive RXP high or connect it
to V
CC
. Note that the receiver threshold is positive
when RXP is high.
The MAX3089E can operate in full- or half-duplex
mode. Drive the H/F pin low, leave it unconnected
(internal pulldown), or connect it to GND for full-duplex
operation, and drive it high for half-duplex operation. In
full-duplex mode, the pin configuration of the driver and
receiver is the same as that of a MAX3080E (Figure 4).
In half-duplex mode, the receiver inputs are switched to
the driver outputs, connecting outputs Y and Z to inputs
A and B, respectively. In half-duplex mode, the internal
full-duplex receiver input resistors are still connected to
pins 11 and 12.
±15kV ESD Protection
As with all Maxim devices, ESD-protection structures
are incorporated on all pins to protect against electro-
static discharges encountered during handling and
assembly. The driver outputs and receiver inputs of the
MAX3080E–MAX3089E family have extra protection
against static electricity. Maxim’s engineers have
developed state-of-the-art structures to protect these
pins against ESD of ±15kV without damage.
±15kV ESD-Protected, Fail-Safe, High-Speed (10Mbps),
Slew-Rate-Limited RS-485/RS-422 Transceivers
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