Introduction to input-output ports
Input-output ports are material elements on the computer, allowing the system to communicate with exterior elements, in other words to exchange data, hence the name input-output interface (sometimes known as I/O interface).
Serial port
Serial ports (also called RS-232, after the name of the standard they refer to) represent the first interfaces to allow computers to exchange information with the "outside world". The term serial refers to data sent via a single wire: the bits are sent one after the other (refer to section on data transmission for a presentation on transmission modes).
Serial ports were originally able to only send data and not receive it, hence two-way ports were developed (the ports on current computers are two-way); two-way serial ports therefore need two wires for communication.
Serial communication takes place asynchronously, meaning that no synchronisation signal (or clock) is required: the data may be sent at random intervals. In return, the peripheral must be able to distinguish the characters (one character is 8 bits in length) among the succession of bits which is sent.
This is why, in this type of transmission, each character is preceded by a START bit and followed by a STOP bit. These control bits, which are needed for serial transmission, waste 20% of the bandwidth (for 10 bits sent, 8 are used to code the character and 2 are used for reception).
Serial ports are generally built into the mother board, which is why the connectors behind the casing and connected to the mother board by a wire cable can be used to connect an exterior element. Serial connectors generally have 9 or 25 pins and take the following form (DB9 and DB25 connectors respectively):
A personal computer generally has between one and four serial ports.
Parallel port
Parallel data transmission involves sending data simultaneously on several channels (wires). The parallel ports on personal computers can be used to send 8 bits (one octet) simultaneously via 8 wires.
The first two-way parallel ports allowed for speeds of 2.4Mb/s. Enhanced parallel ports have been developed however to achieve higher speeds:
The EPP (Enhanced Parallel Port) achieves speeds of 8 to 16 Mbps
The ECP (Enhanced Capabilities Port), developed by Hewlett Packard and Microsoft. It has the same characteristics as the EPP with in addition a Plug and Play feature, allowing the computer to recognise the connected peripherals.
Parallel ports, like serial ports, are built into the mother board. DB25 connectors allow connection to an exterior element (e.g. a printer).
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