Programming X10 devices

Hi,

I'm trying to program an X10 device (an activehome product) via an
ultralite E iT GSM device using ethernut. I'm new to x10 and *all* the
stuff found on the internet hadn't helped me one bit with the problem
I'm having. The X10 protocol says that for a CM11a device, I should send
0x9b to stop the device from polling. Then I send the header:code
combination for which I receive a checksum. After verifying that the
checksum si correct, I then send it a 0x00 signal for which I should
receive 0x55 (interface ready signal). This is where I keep getting
-91!! I've pulled my hair out and I still haven't gotten around the
problem. I tried sending the device a raw hex file in which I send all
the bytes together and sometimes I do get a 0x55 signal. However, this
is not working programatically. Can someone please please give me a hand
here if they've worked on X10? Thanks a lot!

Sona