Thanks to all those who responded!
I did get myself a null-modem cable with proper connectors on both ends. 
The cable I got has both 25pin and 9pin connectors on both ends so I can 
use it between two PCs as well as between a PC and a Mac (using the Macs 
modem cable too)

I plugged it in and was able to "cat" files into one computers device 
and right out of the other computers other device. I'll setup ppp and stuff 
when I finish exams.

I was also thinking it would be cool to use cables like that to setup a 
"ring" network among a few machines (say 4) that dont have ethernet. Each 
machine would use both its serial ports  for connecting to both neigbors 
with cables. Then you could use some traditional ring routing protocols 
to get more speed out of it then a simple daisy chain. One of the machine 
could have an ethernet that serves as a gateway to the rest of the world. 
But that's another story (or trip) that belongs elsewhere. :)

A more sober idea would simply to use both ports to get twice as much 
bandwith between the two machines. You could make it transparent to pppd 
by having processes on each side that split the stream in two and put it 
back together on the other side. This would be nicer with threads since 
you probably end up needing several processes to do the blocking reads 
and writes; but oh well. When exams are over, if I can get my hands on 
another pair of cables, I'll give it a try and let you know what I got.

Of course, if someone has already done this, I'd like to hear from them 
but it doesn't sound like it would be very popular (most people need the 
other serial for printing or something).

Thanks again for the cabling info since I dont know anything about 
hardware I didn't know where to start. Now, I have a device driver level 
interface, I feel at home already.

On Sat, 19 Apr 1997, Dave Thurstan wrote:

> Mark Andres wrote:
> 
> > You will probably need 2 cables, unless you can find one that matches your
> > needs perfectly.  Here is how I do it in 2 cables.
> > 
> > 1) Standard Macintosh modem cable.  This is probably going to be 9-pin
> > (Mac style) male to 25-pin (PC-style) male.
> > 
> > 2) A null modem (or cross) cable which is 25-pin (PC-style) female to
> > whatever connector will connect to the serial port (COMM port) on your PC.
> 
> Yep, that worked for me. I was also able to use a straight modem cable
> on both
> machines with a gender-changer/null-modem-crossover dongle in the
> middle.
> 
> This worked for Linux-Mac, but  I don't see why it couldn't work for
> NetBSD to
> Mac:
> 
> I set up the PC to run pppd on the second serial with the -passive and
> -login
> options set. I also specified an arbitrary IP address for each machine -
> in the
> TCP control  panel on the Mac and on the pppd command line on the PC.
> 
> Then from the mac end I used FreePPP (MacPPP/OT-PPP are fine) to connect
> 'direct' via the modem port with the terminal checkbox set. I logged in
> using
> a known Linux user/password and the mchines did the rest.
> 
> I was able to ftp to/from the mac. Then I got a copy of ftpd and others
> from
> the UMICH archives for the mac and got full TCP/IP service in both
> directions.
> 
> 
> Hope this helps.
> 
> -- 
> :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
> :::::_/_/_/_/::::::::::::::::::::::::::::: Dave Thurstan, J.C.
> Limited::
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> ::
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> 2766::
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> :_/_/_/_/::::_/_/_/_/:::_/:::::_/_/_/:@myrddyn.demon.co.uk:::::::::::::::
> :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
> 

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IBM MVS/XA System Programming Library.