XR33202_17 Datasheet, PDF (13/16 Pages) Exar Corporation – Wide Supply RS-485/RS-422 Transceiver with 1.65V-5.5V I/O Interface

XR33202
Applications Information (Continued)
The XR33202 RS-485/RS-422 devices are part of Exar’s
high performance serial interface product line.
Enhanced Failsafe
Ordinary RS-485 differential receivers will be in an
indeterminate state whenever the data bus is not being
actively driven. The enhanced failsafe feature of the
XR33202 guarantees a logic-high receiver output when
the receiver inputs are open, shorted or when they are
connected to a terminated transmission line with all drivers
disabled. In a terminated bus with all transmitters disabled,
the receivers’ differential input voltage is pulled to 0V by the
termination. The XR33202 interprets 0V differential as a logic
high with a minimum 50mV noise margin while maintaining
compliance with the RS-485 standard of ±200mV. Although
the XR33202 does not need failsafe biasing resistors, it can
operate without issue if biasing is used.
Hot Swap Capability
When V_CC is first applied the XR33202 holds the driver
enable and receiver enable inactive for approximately
10μs. During power ramp-up, other system ICs may drive
unpredictable values or tristated lines may be influenced
by stray capacitance. The hot swap feature prevents the
XR33202 from driving any output signal until power has
stabilized. After the initial 10μs, the driver and receiver
enable pins are weakly pulled to their disabled states (low
for DE and high for RE) until the first transition. After the first
transition, the DE and RE pins operate as high impedance
inputs.
If circuit boards are inserted into an energized backplane
(commonly called “live insertion” or “hot swap”) power
may suddenly be applied to all circuits. Without the hot
swap capability, this situation could improperly enable the
transceiver’s driver or receiver, driving invalid data onto
shared buses and possibly causing driver contention or
device damage.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or by bus contention.
First, a driver current limit on the output stage provides
immediate protection against short circuits over the whole
common-mode voltage range. Second, a thermal shutdown
circuit forces the driver outputs into a high impedance state
if junction temperature becomes excessive.
Line Length
The RS-485/RS-422 standard covers cable lengths up to
4000 feet. Maximum achievable line length for a specific
application is a function of many factors, the data rate,
cable properties, the driver/receiver characteristics and
whether or not termination or stubs are used. For lower
speed applications, data rates <150Kbps, the maximum
cable length is mainly a function of the DC impedance of
the cable being used. As data rates increase the capacitive
and inductive characteristics tend to dominate and limit the
cable lengths that can be achieved. At higher data rates the
cable properties will tend to damper the signal resulting in
increased rise/fall times at the far end of the cable.
The XR33202 has been designed with stronger drivers to
help compensate for these cable affects and is intended for
high speed applications.
±15kV ESD Protection
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 XR33202 has extra protection against static
electricity. Exar uses state-of-the-art structures to protect
these pins against ESD of ±15kV without damage. The
ESD structures withstand high ESD in all states: normal
operation, shutdown and powered down. After an ESD
event, the XR33202 keeps operating without latch up or
damage.
ESD protection can be tested in various ways. The
transmitter outputs and receiver inputs of the XR33202 is
characterized for protection to the following limits:
■■ ±15kV using the Human Body Model, RS-485
bus pins
■■ ±4kV using the Human Body Model, all other pins
ESD Test Conditions
ESD performance depends on a variety of conditions.
Contact Exar for a reliability report that documents test
setup, methodology and results.
Maximum Number of Transceivers on the Bus
The standard RS-485 receiver input impedance is 96kΩ (1/8
unit load). A standard driver can drive up to 32 unit loads.
The XR33202 transceiver has 1/8 unit load receiver input
impedance of 96kΩ, allowing up to 256 transceivers to
be connected in parallel on a communication line. Any
combination of the XR33202 and other RS-485 transceivers
up to a total of 32 unit loads may be connected to the bud
line.
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