uplcom a callin only device?

[Norbert Koch]

Hello.

I'm trying to get a sub-to-rs232 adaptor running with FreeBSD-5.
The vendor of this adaptor is some chinese company named
High-Edge Tech, but as usbdevs shows, it identifies as
  USB_VENDOR_PROLIFIC(0x067b) and USB_PRODUCT_PROLIFIC_PL2303(0x2303).

For testing, I connect the adaptor with the same computer's cuaa0 and
do an 'stty speed 9600 clocal' on /dev/ucom0 and /dev/cuaia0.

Now, if I do 'cat < /dev/ucom0' and 'cat > /dev/cuaa0', I see typed
characters coming in. If I try the opposite direction, nothing happens.
Is that something, I should've expected, and if so, why?

Does it have to do with the difference of callin and callout 
devices? Can I use /dev/ucom0 only as callin device like /dev/ttyd0?

Thanks,
Norbert