## ASCII character format

#### The asynchronous method of transmission is very effective when the information to be sent is generated at irregular intervals, for example, entering letters from a keyboard, where there is no prior knowledge of when the keys will be pressed. The data receiver must therefore be alerted to the fact that a new data message is arriving and also when the message ends so that it can enter an idle mode (often called marking).

#### Shown here is the common format used for sending ASCII characters over a modem link. When the link is inactive, the data line is kept high; it is pulled low for one bit period to bring the data receiver out of its idle mode. This is called the 'start bit'. Following the start bit are the seven bits representing the ASCII character set, followed by a parity check bit. Finally, one or more stop bits are added forcing the data line high, mimicking the idle state, such that the receiver is then ready to detect the presence of a new start bit. |

#### In an asynchronous receiver, the data bits are sampled at a rate many times the bit rate with typically 16 clock cycles per data bit. Once a transition from high to low representing a start bit is detected, the data is then sampled on the eighth clock cycle, with the expectation that this will be close to the centre of the bit period and hence a reliable measure.