XR33032/35/38
15
REV. 1.0.0 WIDE SUPPLY 18V TOLERANT RS-485/RS-422 TRANSCEIVERS
PRODUCT DESCRIPTION
The XR33032/35/38 RS-485/422 devices are part of Exar’s high performance serial interface product line. The
analog bus pins can survive direct shorts up to ±18V, and are protected against ESD events up to ±15kV.
ENHANCED FAILSAFE
Standard RS-485 differential receivers will be in an indeterminate state whenever the analog bus pins are not
being driven. The enhanced failsafe feature of the XR33032/35/38 family 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 XR33032/35/38 family interprets 0V differential as a logic high
with a minimum 50mV noise margin while maintaining compliance with the EIA/TIA-485 standard of ±200mV.
Although the XR33032/35/38 family does not need failsafe biasing resistors, it can operate without issue if
biasing is used.
RECEIVER INPUT FILTERING
XR33032 and XR33035 receivers incorporate internal filtering in addition to input hysteresis. This filtering
enhances noise immunity by ignoring signals that do not meet a minimum pulse width of 30ns. Receiver
propagation delay increases slightly due to this filtering. The high speed XR33038 device does not have this
input filtering.
HOT-SWAP CAPABILITY
When V
CC
is first applied the driver enable and receiver enable (DE and RE) are held inactive for
approximately 10 microseconds. 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 these devices 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, 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 busses 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 and TERMINATIONS
The RS-485/RS-422 standard covers line lengths up to 4000ft. Maximum achievable line length is a function of
the data rate, cable characteristics and environmental noise. Termination prevents signal reflections by
eliminating the impedance mismatches on a transmission line. Line termination is generally used if rise and fall
times are shorter than the round-trip signal propagation time. Higher supply voltage will allow longer cables to
be used.
