Serial line

A serial line is a means of transmitting data in bit serial fashion, often over a communication link such as a telephone line. There are two principal forms:

• asynchronous serial lines, in which the format is self-clocking (i.e. the start location of each character can be inferred from the data stream), and
• synchronous serial lines, where each character starts immediately after the previous one, and is therefore not self-clocking

Both serial line protocols requires two signal levels: idle (mark, high), and asserted (space, low). (The polarity is a legacy from telegraphy, where the line was held high in order to show that the line was not broken, and that the transmitter was functional.)