Date: 12/01/2012

RS-232/485/422 serial tx/rx IC allows single connector for multiple serial protocols

Exar Corporation has launched RS-232/RS-485/RS-422 serial transceiver SP338 for industrial or embedded PCs with dual protocol serial ports. The RS-232 mode provides three drivers and five receivers to support all eight signals required for the DB9 serial port (3TX/5RX). The RS-485/422 modes include half, full, and mixed duplex configurations with up to two drivers and four receivers (2TX/4RX).

"The new SP338 and the recently announced SP339 solve the most difficult design issue of sharing a single connector between multiple serial protocols - enabling and disabling the termination resistor when switching protocols," said Jack Roan, technical marketing manager, Interface Product Line. "These devices provide multiple options for our customers to minimize the number of external connectors they use on their end-products, lowering part count and reducing overall system cost."

The SP338 integrates the resistors and control switches for multiple RS-485/422 channels, 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, eliminating the need for external resistor biasing networks and switching components.

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 circuit due to the wide range of common mode voltage possible on a RS-485 network (anywhere from -7V to +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.

In addition to the on-chip switched cable terminations, the SP338 features eight configuration modes including diagnostic loopback, RS-232 (3TX/5RX), several half duplex RS-485 and full duplex RS-485/422 modes, as well as a mixed duplex mode with two drivers and four receivers (2TX/4RX). 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 SP338 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 $5.58 per unit for 1K pieces.
Additional information on this product can be found at: http://www.exar.com/interface/transceiver/multiprotocol/SP338/.