Product How-to: Software selectable serial protocols over a single connector: Page 2 of 3

August 13, 2013 //By Jack Roan
Product How-to: Software selectable serial protocols over a single connector
Jack Roan of Exar Corporation looks at how to use software selectable serial protocols over a single connector.
devices can drive signal levels up to ±15V, which could damage RS-485/422 devices that are only tested from -7V to +12V.
Older and lower cost RS-485 transceivers require external biasing resistors to hold the bus in a known state when transmitters are idle or powered off. The RS-232 protocol was not designed to drive these resistive loads, so they must be removed from the lines for RS-232 communication. Also, the RS-485 and RS-422 standards specify a termination impedance at each end of the bus to reduce signal reflections at higher data rates. Switching these cable terminators and biasing resistors in and out of circuit requires bulky and costly relays, due to the range of common-mode voltage that the RS-485 standard allows.
Multiprotocol serial transceivers combine RS-232, RS-422, and RS-485 drivers and receivers into a single chip to address these common issues. For example, the SP338 and SP339 devices from Exar include switchable termination resistors for high speed RS-485/422 (up to 20Mbps) and full support of all eight RS-232 signals specified for the popular DB9 connector. They seamlessly handle the impedance switching necessary when sharing a single connector and communication bus with multiple serial standards, and do not require any external resistors or switches. 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. Packaged in 40 QFN, these devices are significantly smaller and much easier to implement and control than multiple discrete transceivers for each protocol.

The SP338 and SP339 have pinouts to the DB9 connector


Device shown in MODE 111 - RS-485/422 MIXED DUPLEX


Industrial serial designers can now use these highly integrated, multiprotocol devices to save board space, increase functionality, and provide more reliable communication.  A small innovation in the world of industrial design, but a cost and time-saving innovation

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