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It seems that the osx-pl2303-0.2.1-10.4-universal driver names the device name as /dev/tty.PL2303-XXXXXXXX, where X = the address of the port the adapter is plugged in to.
I don't know if this is fallback behaviour for a device that does not have an embedded serial number - "USB Prober" developer utility reports "Serial Number String: 0 (none)" for my ATEN USB Serial adapter (0x0557/0x2008). I believe it would be preferable for an adapter with a serial number to be named per the serial number regardless of where it's plugged in.
(BTW, where's the source code?)
You can download the source code with subversion:
svn co https://svn.sourceforge.net/svnroot/osx-pl2303 osx-pl2303
The naming of the device is based on the serial number except when there is no serial number ;-) The ATEN device doesn't provide this number so the suffix is based on the USB location.
Apple provides a solution to determine available serial ports. See the documentation at developer.apple.com. In that case the suffix of the adapter is less important.
Source in SVN - oh, of course :-)
Thanks for clarifying the naming behaviour, and the pointer re. the apple doc:
ADC Home > Reference Library > Guides > Hardware & Drivers > Accessing Hardware From Applications > Finding and Accessing Devices
And for the curious, 'ioreg -w 0 -c IOSerialBSDClient' enumerates the available serial devices (though not in an easily parseable way, so I guess the idea is to write some code for that...).
Thanks again for the great work.