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Cheat sheet for RS-485

Posted: 02 Aug 2012 ?? ?Print Version ?Bookmark and Share

Keywords:RS-485? RTS control? Send-Data control? surge protection?

Guide to troubleshooting RS-485 has been previously published in this site. This Cheat Sheet is a quick reference for designers tasked with implementing RS-485, designed to keep them out of trouble from the get go.

Here are three important bits of info on the RS-485 followed by tips for best performance.

Use it anywhere
The EIA/TIA RS-485 communications standard, an upgrade of RS-422, supports 32 devices (driver/receiver pairs) in a party line or multidrop mode, on a cable of up to 1219.2 m (4,000 feet) for balanced differential signal transmissions at a common mode voltage (Vcm) of -7 to +12V.

You can internally or externally configure RS-485 devices. Four wire connections, which require an additional ground, need a 'master' node (e.g. a PC) to communicate to all others (slaves), which in turn can only communicate with the master.

To terminate or not?
The RS-485 spec says to use termination. For high baud rates and long cable runs, this is true. In most equipment, however, with max speeds of 115kbit, it is unnecessary. Adding termination dramatically increases power consumption and requires that the network be re-biased, which is rarely done. Termination complicates system design and rarely solves problems when used in the kilobit data range.

Extend the network easily
By adding repeaters, you get longer distanceseach 'refreshed' signal can driver another 1219.2 m (4,000 feet) of cable and 32 more RS-485 loads (driver/receiver pairs) per repeater.

Long networks are especially vulnerable to grounding and surge problems. This is easily addressed by isolating the nodes. Use optically isolated repeaters and isolated converters to attach the nodes of your network and you'll have reliable long-distance applications.

Tips for best performance
???Check the converter data sheet to see how the receiver's 'enable' function is connected.
???Test the interval after the last bit is transmitted to ensure complete transmission. A too-short interval causes missed parts of each character being sent. A too-long interval may cause the system to switch the data line from transmit to receive.
???Select appropriate isolation or shunting-type suppression to protect against short circuits to power conductors.
???Use appropriate signal groundsa must haveand shielded cable, which is desirable for safety.
???Check signal types and related issues before writing or purchasing software protocols.
???Device communication characteristics must be checked before completing system design
Get a schematic of each serial port to assist in troubleshooting and repairs.

About the author
Mike Fahrion, director of product management at B&B Electronics, is an expert in data communications with 20 years of design and application experience. He oversees development of the company's rugged M2M connectivity solutions for wireless and wired networks based on serial, Ethernet, wireless and USB communication technologies. Fahrion has particular expertise in reliable connectivity solutions for devices deployed at the "edge" of networks in remote, harsh or uncontrolled environments. Fahrion is a speaker and widely-published author, including his politically-incorrect newsletter, uConnections, with over 50,000 monthly subscribers. Fahrion holds a BSEE from Iowa State University.

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