I bought a USB Serial port that had MAC drivers. Check before you buy it that it has drivers. Not all USB Serial have MAC support. I assume that you have installed the correct drivers from here.
First, open Terminal (Applications/Utilities/Terminal). Next, type
to get a list of your serial lines. Pick the one that you want to open. For example, my list looks like this:
gf:dev gf$ ls -al tty.*
crw-rw-rw- 1 root wheel 18, 4 Jan 25 07:32 tty.Bluetooth-Modem
crw-rw-rw- 1 root wheel 18, 8 Jan 25 07:32 tty.Bluetooth-PDA-Sync
crw-rw-rw- 1 root wheel 18, 10 Jan 25 09:20 tty.PL2303-0000103D
crw-rw-rw- 1 root wheel 18, 0 Jan 25 07:32 tty.SerialPort-1
And I know /dev/tty.PL2303-0000103D is the USB-to-serial adaptor becuase it’s connected, if you unplug it the tty.PL2303-0000103D stops being listed.
Knowing the serial port, you can just type screen portname datarate to show the serial data on the screen. In my case, it was:
screen /dev/tty.PL2303-0000103D 9600
To quit the screen app, type control-A, then control-.
I don’t think that this is very complete, but it worked for me and might help you to get your USB serial working. Please let me know if there anything I can add to complete this piece.
Update 2008/12/28: A reader has pointed out this article is based on a similar article at†http://www.tigoe.net/pcomp/resources/archives/avr/000749.shtml. I must have copied some of the text and then mistakenly believed it was my own. Apologies for this mistake.