Simulink: how to send data via serial interface to a device?

It is the first time for me at all to raise a query in a bulletin board. I hope I do it in a right way.

I want to send data only once via serial interface to an external device. With MATLAB it works fine. Now I try to do the same with Simulink, but it does not work. I only receive zeros. My first question is: which blocks from Instrument Control Toolbox do I have to use - To Instrument/Query Instrument or Serial Send/Serial Receive/Serial Configuration? My data I want to send to the device looks like testdata = ['7E'; '31'; ...]. Before I send these data with my MATLAB-program I converted these data with hex2dec. Do I have to do the same in Simulink at first? I hope there is anybody who can give me a hint. I'm really at my wit's end.

Cheers, Michael