Problems with USB to serial adaptor

[david at thekramers.net (David Kramer)]

I'm using a Prolific PL2303HX-based Sewell USB to serial cable to go from
my laptop to an ARM7 embedded development board. The adapter talks ok to a
modem and other devices, so I know I have the device and the permissions
right.

When I try to talk to the board through the cross-platform version of the 
gdb debugger, arm-elf-gdb, I get:

(gdb) target /rdi /dev/ttyUSB0
(gdb)
Could not open device "dev/ttyUSB0"

I've tried it both as root and as a mortal.  

What tools do people use to work with serial port problems?  I fooled 
around with setserial, but it seemed rather displeased at the device, 
saying "Cannot get serial info: Invalid argument".  I tried fooling around 
with Minicom, but it REALLY REALLY wants to talk to a modem, and doesn't 
want to let me talk to the serial port until I've dialed a number.

Any clues?


Thanks.