COM Port Settings (DOS/Windows)

The COM port is the port through which computers communicate with devices such as modems and mice. Standard PCs have 4 COM ports, with COM ports 1 and 2 generally used by the computer for external ports. However, the four COM ports by default have 2 IRQs, with COM 1 and COM 3 sharing IRQ 4 and COM 2 and COM 4 sharing IRQ 3. This can lead to potential conflicts. However, the external ports of some I/O and controller cards as well as many devices are capable of using alternate IRQs. When assigning COM ports or IRQs to modems, it is important to make sure that no two devices will be using the same COM port or IRQ at the same time. Finally, COM ports communicating with modems must be given several settings by the communications software. The COM port needs to be told the number of bits per character, parity, the number of stop bits to be used, and the speed at which to communicate. The ASCII standard dictates codes up to the range of 255. This means that 8 bits are necessary to send all ASCII characters. Parity is a simple mechanism to detect characters corrupted by line noise. With today's error-correcting modems, this is unnecessary. Finally, stop bits are issued to separate bytes of data. Modern modems do not transfer stop bits, but stop bits are still generated. Because of this, it is wise to set the COM port at a speed higher than the modem is actually capable of transmitting, so the COM port does not slow itself due to useless bits. The COM port settings almost universally used today are 8-bit characters, no parity, and one stop bit.