И да и нет. Open circuit driver voltage, with respect to signal
ground, must not exceed 25 volts with respect to ground. The
potential at the interface point must not be less than 5 volts nor more than 15 volts in magnitude when the terminator resistance is between 3000 and 7000 ohms.
http://duinorasp.hansotten.com/uploads/2740_EIA_RS-232-C.pdf