Or, is there an easier way to do that?
Maybe. The values starting with FF are clearly negative values. So, it appears that the device you are talking to is sending a mix of binary and ASCII data. You could store only those characters that are meaningful to humans ('0' to '9', 'A' to 'Z', and 'a' to 'z'). Stop storing when the '*' arrives. This will exclude the ',', but that might be OK. You could elect to use a different range (or ranges) for the characters to store (maybe ' ' to 'z'), which would then include the ','.