Hex manipulation

[buuuudzik]

Hello,

maybe do you know functions for hex<->string conversion? I am preparing a full integration with Somfy blinds via RS485 and I want receive some data. After this I must delete last 2 bytes(checksum) and inverse all other bytes.

I've tried used this function which I've found in some forum:

Code:

function string.fromhex(str)
   return (str:gsub('..', function (cc)
       return string.char(tonumber(cc, 16))
   end))
end

function string.tohex(str)
   return (str:gsub('.', function (c)
       return string.format('%02X', string.byte(c))
   end))
end

but unfortunately after using string.fromhex(str) for data from the serial port I have such error:
bad argument #1 to '?' (number expected, got nil)

and this is raw data from the serial port when I've log it via log(data)
string: O�C��C����� �