From owner-freebsd-hardware@FreeBSD.ORG  Thu May 15 16:52:51 2008
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To: "Aryeh M. Friedman" <aryeh.friedman@gmail.com>
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Date: Thu, 15 May 2008 08:56:15 +0100
From: Dieter <freebsd@sopwith.solgatos.com>
Cc: freebsd-hardware@freebsd.org
Subject: Re: software null modems 
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> I need a null modem and all I can find is straight through db25 cables 
> and I want to avoid the paper clip trick if I can. So is there any way 
> to create a null modem in software only?

Google can find lots of them, 25 pin or 9 pin, dongle or cable, ...

http://www.google.com/products?as_q=&num=30&scoring=p&btnG=Search+Products&as_epq=null+modem&as_oq=&as_eq=&price1=&price2=&as_occt=any&show=dd&safe=active

Products Results 1 - 30 of about 4,711 for "null modem"

$1.68 and up.

There is more than one null modem pinout, be sure to get the one you need.

For a software solution to work, the pins would need to be bidirectional.
I doubt that anyone makes a RS-232 chip with bidirectional pins.