▷ What is a serial port and parallel port: technical level and differences

Serial port and parallel port is something that we have all heard many times, but it is very likely that the youngest do not even know what these ports have been with us for so many years. In this article we explain what the serial port is and what the parallel port is, as well as its differences.

Index of contents

What is a serial port and a parallel port

In computing, a serial port is a serial communication interface through which information is transferred on or off one bit at a time, in contrast to a parallel port. Throughout most of the history of personal computers, data was transferred through serial ports to devices such as modems, terminals, and various peripherals.

We recommend reading our article on How to connect with remote desktop Windows 10

The importance of the serial port and its operation

While interfaces such as Ethernet, FireWire, and USB send data as a serial stream, the term "serial port" generally identifies hardware more or less compliant with the RS-232 standard, intended to interact with a modem or device. of similar communication. Modern computers without serial ports may require USB to serial converters to allow compatibility with RS-232 serial devices. Serial ports are still used in applications such as industrial automation systems, scientific instruments, point of sale systems, and some industrial and consumer products.

Servers can use a serial port as a control console for diagnostics. Network equipment such as routers and switches often use a serial console for configuration. Serial ports are still used in these areas, as they are simple, inexpensive, and their console functions are highly standardized and widespread. A serial port requires very little support software from the host system.

Some computers, such as the IBM PC, use an integrated circuit called UART. This IC converts characters to and from the asynchronous string form, implementing synchronization and framing of the data in the hardware. Very low-cost systems, like some of the first home computers, would use the CPU to send the data through an output pin, using the bit banging technique. Before large-scale integration (LSI) UART integrated circuits were common, a minicomputer or microcomputer would have a serial port consisting of multiple, small-scale integrated circuits to implement shift registers, logic gates, counters, and all other logic for a serial port.

Low-cost processors now allow faster, but more complex, serial communication standards such as USB and FireWire to replace RS-232. This makes it possible to connect devices that would not have feasibly operated on slower serial connections, such as mass storage, sound, and video devices. Many motherboards still have at least one serial port, even if it can be accessed only through a pin header. Small form factor systems and notebooks can bypass the RS-232 connector ports to save space, but the electronics are still there. RS-232 has been standard for so long that the circuits needed to control a serial port became very cheap and often exist on a single chip, sometimes also with circuits for a parallel port.

While the RS-232 standard originally specified a 25-pin D-type connector, many designers chose to implement only a subset of the full standard, swapping compatibility with the standard for the use of less expensive and more compact connectors (in particular, the DE-9 version used by the original IBM PC-AT). The desire to supply dual-port serial interface cards required IBM to reduce the size of the connector to fit on a single-card rear panel. A DE-9 connector also fits on a card with a second DB-25 connector. Starting around the time of the introduction of IBM PC-AT, serial ports were commonly built with a 9-pin connector to save on cost and space. However, the presence of a 9-pin sub-miniature D connector is not sufficient to indicate that the connection is actually a serial port, as this connector is also used for video, joysticks, and other purposes. Some miniaturized electronic devices, particularly graphing calculators and two-way and amateur radio laptops, have serial ports that use a phone jack, usually the smaller 2.5 or 3.5mm connectors, and use the more basic 3-wire interface.

Many Macintosh models prefer the related RS-422 standard, mostly with German mini-DIN connectors, except for the early models. Macintosh included a standard set of two ports for connection to a printer and a modem, but some PowerBook laptops had only one combo port to save space. Since most devices do not use all of the 20 signals defined by the standard, smaller connectors are often used.

For example, the 9-pin DE-9 connector is used by most IBM-compatible PCs from the IBM AT PC, and has been standardized as TIA-574. More recently, modular connectors have been used. The most common are 8P8C connectors, for which the EIA / TIA-561 standard defines a pinout, while the "Yost Serial Device Wiring Standard" invented by Dave Yost is common on Unix computers and newer devices from Cisco Systems. 10P10C connectors can also be found on some devices. Digital Equipment Corporation defined its own DECconnect connection system that is based on the Modular Modular Jack (MMJ) connector. This is a modular 6-pin connector where the key is displaced from the center position. As with the Yost standard, DECconnect uses a symmetrical pin design that allows direct connection between two DTEs. Another common connector is the common DH10 header connector on motherboards and add-in boards, which is generally cabled to the more standard 9-pin DE-9 connector (and is frequently mounted on a free slot board or other part of the case).

Parallel port works very differently

A parallel port is a type of interface found on PCs to connect peripherals. The name refers to the way in which data is sent, since parallel ports send multiple bits of data at the same time, in parallel communication, unlike serial interfaces that send bits one by one. To do this, parallel ports require multiple data lines on their port cables and connectors, and they tend to be larger than contemporary serial ports that only require one data line.

There are many types of parallel ports, but the term has been more closely associated with the printer port or Centronics port found on most personal computers from the 1970s through the 2000s. It was a de facto industry standard for many years and was finally standardized as IEEE 1284 in the late 1990s, which defined the two-way versions of Enhanced Parallel Parallel Port (EPP) and Extended Capacity Port (ECP) . Today, the parallel port interface is virtually non-existent due to the rise of universal serial bus (USB) devices, along with network printing using connected Ethernet and Wi-Fi printers.

The parallel port interface was originally known as the Parallel Printer Adapter on IBM PC compatible computers. It was primarily designed to operate printers that used IBM's eight-bit extended ASCII character set to print text, but could also be used to adapt other peripherals. Graphic printers, along with a number of other devices, have been designed to communicate with the system.

Before the advent of USB, the parallel interface was adapted to access a number of peripheral devices other than printers. One of the earliest uses of the parallel port was for dongles used as hardware keys that were supplied with the application software as a form of software copy protection. Other uses include optical disc drives such as CD players and writers, Zip drives, scanners, external modems, game controllers, and joysticks. Some of the earliest portable MP3 players required a parallel port connection to transfer songs to the device. Adapters were available to run SCSI devices through parallel. Other devices like EPROM programmers and hardware drivers could connect through the parallel port.

Most PC-compatible systems in the 1980s and 1990s had one to three ports, with communication interfaces defined this way:

• Logical parallel port 1: I / O port 0x3BC, IRQ 7 (usually on monochrome graphics adapters) Logical parallel port 2: I / O port 0x378, IRQ 7 (dedicated IO cards or via a controller built into the motherboard) Logical parallel port 3: I / O port 0x278, IRQ 5 (dedicated IO cards or via a controller built into the motherboard)

If there is no printer port at 0x3BC, the second port in row (0x378) becomes logical parallel port 1 and 0x278 becomes logical parallel port 2 for the BIOS. Sometimes the printer ports are bridged to share an interrupt despite having their own I / O addresses, i.e. only one can be used at a time. In some cases, the BIOS also supports a fourth printer port, but the base address differs significantly between providers. Since the reserved input for a fourth printer logical port in the BIOS Data Area is shared with other uses on PS / 2 machines and with S3-compatible graphics cards, it generally requires special drivers in most environments. Under DR-DOS 7.02, BIOS port assignments can be changed and overridden using the CONFT.SYS directives LPT1, LPT2, LPT3, and optionally LPT4.

DOS-based systems make the logical parallel ports detected by the BIOS available under device names such as LPT1, LPT2, or LPT3 corresponding to parallel logical port 1, 2, and 3, respectively. These names are derived from terms such as Line Print Terminal, Local Print Terminal or Line PrinTer. A similar naming convention was used on ITS, DEC, as well as CP / M and 86-DOS (LST) systems.

In DOS, parallel printers can be accessed directly from the command line. For example, the command "TYPE C: \ AUTOEXEC.BAT> LPT1:" would redirect the contents of the AUTOEXEC.BAT file to the printer port. A PRN device was also available as an alias for LPT1. Some operating systems like DOS allow you to change this fixed allocation by different means. Some versions of DOS use resident driver extensions provided by MODE, or users can change the mapping internally through a CONFIG.SYS PRN = n directive (as in DR-DOS 7.02 and higher). DR-DOS 7.02 also provides optional built-in support for LPT4 if the underlying BIOS supports it.

PRN, along with CON, AUX, and a few others are invalid file and directory names in DOS and Windows, even Windows XP. There is even an MS-DOS device in the path name vulnerability in Windows 95 and 98, which causes the computer to crash if the user types "C: \ CON \ CON", "C: \ PRN \ PRN" or " C: \ AUX \ AUX "in the address bar of Windows Explorer. Microsoft has released a patch to correct this error, but the newly installed Windows 95 and 98 operating systems will still have the error.

This ends our article on what is a serial port and parallel port, you can leave a comment if you have any questions.

Wikipediawikipedia Source