uplcom: can only receive data

[Frederik Sausmikat <frederik.sausmikat%gmx.de@localhost>]

Hi,

I'm currently trying to get USB to serial converters to work with
NetBSD-4.0 and -current. When plugging the adapters in they get recognized
by the kernel...

   uplcom0 at uhub0 port 1
   uplcom0: Prolific Technology Inc. USB-Serial Controller D, rev
   1.10/3.00, addr 2
   ucom0 at uplcom0

or

   uplcom0 at uhub3 port 1
   uplcom0: ATEN International Serial adapter, rev 1.10/0.01, addr 2
   ucom0 at uplcom0

...and I am able to send data *from* a regular built-in serial port *to* 
the USB adapter but not the other way round (using cat, cu or minicom). 
This is the case on an Asus EeePC 900 running 4.99.72 GENERIC and my 
home PC running 4.0-STABLE (both i386).

Additionally I tried the adapters with my machine at work, running 
4.0-GENERIC (looping tty00 back to ttyU0 via null modem cable) and there 
they work without any problems (this machine is also i386).

They also work with Debian ("testing", kernel 2.6.26?) on all machines, 
so it doesn't seem to be hardware related.

Now the question is, does anybody have an idea what the problem might be 
or any tips on where to start looking in the sources or debugging the 
problem?

I'll happily provide any additional information you might need, but as 
far as I can see, dmesg, usbdevs or /var/log/messages don't show up 
anything unusual, which is why I don't post their output by now.

Regards, Freddy