A while ago someone told me to use /dev/cu.usbserial rather than /dev/tty.usbserial as the former did not require hardware handshaking. I was looking for this justification again as my memory had gone a little hazey on the exact reason for using cu over tty. I found the following at http://lists.berlios.de/pipermail/gpsd-dev/2005-April/001288.html. Thought it a good thing to know and thought I’d remember it here for next time.
“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.”
So this is what I use when programming with AVRDude as the butterfly doesn’t have any DCD/DTR lines to assert. Its just TX, RX, and GND. If you have problems on the mac using a tty serial port you should try its cu equivalent.