Parallel port

Parallel Port

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Parallel port

A parallel port is a type of interface found on computers (personal

Personal computer

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A parallel port is a type of interface found on computers (personal

Personal computer

A personal computer is any general-purpose computer whose original sales price, size, and capabilities make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator....
and otherwise) for connecting various peripherals. It is also known as a printer port or Centronics port
Centronics

Centronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered primarily for the Centronics printer port that bears its name....
. The IEEE 1284

IEEE 1284

IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices. In the 1970s, Centronics developed the now-familiar printer parallel port that soon became a de facto standard....
standard defines the bi-directional version of the port.

History

The Centronics

Centronics

Centronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered primarily for the Centronics printer port that bears its name....
Model 101 printer was introduced in 1970 and included the first parallel interface for printers. The interface was developed by Dr. An Wang

An Wang

Dr. An Wang was a Chinese American computer engineer and inventor, and co-founder of computer company Wang Laboratories....
, Robert Howard and Prentice Robinson at Wang Laboratories

Wang Laboratories

Wang Laboratories was a computer company founded in 1951 by Dr. An Wang and Dr. G. Y. Chu. The company was successively headquartered in Cambridge, Massachusetts , Tewksbury, Massachusetts , and Lowell, Massachusetts ....
. The now-familiar connector was selected because Wang had a surplus stock of 20,000 Amphenol

Amphenol

Amphenol Corporation is a major producer of electronic and fiber optic connectors, cable and interconnect systems. Amphenol is a portmanteau from the corporation's original name, American Phenolic Corp....
36-pin micro ribbon

Micro ribbon

The micro ribbon connector is a common type of electrical connector used particularly in computer and telecommunications applications. Popularly referred to as a Centronics connector due to the widely used Centronics parallel interface, it is also known as a Telco, Amphenol, 25-pair, Delta ribbon or CHAMP conn...
connectors that were originally used for one of their early calculators. The Centronics parallel interface quickly became a de facto

De facto

De facto is a Latin expression that means "concerning the fact" or in practice but not necessarily ordained by law. It is commonly used in contrast to de jure when referring to matters of law, governance, or technique that are found in the common experience as created or developed without or contrary to a regulation....
industry standard; manufacturers of the time tended to use various connectors on the system side, so a variety of cables were required. For example, early VAX

VAX

VAX was an instruction set architecture developed by Digital Equipment Corporation in the mid-1970s. A 32-bit complex instruction set computer ISA, it was designed to extend or replace DEC's various Programmed Data Processor ISAs....
systems used a DC-37

D-subminiature

NCR Corporation

NCR Corporation is a technology company specializing in products for the retail and financial sectors. Its main products are point of sale, automatic teller machines, cheque processing systems, barcode reader, and business consumables....
used the 36-pin micro ribbon connector, Texas Instruments

Texas Instruments

Texas Instruments , better known in the electronics industry as TI, is an United States company based in Dallas, Texas, Texas, United States, renowned for developing and commercializing semiconductor and computer technology....
used a 25-pin card edge connector and Data General

Data General

Data General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation....
used a 50-pin micro ribbon connector.

Dataproducts introduced a very different implementation of the parallel interface for their printers. It used a DC-37

D-subminiature

The D-subminiature or D-sub is a common type of electrical connector used particularly in computers. Calling them "subminiature" was appropriate when they were first introduced, but today they are among the largest common connectors used in computers....
(sometimes incorrectly referred to as a "DB50") or the block shaped M-50 connector; the M-50 was also referred to as Winchester. Dataproducts parallel was available in a short-line for connections up to and a long-line version for connections from to . The Dataproducts interface was found on many mainframe systems up through the 1990s, and many printer manufacturers offered the Dataproducts interface as an option.

IBM

IBM

International Business Machines Corporation, abbreviated IBM and nicknamed "Big Blue" , is a multinational corporation computer technology and consulting corporation headquartered in Armonk, New York, New York, United States....
released the IBM Personal Computer in 1981 and included a variant of the Centronics interface— only IBM logo printers (rebranded

Original Equipment Manufacturer

OEM stands for "Original Equipment Manufacturer".An original equipment manufacturer, or OEM is typically a company that uses a component made by a second company in its own product, or sells the product of the second company under its own brand....
from Epson) could be used with the IBM PC. IBM standardized the parallel cable with a DB25F

D-subminiature

The D-subminiature or D-sub is a common type of electrical connector used particularly in computers. Calling them "subminiature" was appropriate when they were first introduced, but today they are among the largest common connectors used in computers....
connector on the PC side and the Centronics connector on the printer side. Vendors soon released printers compatible with both standard Centronics and the IBM implementation.

IBM implemented an early form of bidirectional interface in 1987. HP

Hewlett-Packard

The Hewlett-Packard Company , commonly referred to as HP, is a technology corporation headquartered in Palo Alto, California, United States....
introduced their version of bidirectional, known as Bitronics, on the LaserJet 4

HP LaserJet 4

The HP LaserJet 4 is a group of monochrome laser printers produced in the early to mid-1990s as part of the LaserJet series by Hewlett Packard ....
in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284

IEEE 1284

Uses

Before the advent of USB

Universal Serial Bus

In information technology, Universal Serial Bus is a Serial communications computer bus standard to electrical connector devices to a host computer....
, the parallel interface was adapted to access a number of peripheral devices other than printers. Probably one of the earliest devices to use parallel were dongle

Dongle

A dongle is a small piece of Computer hardware that connects to a computer. Electrically dongles mostly appear as two-interface security tokens with transient data flow that does not interfere with other dongle functions and a pull communication that reads security data from the dongle....
s used as a hardware key form of software copy protection. Zip drives and scanners

Image scanner

In computing, a scanner is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass window for scanning....
were early implementations followed by external modem

Modem

Modem is a peripheral device that modulation an analog carrier wave Signal to encode digital information, and also demodulation such a carrier signal to decode the transmitted information....
s, sound card

Sound card

A sound card is a computer expansion card that facilitates the input and output of sound to/from a computer under control of computer programs....
s, webcam

Webcam

File:Logitech E2500 webcam.jpgWebcams are video capture connected to computer or computer network, often using Universal Serial Bus or, if they connect to networks, ethernet or Wi-Fi....
s, gamepad

Gamepad

A gamepad, also called joypad or control pad, is a type of game controller held in the hand, where the digits are used to provide input....
s, joystick

Joystick

A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. Joysticks are often used to control video games, and usually have one or more push-buttons whose state can also be read by the computer....
s and external hard disk drives and CD-ROM

CD-ROM

CD-ROM is a pre-pressed Compact Disc that contains Computer data storage accessible to, but not writable by, a computer. While the Compact Disc format was originally designed for music storage and playback, the 1985 Yellow Book standard developed by Sony and Philips adapted the format to hold any form of Binary file....
drives. Adapters were available to run SCSI

SCSI

Small Computer System Interface, or SCSI , is a set of standards for physically connecting and transferring data between computers and peripheral devices....
devices via parallel. Other devices such as EPROM

EPROM

An EPROM, or Erasable Programmable Read Only Memory, is a type of memory integrated circuit that retains its data when its power supply is switched off....
programmers and hardware controllers could be connected parallel.

Current use

At the consumer level, the USB

Universal Serial Bus

In information technology, Universal Serial Bus is a Serial communications computer bus standard to electrical connector devices to a host computer....
interface—and in some cases Ethernet

Ethernet

Ethernet is a family of Data frame-based computer networking technologies for local area networks . The name comes from the physical concept of the Luminiferous aether....
—has effectively replaced the parallel printer port. Many manufacturers of personal computers and laptops consider parallel to be a legacy port

Legacy port

A legacy port is a port or connector on a IBM PC clone that is considered fully or partially obsolete. Some manufacturers, particularly in laptops, remove the legacy ports, making way for modern ports such as Universal Serial Bus....
and no longer include the parallel interface. USB to parallel adapters are available to use parallel-only printers with USB-only systems. However, due to the simplicity of its implementation, it is often used for interfacing with custom-made peripherals.

Implementation on IBM personal computers

Port addresses

Traditionally IBM PC systems have allocated their first three parallel ports according to the configuration in the table below.

PORT NAME	Interrupt Interrupt In computing, an interrupt is an asynchronous communication signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change in execution.... #	Starting I/O	Ending I/O
`LPT1`	`IRQInterrupt request The computing phrase "interrupt request" is used to refer to either the act of interrupting the Computer bus lines used to signal an interrupt, or the interrupt input lines on a Programmable Interrupt Controller .... 7`	`0x378`	`0x37f`
`LPT2`	`IRQInterrupt request The computing phrase "interrupt request" is used to refer to either the act of interrupting the Computer bus lines used to signal an interrupt, or the interrupt input lines on a Programmable Interrupt Controller .... 7`	`0x278`	`0x27f`
`LPT3`	`IRQInterrupt request The computing phrase "interrupt request" is used to refer to either the act of interrupting the Computer bus lines used to signal an interrupt, or the interrupt input lines on a Programmable Interrupt Controller .... 5`	`0x3bc`	`0x3bf`

If there is an unused LPTx slot, the port addresses of the others are moved up. (For example, if a port at 0x3bc does not exist, the port at 0x378 will then become LPT1.) The IRQ lines, however, remain fixed (therefore, 0x378 at LPT1 would use IRQ 7). Unfortunately the default IRQ used by the first two addresses is the same, and it's difficult to get correct interrupt behaviour if both of these addresses are in use. The port addresses assigned to each LPTx slot can be determined by reading the BIOS Data Area (BDA) at 0000:0408.

Bit to Pin Mapping for the Standard Parallel Port (SPP):

Address		MSB Most significant bit In computing, the most significant bit is the bit position in a Binary numeral system having the greatest value. The msb is sometimes referred to as the left-most bit on big-endian architectures, due to the convention in positional notation of writing more significant digits further to the left....							LSB Least significant bit In computing, the least significant bit is the bit position in a Binary numeral system integer giving the units value, that is, determining whether the number is even or odd....
	Bit:	7	6	5	4	3	2	1	0
`Base (Data port)`	Pin:	9	8	7	6	5	4	3	2
`Base+1 (Status port)`	Pin:	~11	10	12	13	15
`Base+2 (Control port)`	Pin:					~17	16	~14	~1

~ indicates a hardware inversion of the bit.

Program interface

In versions of Windows

Microsoft Windows

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces ....
that did not use the Windows NT

Windows NT

Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was originally designed to be a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix....
kernel (as well as DOS

DOS

DOS, short for "Disk Operating System", is a shorthand term for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions Windows 95, Windows 98, and Windows Me....
and some other operating systems), programs could access the parallel port with simple outportb and inportb subroutine commands. In operating systems such as Windows NT

Windows NT

Unix

Unix is a computer operating system originally developed in 1969 by a group of American Telephone & Telegraph employees at Bell Labs, including Ken Thompson , Dennis Ritchie, Douglas McIlroy, and Joe Ossanna....
(NetBSD

NetBSD

NetBSD is a freely redistributable, open source version of the Unix-derivative Berkeley Software Distribution computer operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed....
, FreeBSD

FreeBSD

FreeBSD is a Unix-like free software operating system descended from AT&T Unix via the Berkeley Software Distribution branch through the 386BSD and Berkeley Software Distribution#4.4BSD and descendants operating systems....
, Solaris, 386BSD

386BSD

386BSD, sometimes called "JOLIX", was a Free software Berkeley Software Distribution Unix operating system first released in 1992. It ran on PC compatible computer systems based on the Intel 80386 microprocessor....
, etc), the microprocessor is operated in a different security ring, and access to the parallel port is inhibited, unless using the required driver. This improves security and arbitration of device contention. On Linux, inb and outb can be used when a process is run as root and an ioperm command is used to allow access to its base address

Base address

In computing, a base address is an address serving as a reference point for other addresses.In computers using relative addressing scheme, to obtain an absolute address, the relevant base address is taken and Offset is added to it....
.

Pinouts

Pinout

In electronics, a pinout is a cross-reference between the contacts, or pins, of an electrical connector, and their functions....
s for a parallel port connectors are:

Pin No (DB25)	Pin No (36 pin)	Signal name	Direction	Register - bit	Inverted
1	1	nStrobe	Out	Control-0	Yes
2	2	Data0	In/Out	Data-0	No
3	3	Data1	In/Out	Data-1	No
4	4	Data2	In/Out	Data-2	No
5	5	Data3	In/Out	Data-3	No
6	6	Data4	In/Out	Data-4	No
7	7	Data5	In/Out	Data-5	No
8	8	Data6	In/Out	Data-6	No
9	9	Data7	In/Out	Data-7	No
10	10	nAck	In	Status-6	No
11	11	Busy	In	Status-7	Yes
12	12	Paper-Out	In	Status-5	No
13	13	Select	In	Status-4	No
14	14	Linefeed	Out	Control-1	Yes
15	32	nError	In	Status-3	No
16	31	nInitialize	Out	Control-2	No
17	36	nSelect-Printer	Out	Control-3	Yes
18-25	19-30,33,17,16	Ground	-	-	-

Unidirectional parallel ports

In early parallel ports the data lines were unidirectional (data out only) so it was not easily possible to feed data in to the computer. However, a workaround was possible by using 4 of the 5 status lines. A circuit could be constructed to split each 8-bit byte into two 4-bit nibble

Nibble

A nibble is the computing term for a four-bit aggregation, or half an octet . As a nibble contains 4 bits, there are sixteen possible values, so a nibble corresponds to a single hexadecimal digit ....
s which were fed in sequentially through the status lines. Each pair of nibbles was then re-combined into an 8-bit byte. This same method (with the splitting and recombining done in software) was also used to transfer data between PCs using a laplink cable

LapLink cable

A LapLink cable is a cable that allows one to connect two computers together to establish a direct cable connection. The connection is achieved via the parallel ports on the two computers....
.

Parallel port

History

Uses

Current use

Implementation on IBM personal computers

Port addresses

Program interface

Pinouts

Unidirectional parallel ports

See also

External links