Or is Arduino 5V ( that RS232 has) tolerant?
I also don't quite understand but in no way is the Arduino tolerant of RS-232 voltage levels.
There are a few ways to do this but the the best way I reckon is to use the chips designed for the job, ie MAX232 or equivalents.
I do not need the full ( true) RS232 protocol .Only convert voltage level
As Lefty said, give is a link to the device, we can probably sort it out for you.
Rob