A serial port allows a PC or network devices to transmit or receive data one bit at a time. A serial interface is the oldest types of an interface that was also used to connect printers and external modems to a PC at that time. Modern serial ports are used in scientific instruments such as cash registers and applications like industrial machinery systems. The data transfer rate is very slow then the parallel port.
The serial port is used for point-to-point connection such as LAN-to-WAN connection. The LAN-to-WAN connection over a serial interface is also known as leased line connection because the line is usually leased from the telephone companies and the subscribers pay for those lines to hold the continuous connection between two sites.
Usually serial port is a male port and system resource configurations are chosen for each port and are recognized by COM1, COM2, COM3, COM4, and so forth. Each COM position represents an input/output (I/O) and an interrupt request (IRQ) address. The I/O address sends and receive packets to and from a peripheral device such as a mouse or keyboard or any network devices.
Using the serial connection on the network, we can send and receive data bits sequentially over a single channel. The serial port is bidirectional, and usually known as a bidirectional port or a communications port.
A serial connection was primarily used for signal exchange because of their bidirectional ability. Serial communications are less costly to implement because it is used fewer wires, cheaper cables, and fewer connector pins. On most computers, parallel ports and serial ports have been replaced by the higher speed serial Universal Serial Bus (USB) interfaces. Though, for long-distance communication, many WANs still uses serial transmission. This standard used for serial transmission between devices is RS 232 and usually called data communications equipment (DCE) and data terminal equipment (DTE). The serial port uses a nine-pin connector which is known as DB-9 connector.
Today the Higher speed communication port Universal Serial Bus (USB) and FireWire introduced faster interfaces; therefore, the serial port is hardly ever used but can be found as a communication device for Radios, GPS receivers, LED and LCD text displays, bar-code scanners and flat-screen monitors.
Serial Port DB-9 Connector
The figure below illustrates the serial connector DB-9. The figure shows each of the pins located on the DB9 connector. The Pin 1 starts from the top left and pin 9 is in the bottom right.
The serial ports carry data signals and control signals. To support these signal types including ground signals, RS-232 standard defines a 25-pin connection known as a parallel port. However, most PC’s and UNIX platforms use a 9-pin connection. Actually, only three pins are needed for serial port communications: pin for sending data, pin for receiving data, and pin for the signal ground. The table below illustrates the signals associated with the DB-9 pin connectors.
The RTS and CTS Pins
The RTS and CTS pins transmit signals that the devices are ready to send or receive data. It is providing hardware handshaking and control data loss during transmission. The TD pin is used to send data between DCE and DTE.
The DTR and DSR Pins
These Pins are used for showing that the device is connected and powered. These pins provide an alternative method of hardware handshaking.
The CD and RI Pins
The CD and RI pins are used to show different signals during modem to modem connections. The modem gets indication from CD pin that it is connected to another modem. RI is used to indicate the presence of an audible ringing signal.