There are two methods of serial data transmission for correct work and possible transmission errors detection - they are the synchronous and asynchronous methods. Today the asynchronous transmission method is widely spread.

When the connection is asynchronous the PC sends or receives data bytes bitwise. The time intervals between bytes are here inessential but timing between separate bits of the byte is very important. The line sygnal can be of high or low level which corresponds to logical 0 and 1 and means that the line is marking, when the level is high , or spacing, when the level is low.

The line is in the marked state, when there's no data transmission. When the data byte transmission starts the signal goes into 0, marking the start bit. Then go eight data bits (sometimes less) presented as a set of high and low levels. The last data bit can be accompanied by the parity bit used for errors detection, then 1 ore more stop bits are included into succession (high level corresponds to these stop bits). These stop bits start the marked state which will be preserved until the transmission of the next data byte starts.The number of stop bits is important as they set minimum time to pass before the next start bit. This succession is shown in the picture. Of course the seding and receiving stations must use the same protocol for these bit trains and their baud rates must be the same (measured in bits per second, also called bauds). There may be errors when transmitting/receiving data, so communications equipment gives various information about the port state as well as state of the modem connected to it.