There are many reasons why you would want to have a serial port on your Macintosh. Especially on iBooks and PowerBooks, the missing serial port can be a problem for people who often have to work on network equipment like routers, switches and firewalls - because it is often the only possibility to configure such equipment, or at least the serial port is the only way to do the initial configuration.
Additionally servers in co-location often have neither a monitor nor a keyboard. So a serial connection is a convenient way to administer such servers on-site. Okay, you can use your Palm or PocketPC for this, but a full-sized laptop keyboard is often easier to use - but that`s another topic all together.
An easy way to add a serial port to your Mac is to buy a USB-to-Serial Adaptor. These adaptors are quite cheap and work.
A common chip used in USB-to-Serial Adaptors is the PL-2303. Drivers for Windows, Linux and Mac OS X are available. My advice is to get a PL-2303 based adaptor.
I got a generic adapter from eBay for 9.90 euros.
If the manufacturer of your USB-to-Serial Adaptor provides drivers for OS X the installation should be really straightforward. Otherwise you will have to slightly modify another driver.
I took a driver from profilic.com, which offers a nice graphical installer.
Update: Due to a security issue prolific updated the driver. You should at least use version 1.0.9 build 6
Go to tech.prolific.com.tw and search for "2303", then download the "Driver for Mac OS X". I got version 1.0.8, but a few days ago version 1.0.9 was released which should work in the same way.
Update: For usage with OS X 10.4 Tiger at least version 1.0.9b4 is needed, and as stated above, at least 1.0.9b6 is needed because of a security flaw in earlier versions.
Your browser should extract the zip-archive on the fly, so you get a pkg file. Double-click on the pkg file to launch the graphical installer and follow the instructions.
The following steps are optional, it seems that recent versions of the driver do not require this modifications. Just reboot and it should work. You may skip this section and jump to "ZTerm". Original procedure below (for historical purpose):
After the installation has finished, you do not need to immediately reboot. Before rebooting your Mac you should plug in your USB-to-Serial Adaptor and open the Apple System Profiler (Apple-Menu > About this Mac > More Information). In the USB section note the Product-ID and Vendor-ID of your USB-to-Serial Adaptor.
Now open a terminal (Applications > Utilities > Terminal), go to /System/Library/Extensions/ProlificUsbSerial.kext/Contents and open the file Info.plist (plist = Property List) in your favourite editor (e.g. vi). Now scroll down to the section starting with <key>067B_2303</key> (it may differ a bit, important is the part "2303"). In this section locate the entries idProduct and idVendor and fill in the values gathered from the System Profiler a few minutes earlier. It should then look something like this:
<key>067B_2303</key> <dict> ... <key>idProduct</key> <integer>8963</integer> <key>idVendor</key> <integer>1659</integer> </dict>
ZTerm is available from http://homepage.mac.com/dalverson/zterm/. It is a terminal emulator like Terminal.app, but it can talk to serial ports, which is what we need.
First configure ZTerm to use the USB-to-Serial Adaptor as serial port. Open Settings > Modem Preferences and select usbserial0 as default modem. Then edit your connection in Settings > Connection and just connect the serial cable to start using it. The following pictures should explain the process.
The settings screen of ZTerm
A serial connection to my Linux Box
Because some people use Linux on Macs, here is a short explanation of the procedure to get the USB-to-Serial Adaptor working on Linux systems.
Installation on Linux is really plug `n play. All recent kernels provide a driver for the PL-2303 chip.
The device is /dev/ttyUSB0 for the first PL-2303 adaptor. If the device is not automatically created, create it with
mknod c 188 0 /dev/ttyUSB0Actually on my machine, ttyUSB0 is a link to /dev/usb/tts/0.
If you ask yourself what you need a serial port for, it is likely that you will never need one. If you already need a serial port on your Mac, than a USB-to-Serial Adaptor based on the PL-2303 chip is a cheap and convenient way to add a serial port to your Macintosh.