RS-232 defines the voltages and circuits used by all serial connections.
Binary 1 (mark condition) is defined as any voltage between -3V and -25V. Lines with voltages in this range are considered to be off.
Binary 0 (space condition) is defined as any voltage between +3V and +25V. Lines with voltages in this range are considered to be on.
Voltages between -3V and +3V are undefined (neither on nor off).
RS-232 also specifies data transmission over a maximum cable length of 50 feet at 20,000 bps. (In practice, greater lengths and higher speeds are possible.)