There seems to be a lot of misunderstanding regarding what kind of USB/Serial adapter to use for OS X and Xastir. There are two answers: use either a FTDI device (which requires some amount of effort if you are continually plugging in your TNC into different USB ports; but who does this, really?) or a Keyspan one. I favor the Keyspan because I had one to use, it’s drivers worked the first time they were installed and because it is very rugged. Also, using the Keyspan is easier, because each USB port is aliased to “/dev/tty.Keyspan1”.
What this means is that, no matter which USB port you plug the Keyspan USB/Serial Port adapter into, you can always use the “/dev/tty.KeySerial1” address in the Interface Control panel of Xastir.
While the nerds get all hot in the pants about how you could just as easily use an FTDI USB/Serial connector for Xastir, what they fail to mention is that you must use the “ls -l /dev/tty*” command in Terminal, then take that answer and plug it into Xastir every time you change ports (and remember, these guys love the interoperability and champion this as a feature, but they don’t seem to mention the tedious nature of what said entails). I just recently witnessed some amateur argue that it was better to use an FTDI USB/Serial adapter because it was cheaper to own and did not handle ports any differently than the Keyspan. This simply isn’t so, as I mentioned above. If changing USB ports is a priority for you, and it was argued that this was the single feature by which to choose among USB/Serial connectors, then the answer is to get a Keyspan and NOT an FTDI.
This brings up a good question: do we eschew convenience and more expense because some other more tedious solution is cheaper? Where is the tipping point?
Nothing wrong with wanting to hack around and have fun. But sometimes, things are worth the extra money if they simply work and are trouble free in the future.
If you are looking for the feature of using the multiple ports on your Mac for your TNC/USB/Serial connection with Xastir, then get the Keyspan. If you want to hook up your TNC and leave it alone once you do so, then get the FTDI.