XR34350 Datasheet, PDF (18/20 Pages) Exar Corporation – Internal Termination and Wide Output Swing

XR34350
Applications Information
Product Summary
The XR34350 is an advanced multiprotocol transceiver
supporting RS-232, RS-485, and RS-422 serial standards
in a 40-pin QFN package. Integrated cable termination and
four configuration modes allow all three protocols to be
used interchangeably over a single cable or connector with
no additional switching components. The RS-485/RS-422
modes feature one driver and one receiver (1TX/1RX) in
both half and full duplex configurations. The RS-232 mode
(3TX/5RX) provides full support of all eight signals commonly
used with the DB9 RS-232 connector. A dedicated mode is
also available for diagnostic loopback testing.
Internally Switched Cable Termination
Enabling and disabling the RS-485/RS-422 termination
resistor is one of the largest challenges system designers
face when sharing a single connector or pair of lines across
multiple serial protocols. A termination resistor may be
necessary for accurate RS-485/RS-422 communication,
but must be removed when the lines are used for RS-232.
XR34350 provides an elegant solution to this problem by
integrating the termination resistor and switching control,
and allowing it to be switched in and out of the circuit with a
single pin. No external switching components are required.
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
XR34350 guarantees a logic-high receiver output when the
receiver inputs are open, shorted, or terminated but idle/
undriven. The enhanced failsafe interprets 0V differential
as a logic high with a minimum 50mV noise margin, while
maintaining compliance with the EIA/TIA-485 standard
of ±200mV. No external biasing resistors are required,
further easing the usage of multiple protocols over a single
connector.
±15kV ESD Protection
ESD protection structures are incorporated on all pins to
protect against electrostatic discharges encountered during
handling and assembly. The bus pins (driver outputs and
receiver inputs) have extra protection structures, which have
been tested up to ±15kV without damage. These structures
withstand high ESD in all states: normal operation, shutdown
and powered down.
ESD protection is be tested in various ways. Exar uses
the following methods to qualify the protection structures
designed into XR34350:
■■ ±15kV using the Human Body Model (HBM)
■■ ± 8kV using IEC 61000-4-2 Contact Discharge
■■ ± 15kV using IEC 61000-4-2 Air Gap Discharge
The IEC 61000-4-2 standard is more rigorous than HBM,
resulting in lower voltage levels compared with HBM for
the same level of ESD protection. Because IEC 61000-4-
2 specifies a lower series resistance, the peak current is
higher than HBM. The XR34350 has passed both HBM and
IEC 61000-4-2 testing without damage.
Diagnostic Loopback Mode
The XR34350 includes a diagnostic digital loop back mode
for system testing as shown in Figure 2. The loopback mode
connects the TTL driver inputs to the TTL receiver outputs,
bypassing the analog driver and receiver circuitry. The
analog/bus pins are internally disconnected in this mode.
REV1B
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