In odd parity, a bit is sent so that the entire frame always contains an odd number of 1 bits. Conversely, in even parity, the parity bit results in an even number of 1 bits. No parity means that no additional bit is sent. To signify the end of the character frame, the sender places the line back to the mark state for a minimum specified time interval. This signal is known as the stop bit, and returns the transmission line back to the mark state.
Electrical lines are always subject to environmental perturbations known as noise.
This noise can cause errors in transmission by altering voltage levels so that a bit is reversed, shortened, or lengthened. When this occurs, the ability of the receiver to distinguish a character frame may be affected, resulting in a framing error. The break signal is a special signal that falls outside the character frame. The break signal occurs when the line is switched from the mark state to a space and held there for longer than a character frame. The electrical characteristics of a serial communications connection are specified by various interfacing standards.