Parallel port

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It has been suggested that this article or section be merged with Parallel communications. (Discuss)

It has been suggested that this article or section be merged with Parallel transmission. (Discuss)

In computing, a parallel port is a type of physical interface used in conjunction with a cable to connect separate peripherals in a computer system. Over a parallel port, binary information is transferred in parallel: each bit in a particular value is sent simultaneously as an electrical pulse across a separate wire (see parallel communications), in contrast to a serial port, which requires each bit to be sent in series over a single wire. The number of wires and the type of connector on a parallel port can vary.

Although there are many types of parallel interfaces, the unqualified term "parallel port" usually refers to a more or less IEEE 1284 compliant interface, also known as a "Centronics-style" interface.

Image:Parallel computer printer port.jpg

For the most part, the USB interface has replaced the Centronics-style parallel port — as of 2006, most modern printers are connected through a USB connection, and often don't even have a parallel port connection. On many modern computers, the parallel port is omitted for cost savings, and is considered to be a legacy port.

1 Uses
2 Port addresses
3 Interfacing to
4 References
5 External links

[edit] Uses

Printer
ZIP drive
peripheral devices such as EPROM programmers, emulators

[edit] 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`	`0x3bc`	`0x3bf`
`LPT2`	`IRQ 5`	`0x378`	`0x37f`
`LPT3`	`IRQ 5`	`0x278`	`0x27f`

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). 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`	Pin:	9	8	7	6	5	4	3	2
`Base+1`	Pin:	~11	10	12	13	15
`Base+2`	Pin:					~17	16	~14	~1

~ indicates a hardware inversion of the bit.

[edit] Interfacing to

In Windows 98 and previous operating systems, programs could access the parallel port with simple outportb() and inportb() subroutine commands. In Windows NT, 2000 and XP, the microprocessor is operated in a different security ring, and accesses to the parallel port are inhibited, unless using the required driver. This greatly improves the security, since previously hackers could gain access to the hard disk from the parallel port. However this also means that many legacy products will not work with later operating systems such as Windows XP.