I hate to say this, but it seems that OS X does not support serial ports. It is possible that support might come out in later updates. However, I would consider getting a USB card for printers and modems. They're fairly cheap, but you would also have to get a new modem and new printer.
Mac OS X Legacy programs might not support the serial ports, but its BSD/UNIX parts do. I've got two serial-port terminals on my G3/266. If you want to know how to do that, just look it up in this list (search it, its there).
If you want to just use a modem or a dot-matrix printer, you can test the device like this:
#su (become root)
#echo "Hi there." > tty.modem (modem port)
#echo "Hi there." > tty.printer (printer port)
The above commands will echo the text "Hi there." to the respective ports. To set the speed and parity of agiven port, see the manual for the program "/bin/stty" (#man stty).
You might be thinking ...How is any of that usefull....
Lets say you have a text file, and you want to print it to a line printer (that is, bubble jet or dot matrix, I think).
#cat filename.tx > /dev/tty.printer
You can even put stuff like that into login scripts. (So when your 'friends' login to your computer and su to root, your dot matrix wakes up the whole neiborhood...)
And, if you wanted to connect to an ISP with a serial port modem, it might be hard, but you *could* hack pppd (the point-to-point daemon) and connect via wvdial (world-view dial).
And, those ligacy serial ports may allow you to use serial mice in XF86. Maybe even useing formally 'pc-only' hardware on your mac (just wait for a few geeks to compile it...).
Basically, what I am trying to say is that those 'unsupported' serial ports ARE detected and configured by the kernel. Its just a question of convincing programs to use them. And believe me, the programs will pop up.
Modem will work on Modem port.
Printer won't work on Printer port.
Right now I'm logged into my ISP with my external GlobalVillage 56K from OS X, and it works fine.
Unfortunately my printer - an Epson Stylus Color 850 does not work with the printer port, and I suspect it may never work. I'll wait until July/Macworld to see what the printer driver SDK will be like to see if anyone will write a driver for it. So far, I have yet to see a single serial port printer to be supported under OS X.
I've purchased a combined USB/Firewire PCI card, and it is automatically recognized by OS X. So I may have to get a new printer. A Postscript Ethernet printer should work, as well as supported USB printers.
If you have installed some USB ports in your mac then you might be able to get some third-party adapters to convert your serial-priter port to a USB port. Umm... I believe Keyspan has some serial adapter, not sure if they work for the modem port or not.
Powerprint from Strydent.com used to make serial to parallel converters, they might have something you could use. You might have to jump through a couple hoops but there's probably a workaround.
There should be a cheaper way to print with Mac OS X without having to go buy a usb printer.
Epson actually makes a USB to parallel converter for the Stylus Color 800, 850, 1520. However, Epson introduced this adapter back in the days when the iMac first came out so that iMac users could print to these printers. Unfortunately there are no printer drivers listed which support printers using USB to parallel adapters. This option is worth checking out, I don't know if anyone has tried to get a Stylus Color printer working with such an interface...