Decoder,Converter & Validator

Hi All,
I’m having a doubt that what is the purpose of Converter & validator in payload function field. Is it to convert the decoded value?

Actually I’m sending a data as a string format from the LoRa node to TTN. I need to decode the data without any change in the value.

For Example: The data from the LoRa Node is 12 34 56 78. I need this data to be decoded as 78 56 34 12 without any conversion.

Can any body help me with this?