Exar introduces multi-protocol serial transceiver

     Exar has added the SP338 to its family of single-chip RS-232/RS-485/RS-422 multi-protocol serial transceivers. This new product complements Exar's existing serial transceiver product line and further extends Exar's leadership in the single-chip, multi-protocol transceiver market. The SP338 is ideal for industrial or embedded PCS that require a multi-protocol serial interface. The RS-232 mode has three drivers and five receivers to meet the eight signal channels required for the DB9 (3TX/5RX). The RS-485/422 mode includes half duplex, full duplex, and hybrid duplex configurations with up to two drivers and four receivers (2TX/4RX).

     "The newly released SP338, as well as the recently released SP339, solve the most difficult design problem when sharing a single connector between multiple serial protocols, which is to enable and disable terminal resistors when converting protocols." Jack Roan, technical marketing Manager, Interface product Line, EXAR, said: "These two products allow our customers to reduce the number of external connectors used in their products, reduce the number of devices, reduce product cost and size."

     The SP338 integrates terminal resistors and control switches that allow the system processor to enable or disable terminal resistors with a single pin. The RS-485/422 mode includes open circuit, short circuit, and EXAR enhanced receiver fail-safe when terminal resistors are used but the bus is idle, avoiding the need for external resistance bias networks and switching elements.

     When a connector is shared between RS-232 and RS-485/422 protocols, the RS-485 terminal and bias resistor must be disconnected in order for RS-232 to communicate properly. Due to the wide common-mode voltage range of the RS-485 specification (-7V to +12 V), in RS-485 networks, large and expensive relays must be used to connect or disconnect resistors in the circuit. Most designers will have a separate external connector for each protocol, or manually switch resistor connections/disconnections, and additional size and cost will come with it.

     In addition to the integrated switchable terminal resistor, the SP338 features eight configurable modes, including diagnostic loop, RS-232 (3TX/5RX), half-duplex RS-485 and full-duplex RS-485/422, and hybrid duplex with 2 drivers and 4 receivers (2TX/4RX). Maximum transfer rates of 20Mbps and 1Mbps (for RS-485/422 and RS-232, respectively) can be limited to 256kbps by switching a single control pin in any mode. The internal integrated charge pump can generate the required bipolar voltage level of RS-232 from a single 3.3V or 5V power supply with only four external capacitors, without any inductors or magnetic components.