VOLTAGES INVOLVED IN RS-232

 Voltages between 3 to 25 volts are considered ON, Spacing, or
 Binary 0.

 Voltages between -25 to -3 volts are considered OFF, Marking or
 Binary 1.

 In theory RS232C interfaces are robust enough that you can short
 any two pins together without smoke pouring out, but don't count
 on it.