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SP339E
REV. 1.0.3
RS-232/RS-485/RS-422 TRANSCEIVER WITH INTERNAL TERMINATION
PRODUCT SUMMARY
The SP339 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-
485/422 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-485/422 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-485/422 communication, but must be removed when the lines are used for
RS-232. SP339 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 SP339 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 SP339:
±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 SP339 has passed both HBM and IEC 61000-4-2 testing without damage.
DIAGNOSTIC LOOPBACK MODE
The SP339 includes a diagnostic digital loop back mode for system testing as shown in Figure 1 . 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.
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