Ports, Serial Ports, Serial Port Specifications

Serial Ports

All computers have a minimum of one or two serial ports and one parallel port. Counting the pins and determining if the port is male or female can identify ports. Serial ports were originally intended for input and output devices, and parallel ports were intended for printers

A serial port conforms to the standard interface called Standard 232 revision c (RS-232c) and is sometimes referred to as RS-232 port. Today, computers can have a 9-pin serial port, a 25-pin serial port, or both. Both ports work the same way. The 25-pin port uses only 9 pins; the other pins are unused.

Serial ports are almost always male ports, and parallel ports are almost always female ports.

The figure below shows a typical configuration that might be found on a PC:

One 9-pin male serial port, one 25-pin male serial port

One 25-pin female parallel port, one 15-pin female game port

The serial ports in the figure can be configured for COM1, COM2, COM3, or COM4.

The parallel ports can be configured as LPT1:, LPT2:, or LPT3:.

These ports are sometimes called DB-9 and DB-25 connectors. (DB stands for data bus and refers to the number of pins on the connector.)

Typically, the 9-pin serial port would be configured as COM 1, and the 25-pin serial port would be configured as COM 2. To know for sure, open the case and look for the cables leading from these ports, which should connect to connections on the systemboard or I/O card labeled COM 1 and COM 2. Possibly, the ports come directly off an I/O card, which, in this case, should have the ports labeled somewhere on the card.

Port Assignments

Each port is assigned a unique IRQ and a unique I/O address. The UART chip controlling the port determines the speed of the port. Serial ports are almost always male ports, and parallel ports are almost always female ports. The table below provides a list of ports and IRQs. The I/O addresses in the table are the memory addresses of the I/O Address Table where the address of the request handler is stored. The numbers are written in hex.

Port IRQ Type I/O Address

COM1 IRQ 4 Serial 03F8

COM2 IRQ 3 Serial 02F8

COM3 IRQ 4 Serial 03E8

COM4 IRQ 3 Serial 02E8

LPT1: IRQ 7 Parallel 0378

LPT2: IRQ 5 Parallel 0278

Serial Port Specifications

One of the 9 pins on a serial port transmits data in a sequence of bits, and a second pin receives data sequentially. The other 7 pins are used to establish the communications protocol.

A protocol is a set of agreed-upon rules for communication that is established before data is actually passed from one device to another. The protocol for using the 9 pins on the port is given in the table below.

The table also describes the functions of the pins of a serial port connection to a modem that is connected to another remote modem and computer. External modems sometimes use lights on the front panel to indicate the state of these pins. The labels on these modem lights are listed in the last column. When using serial ports, one of the devices is called the DTE (Data Terminal Equipment), and the other device is called the DCE (Data Communications Equipment). For example, for a modem, the modem is called the DCE and the computer to which it is installed is called the DTE.