Understanding Serial Ports

A serial port is either an input or output port that supports serial communication. Serial communication is the process of transmitting and processing data one bit at a time.

Data transmitted in a serial fashion can be sent by using one of two methods: synchronous data transmission or asynchronous data transmission.

In synchronous data transmission, a clock signal regulates the flow of data over a cable or wire. This transmission method is used when large amounts of data must be transferred in a short period of time.

Asynchronous data transmission uses a single information bit, referred to as the start bit, to tell the computer when to start transmitting data and a stop bit to tell it when to stop. This method is used when transferring smaller amounts of data. Both the computer sending the data and the one receiving the data must agree on the number of start and stop bits for communication to take place.

serial communication

The transmission of data one bit at a time.

clock signal

Controls the rate at which synchronous data is transmitted.

start bit

The bit that synchronizes the clock on the computer receiving the data. In asynchronous data transmission, the start bit is a space.

stop bit

The bit that identifies the end of the character being transmitted so that the character is clearly recognized.

Most PCs are manufactured with two serial ports that are used to connect serial devices, such as a mouse, modem, or line printer, by way of a serial cable.

Serial port connectors come in two types: 9-pin and 25-pin. A 9-pin connector is referenced as DB-9 and is a common serial connector for mice and handheld devices such as the PalmPilot. DB-25 refers to 25-pin connectors and is frequently used by external modems.