Parallel port

This article is about the Centronics style port. For the concept in general, see Parallel communications.

A DB-25 parallel printer port, as on IBM-PC style, and a few other types of computers.

Micro ribbon 36 pin female, such as on printers and on some (particularly industrial and early- and pre-1980s personal) computers.

A parallel port is a type of interface found on computers (personal and otherwise) for connecting various peripherals. It is also known as a printer port or Centronics port. The IEEE 1284 standard defines the bi-directional version of the port.

History

The Centronics Model 101 printer was introduced in 1970 and included the first parallel interface for printers. The interface was developed by Dr. An Wang, Robert Howard and Prentice Robinson at Wang Laboratories. The now-familiar connector was selected because Wang had a surplus stock of 20,000 Amphenol 36-pin micro ribbon connectors that were originally used for one of their early calculators. The Centronics parallel interface quickly became a de facto 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 systems used a DC-37 connector, NCR used the 36-pin micro ribbon connector, Texas Instruments used a 25-pin card edge connector and Data General 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 connector on the host side and a 50 pin connector on the printer side—either a DD-50 (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 50 feet (15 m) and a long-line version for connections from 50 feet (15 m) to 500 feet (150 m). 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 released the IBM Personal Computer in 1981 and included a variant of the Centronics interface— only IBM logo printers (rebranded from Epson) could be used with the IBM PC. IBM standardized the parallel cable with a DB25F 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 introduced their version of bidirectional, known as Bitronics, on the LaserJet 4 in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284 standard in 1994.

Uses

Before the advent of USB, 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 dongles used as a hardware key form of software copy protection. Zip drives and scanners were early implementations followed by external modems, sound cards, webcams, gamepads, joysticks and external hard disk drives and CD-ROM drives. Adapters were available to run SCSI devices via parallel. Other devices such as EPROM programmers and hardware controllers could be connected parallel.

Current use

At the consumer level, the USB interface—and in some cases Ethernet—has effectively replaced the parallel printer port. Many manufacturers of personal computers and laptops consider parallel to be a legacy port 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 #	Starting I/O	Ending I/O
LPT1	IRQ 7	0x378	0x37f
LPT2	IRQ 7	0x278	0x27f
LPT3	IRQ 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							LSB
	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 that did not use the Windows NT kernel (as well as DOS 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 and Unix (NetBSD, FreeBSD, Solaris, 386BSD, 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.

Pinouts

Pinouts 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 nibbles 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.

See also

• Parallel communications
• Parallel transmission
• IEEE 1284

References

• Axelson, Jan (2000). Parallel Port Complete. Lakeview Research. ISBN 0-9650819-1-5.
• The (Linux) Parallel Port Subsystem by Tim Waugh

External links

• Interfacing to the Standard Parallel Port
• Video on basics of standard parallel port
• Parallel Port programming and interfacing
• Parallel Port .NET Programming and Interface with Hardware
• Linux I/O port programming mini-HOWTO
• Parallel Port interfacing with Windows NT/2000/XP
• D-subminiature nomenclature and pinouts