Obviously, the MAX3232 requires 5 volts to operate.
Can I draw them from Spark pins when the Core is powered by USB? Can I power the MAX3232 with a different (5V) power source and it will work? Or is there any other way? Thank you very much.
Good news: I can confirm that the MAX3232 level shifter for serial communications work with both 3.3 and 5 Volts directly from the breadboard (powered by USB).