Could you clarify what you mean by "hex string"? I can think of at least two interpretations (and (1) is more likely):
(1) The string is already the binary representation of the number (meaning the string will probably look like garbage unless you select HEX display for the string indicator). In this case you need to know the data type of the number (e.g. U8, I32, etc.) and if it is more than one byte you also need to know if it is big- or little-endian.
---> Use
typecast with the string as input and a constant of the desired data type as type input (assuming big-endian).
Check the documentation of your serial device for the exact specifications.
(2) The string is formatted text using the letters 0-9 and A-F to represent the hex values.
---> In this case you would use
Hexadecimal string to number.
If you have problems, please attach a small VI with a string diagram constant containing some raw data as received. We should be able to figure it out.