The COM port is the serial port in personalcomputers(PCs) for communicating with PDAs, modems, computer mouse, or other serial peripherals.
Personal computers have at least two serial communication ports, but four by default. These are designated COM1 through COM4. As connectors, the COM port uses Sub-D connectors in both the 9- pin and 15-pin versions.
The data transfer ratesare determined by the width of the data bus and the UART, which handles the parallel/serial conversion. They are between 75 bit/s and more than 100 kbit/s.
The bidirectional COM interface has only two data lines for transmit data( TxD) and receive data( RxD), the other interface lines are for control and message signals.