Problems with usb to serial adapter with arm-elf-gdb/Angel

[David Kramer]

We have a setup using an ARM 7T evaluator board with Angel debugger.  We're
using the GNU toolchain for cross-compiling under Linux.  On a workstation
with a real serial port, it works great.  When we try it on a laptop without a
built-in serial port, using a USB to serial adapter (prolific chipset, it it
matters), arm-elf-gdb can't open the serial port.

I've tried other devices (modem, X10, etc) with the adapter, and it works fine
with them.  So I know the serial port is otherwise working.

Is there something unusual about how gdb uses the serial port that would make
it not work with one of these adapters?  Would a PC Card serial adapter work
better?

Thanks.