What is the difference between a serial port and a parallel port?

The serial ports and parallel ports they are an example of technology that was once avant-garde, since in their day they were the most used connections for data transfer.

Currently, both have been superseded by USB ports, such as micro USB, fiber optics, ethernet connections or wireless connections such as WiFi or Bluetooth.

Very few devices manufactured today include a serial or parallel portand for old computers that only have these types of ports, it is usually necessary to use adapters to be able to use most modern peripherals.

The main difference between a serial port and a parallel port is the how information is transferred. The serial port transmits the information bitwise, one after another. Parallel ports, on the other hand, can transmit multiple bits in parallel through various channels.

In this way, a parallel port can transfer 8 bits at a time, which is equivalent to 1 byte (1 byte = 8 bits), although there are parallel ports of variable capacity.

Parallel and serial transfer
Parallel and serial bit transfer scheme

parallel ports

The parallel ports They generally have a 25-pin connector, each with a different function. In each connector transfer 1 byte (8 bits at a time), which makes them faster than serial ports.

Parallel ports were commonly called printer ports, as this function is the most common since they began to be used. Some devices, like the first external modems or old external storage devices, also used the parallel port for connection to the computer.

The parallel port is older than the serial port being first used in 1970 on the Centronics Model 101 printer. This first parallel port allowed one-way data transmission from a source to a secondary device, for example, from a computer to a printer.

After the parallel ports developed by Centronics, other types of parallel ports were developed that allowed communication in two directions, for example the IDE port (or Parallel ATA, abbreviated PATA) used for the connection of hard drives, optical readers/recorders (CD/DVD/Blu-Ray), etc.

One of the main drawbacks of parallel ports was that for years they did not have a standardized design. Each company manufactured parallel ports with their own specifications that were not compatible with other companies' devices.

For example, the first parallel ports manufactured by IBM in 1981, known as Parallel Printer Adapterthey only worked on IBM and compatible PCs.

The first parallel port standard was the IEEE 1284from the late 1990s, and defined five versions or modes:

  • compatibility mode: Also known as the Centronics standard or SPP, it is an implementation of unidirectional parallel ports. They were used almost exclusively in printers, and the printer could only transmit status information back to the computer, for example the "out of paper" error.
  • nibbling mode: introduced by HP, allowed to transmit 4 bits of data (a nibble).
  • byte mode: Also known as bi-directional mode, it can transmit 8 bits at a time (1 byte).
  • EPP mode (Enhanced Parallel Port): bi-directional implementation that allows the transmission of larger amounts of data and the rapid change of the direction of transmission in each channel.
  • ECP mode (Extended Capability Port): Improved EPP implementation with faster transmission than EPP mode by using, among other improvements, direct memory access on channel 3.

serial ports

The serial port, or serial port, transmits information bit by bit, one after another, unlike the parallel port that can transmit several bits at once. Consequently, serial ports are slower than parallel ports.

Although many other types of connections use a serial transmission streamfor example ethernet and USB, the term serial port is used to refer to hardware connections using the RS-232 standard and its variants.

Serial ports can be built with 25-pin connectors, but not all pins are typically used. Initially, the 9-pin serial ports were the most common because they were believed to be more compact, effective and less expensive.

Currently they are still widely used, although in the field of personal computers they have been greatly reduced. For example, serial ports are common in industrial automation systems, certain scientific instruments, and point-of-sale terminals.

Go up