Product How-to: Software selectable serial protocols over a single connector

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.

While the RS-232, RS-422, and RS-485 serial standards are hardly new, combining them over a single connector creates challenges unforeseen when these standards were first drafted years ago. Multiprotocol transceivers with integrated termination resistors greatly simplify the development of modern serial controllers, resulting in smaller, lower cost, and quicker-to-market solutions.
 
The RS-232 serial standard was first introduced in 1962 to connect teletypewriters with modems, printers, and other peripheral devices. In the years since, additional serial standards such as RS-422 and RS-485 emerged, offering higher data rates, improved noise immunity, longer transmit range, and multi-drop and multi-node bus communication.
 
Today RS-232 has largely been replaced by USB in the personal computer market, but RS-232, RS-422, and RS-485 are still used heavily in industrial applications like medical devices, factory and building automation, and robotics. These protocols are simple and low cost to implement, are time tested, and deliver rugged and robust communication in noisy environments.
 
Interfacing with an existing network of serial devices may require several of the common protocols, typically with separate external connectors for each. However, with the electronics equipment industry continuously moving in the direction of cheaper, smaller, and quicker to market, designers are being pressed to reduce the number of bulky connectors and deliver a lower cost integrated solution in a shorter time. Since the RS-232 standard specifies the DB9 connector, it is often chosen, requiring the RS-485 and RS-422 standards to co-exist alongside RS-232 in the same connector.
 
Integrating multiple serial standards over a single shared connector introduces a host of issues the standards developers did not conceive or plan for. Proper signaling with one protocol requires the drivers and receivers of the others to be disconnected to avoid excessive loading on the bus. Many standalone RS-232 or RS-485 transceivers offer standby or tri-state modes, but do not completely disconnect from the bus. This creates a long term reliability concern, as legacy RS-232

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