What is a Stop Bit?

A stop bit is a character in asynchronous communication that lets a receiver know that the byte being transmitted has ended. This is vital to the way most information is transmitted over the Internet. Without a stop bit, it is possible that a receiver's computer would likely prompt an error message and make further communication with the sending computer problematic.

Asynchronous communication means that data is transmitted without any external clock. As such, a receiving computer requires knowing when the byte ends in order to get "ready" to receive the next byte. In addition to a stop bit, the other piece of the puzzle is the start bit, which signals the beginning of a new piece of information.

Information transmitted over the Internet is broken down into blocks of data, known as bytes, for transmission. Bytes normally include eight bits of data, plus a start bit and stop bit for a 10-bit character frame. These bytes are then assembled to present the data in its original context. Bytes can be parts of a computer program, or something as simple as words in an e-mail.

Curiously, while a stop and start bit are normally associated with modern technology, including state-of-the-art data transmission, the two terms have their roots in older communications technologies. For example, teletype machines in the late 19th and early 20th centuries had to be resynchronized after the start of each character, called codes. These codes could often include more than one word. However, after each code was transmitted, a stop bit would also be transmitted to indicate to the receiving team that it was time to resynchronize.

In some cases, older teletype machines may have required more than one stop bit. However, in modern asynchronous communication, no more than one stop bit is needed. This may confuse some computer users who stumble onto the Windows communications configuration panel, especially in computers with older operating systems. A myriad of choices are presented, most of which make no sense by today's data transmission standards.

If a stop bit is not present for a computer receiving data, an error message will likely come up. This error is called a framing error and may be caused by the bytes not being properly formatted. It may require the attention of an IT professional. Usually, a framing error is the fault of the sending machine, not the receiving, though it is possible signal degradation could also cause a framing error.