The RS-232 defines the voltage levels that correspond to logical one and logical zero levels for the data transmission and the control signal lines.

Valid signals are either in the range of +3 to +15 volts or the range −3 to −15 volts with respect to the “Common Ground” (GND) pin; the range between −3 to +3 volts is not a valid RS-232 level.

For data transmission lines (TxD, RxD, and their secondary channel equivalents):

• Logic one is defined as a negative voltage (MARK).
• Logic zero is positive (SPACE)

Control signals have the opposite polarity:

• the active state is positive voltage
• the inactive state is negative voltage

The standard specifies a maximum open-circuit voltage of 25 volts.