Send ASCii via RS232 with Stream Output, Why is "Data Error" = 1?

Hi,

I have a problem using the "Stream Output" Block correctly. (Win7 64bit, R2012a 64bit)

I am trying to send three numbers via RS232 to a device. The RS232 is actually a simulated serial port which is connected via USB. I can send data to it using other Programs like arduino or processing. The numbers are 4, 2 and 2 digits long and shall be separated with a comma. They are put in as three arrays containing the single digits. (e.g. 1200 -> [1;2;0;0])

The Stream Output Block has the following configuration:

Board: Standard Device/Serial Port, COM5, 19200, 8, N, 1 (Same options like target device)
Sample Time: 0.02
Max missed ticks: 10000  -> doesn't matter for testing
Input port sizes: [4 2 2]
Format String: '%u%u%u%u,%u%u,%u%u,'  %I also tryed %d, %c, ect...
[x] Show "Data Ready" port
[x] Show "Data Error" port

My goal is to send a 11-byte ASCii array (8 numbers and 3 comma). Does it work this way or does the %u for example in the "Format String" convert my digit into a 2-byte or 4-byte number before translating into ASCii?

Now if I start the Model, The "Data Error" Port shows 0 for the first timestep and 1 for all following timesteps until the end. The "Data Ready" Port shows 1 all the way through.

Why is there a data error? And is it the reason why no signal arrives at my target device?

Thanks in advance for any advice.

Sincerely, Thomas