MAX13410E–MAX13415E
the MAX13410E–MAX13415E. The circuit-board designer
has no control over this parameter.
Fail Safe
The MAX13410E/MAX13411E/MAX13414E/MAX13415E
guarantee a logic-high receiver output when the receiv-
er inputs are shorted or open, or when they are con-
nected to a terminated transmission line with all drivers
disabled. This is done by setting the receiver input
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 differ-
ential input voltage is pulled to 0 by the termination.
With the receiver thresholds of the MAX13410E/
MAX13411E/MAX13414E/MAX13415E, the result is a
logic-high with a 50mV minimum noise margin. Unlike
previous fail-safe devices, the -50mV to -200mV thresh-
old complies with the ±200mV EIA/TIA-485 standard.
AutoDirection Circuitry
The AutoDirection circuitry in the MAX13412E/
MAX13413E is a technique to minimize the number of
signals needed to drive the part. This is especially useful
in very low cost, isolated systems. In a typical isolated
system, an optocoupler is used for each control signal to
cross the isolation barrier. These optocouplers add cost,
size and consume power. Without the AutoDirection cir-
cuitry, three to four optocouplers may be required for
each transceiver. With the AutoDirection circuitry, the
number of optocouplers can be reduced to two.
Typical RS-485 transceivers have four signals on the
control side of the part. These are RO (receiver output),
RE (receiver enable), DE (driver enable), and DI (driver
input). In some cases, DE and RE may be connected
together to reduce the number of control signals to
three. In half-duplex systems, the RE and DE signals
determine if the part is transmitting or receiving. When
the part is receiving, the transmitter is in a high-imped-
ance state. In a fully compliant RS-485 system, all three
or four signals are required. However, with careful
design and Maxim’s AutoDirection feature, the number
of control signals can be reduced to just RO and DI in
an RS-485 compatible system. This feature assumes the
DI input idles in the high state while the receiver portion
of the MAX13412E/MAX13413E is active. It also requires
an external pullup resistor on A and pulldown resistor on
B (see the typical application circuit, Figure 10). The fol-
lowing is a description of how AutoDirection works.
When DI is low, the MAX13412E/MAX13413E always
drive the bus low. When DI transitions from a low to a
high, the drivers actively drive the output until (A - B) >
V
DT
. Once (A - B) is greater than V
DT
, the drivers are
disabled, letting the pullup/pulldown resistors hold the
A and B lines in the correct state. This allows other
transmitters on the bus to pull the bus low.
Pullup and Pulldown Resistors
The pullup and pulldown resistors on the A and B lines
are required for proper operation of the MAX13412E
and MAX13413E, although their exact value is not criti-
cal. They function to hold the bus in the high state (A - B
> 200mV) when all the transmitters are in a high-imped-
ance state due to either a shutdown condition or
AutoDirection. Determining the best value to use for
these resistors depends on many factors, such as termi-
nation resistor values, noise, number of transceivers on
the bus, etc. Size these resistors so that, under all con-
ditions, (A - B) > 200mV for ALL receivers on the bus.
Idle State
When not transmitting data, the MAX13412E/
MAX13413E require the DI input to be driven high to
remain in the idle state. A conventional RS-485 trans-
ceiver has DE and RE inputs that are used to enable
and disable the driver and receiver. However, the
MAX13412E/MAX13413E do not have a DE input, and
instead use an internal state machine to enable and
disable the drivers. DI must be driven high to go to the
idle state.
Enhanced ESD Protection
As with all Maxim devices, ESD-protection structures are
incorporated on all pins to protect against electrostatic
discharges encountered during handling and assembly.
The driver outputs and receiver inputs of the MAX13410E–
MAX13415E have extra protection against static electricity.
Maxim’s engineers have developed state-of-the-art struc-
tures to protect these pins against ESD of ±15kV
(MAX13412E/MAX13413E) and ±14kV (MAX13410E/
MAX13411E) without damage. The ESD structures with-
stand high ESD in all states: normal operation, shutdown,
and powered down. After an ESD event, the MAX13410E–
MAX13415E keep working without latchup or damage.
ESD protection can be tested in various ways. The trans-
mitter outputs and receiver inputs of the MAX13410E–
MAX13415E are characterized for protection to the
following limits:
±15kV using the Human Body Model (MAX13412E/
MAX13413E)
±14kV using the Human Body Model (MAX13410E/
MAX13411E)
RS-485 Transceiver with Integrated Low-Dropout
Regulator and AutoDirection Control
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