Hey guys,
Here is a small update, i thought might be interesting for other people struggeling with this. I found that the numeric decimal veriable length does work. But only in certain situations. For example it is possilbe to construct a receive msg like this:
R[11111111111]
Here [11111111111] is the variable and "R" is static ascii, can be any ascii
As ETX I used 0x0A. It only seems to work when ETX 1 byte long.
And there can be nothing else behind the variable.
I dont know why this is. But it seems to work for me.