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Each serial device shows up twice in /dev, once as a tty.* and once as a cu.*.
What is the cu.* device? How does it differ from the tty.* device?
mh@maru ~ --> ls -l /dev/*.usbmodem621 crw-rw-rw- 1 root wheel 11, 5 Dec 25 18:00 /dev/cu.usbmodem621 crw-rw-rw- 1 root wheel 11, 4 Dec 25 18:00 /dev/tty.usbmodem621 
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So, what's the difference? Well, TTY devices are for calling into UNIX systems, whereas CU (Call-Up) devices are for calling out from them (eg, modems). We want to call-out from our Mac, so /dev/cu. * is the correct device to use. The technical difference is that /dev/tty.
/dev/tty stands for the controlling terminal (if any) for the current process. To find out which tty's are attached to which processes use the "ps -a" command at the shell prompt (command line). Look at the "tty" column. For the shell process you're in, /dev/tty is the terminal you are now using.
Only one TTY (TTY1) is available in OSX. To enable tty use fn + ctrl + alt + F1 .
TTY is a common trademark-free abbreviation for teletype, a device commonly attached to early computers' serial ports, and * represents a string identifying the specific port; the syntax of that string depends on the operating system and the device.
http://lists.berlios.de/pipermail/gpsd-dev/2005-April/001288.html :
The idea is to supplement software in sharing a line between incoming and outgoing calls. The callin device (typically /dev/tty*) is used for incoming traffic. Any process trying to open it blocks within the open() call as long as DCD is not asserted by hardware (i.e. as long as the modem doesn't have a carrier). During this, the callout device (typically /dev/cu* -- cu stands for "calling unit") can be freely used. Opening /dev/cu* doesn't require DCD to be asserted and succeeds immediately. Once succeeded, the blocked open() on the callin device will be suspended, and cannot even complete when DCD is raised, until the cu device is closed again.
That way, you can have a getty listening on /dev/tty*, and can still use /dev/cu* without restrictions.
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