Internally switched cable termination enables RS-232, RS-485/422 protocols to share single connector

Exar says its SP339 solves the most difficult design issue of sharing a single connector between multiple serial protocols -- enabling and disabling the termination resistor.

Nov 14th, 2011

Exar Corporation (NASDAQ: EXAR) has added another device to its single-chip RS-232/RS-485/RS-422 serial transceiver product family. The SP339 device is the company's first serial transceiver with internally switched cable termination for high speed RS-485/422 communication; the device enables dual protocol serial ports using a single shared connector. The chip's RS-232 mode provides three drivers and five receivers to support all eight signals required for the DB9 serial port (3TX/5RX), while RS-485/422 modes include both half and full duplex (1TX/1RX).

"The SP339 solves the most difficult design issue of sharing a single connector between multiple serial protocols – enabling and disabling the termination resistor when switching protocols," explains Jack Roan, technical marketing manager, Interface Products. "This device enables our customers to reduce the number of external connectors they use on their products, lowering their parts count and reducing their product cost and size. We are already seeing encouraging market acceptance of the SP339 at our alpha customers."

Product Features

According to Exar, when sharing a single connector between RS-232 and RS-485/422 protocols, the RS-485 termination and biasing resistors must be disconnected for proper RS-232 communication. Large and expensive relays are required to switch these resistors in and out of the circuit due to the wide range of common mode voltage possible on a RS-485 network (between -7V and +12V). Most designers find it easier to have a separate external connector for each protocol, or to manually switch the resistor in and out, despite the additional size and cost of those approaches.

The SP339 integrates the resistor and control switches, allowing the system processor to dynamically enable or disable the termination with a single pin. The RS-485/422 modes include Exar's enhanced receiver failsafe on open, shorted, and terminated but idle inputs, obviating the need for external resistor biasing networks and switching components.

In addition to the on-chip switched termination, the SP339 also features ±20V fault tolerance on the bus pins; allowing it to withstand direct shorts or signal voltages up to this level without damage. The four configuration modes include diagnostic loopback, RS-232 (3TX/5RX), Half Duplex RS-485 (1TX/1RX), and Full Duplex (1TX/1RX).

The maximum data rates of 20Mbps and 1Mbps (RS-485/422 and RS-232, respectively) can be slew limited to 256kbps in any mode by toggling a single control pin. The on-board charge pump generates the RS-232 bipolar voltage levels from a single 3.3V or 5V supply with only four external capacitors, and does not require any inductors or magnetic components.

All bus pins are protected against severe ESD events up to ±15kV HBM (Human Body Model), and ±8kV IEC 61000-4-2 Contact Discharge.

The SP339 is available now in volume quantities in a RoHS compliant 40-pin QFN package in Commercial (0 to +70ºC) and Industrial (-40 to +85ºC) operating temperature grades. Pricing starts at $4.65 per unit for 1K pieces.

Additional information on this product can be found at: http://www.exar.com/Common/Content/ProductDetails.aspx?ID=SP339.

More in Connectivity