I used to linux USB port naming, aka /dev/ttyUSBxx and currently trying to write some test software to read/wrtie raw data to USB ports on iMac, but i find it hard to detect to
I have a SheevaPlug that acts as a USB serial device and that appears as /dev/tty.usbserial-XXXX
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OS X doesn't create a /dev entry for raw access to USB devices, and there's no way to access them as TTY devices.
Partly this is a different in philosophy between Linux and BSD, partly it's a difference in driver-writing technology, and partly it's a matter of already having a well-defined hierarchy (the IOKit Registry) that would have to be mapped to the /dev hierarchy in some complicated way.
At any rate, if you want to learn how you should access USB devices on a Mac, start with QA1370. If you're looking to do a quick port of some linux code without actually learning IOKit/IOUSB, it's probably going to be impossible. If you're looking for something else, you'll need to give more info about what you're trying to do.
If your USB device has an FTDI chip internally, installing VCP drivers from FTDI might help.