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null modem cable maximum length

Posted on 2006-11-25
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Hi,

Does anyone know the maximum length for null modem cables? Can I order one on the Internet to be shipped in the UK - any suggestions?

Many thanks,

Chris
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Question by:csetzkorn
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by:AlexNek
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here you'l find for 10 meters
http://www.kenable.co.uk/index.php/cPath/23_84
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by:pgm554
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Max distance depends upoin speed ,about 50FT @19.2K.

 http://www.maxim-ic.com/appnotes.cfm/appnote_number/83

As for cables

http://www.pricewatch.com/cables/
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by:TPFLSandy
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Doing the Google thing shows 15M (~50 feet) and 19.2K baud as the design limit for the original EIA RS-232 of 1962, which used -25V and +25V.  RS-232C of 1969 was a big step forward, tightening the signals to +/-15V and specifying the common D25 connector we all know and love, but I don't think that it changed the original design limits.  Rev-D changed it to a total line impedance limit, which makes a lot more sense.
   You can extrapolate as pqm554 said above: if you double the distance to 30 meters, is should still work if you halve the speed to 9.6K baud.  Keep in mind some other factors too:  Modern (packaged in an IC) UARTs produce a lot cleaner signal than the original discrete components did when the RS-232 standard was set, so you should be able to go a lot further and faster than the engineers expected back in 1962.  On the other hand, if you have a bad cable, or there's a lot of RF interference, then line noise will cut your speed down a lot. Routing your unshielded cable on top of a flourescent light will cause you a lot of trouble.
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