...live long and prosper...
The parallel port is the swiss-army knife of hardware.
Here's an inside view,
hardware- and software-wise!
Parallel port explained
#/computronics/lcd/lpt/.
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Overview

The parallel port, also known as the "line printer port", started its career as exactly that: a cheap interface for line printers. Of course, back then, there wasn't any other kind!

As opposed to other computer design approaches, IBM decided to decouple the printer interface from the CPU's physical bus. Instead, they created a simple IO device, following the standard then known as "Centronics interface", traditionally mapped at address 0x378 (or 0x3BC for those equipped with an MDA video card). Additionally, they provided relevant routines through the nowadays obsolete BIOS interrupt 17h.

As time passed, the interface got further enhancements and subsequently got standardised through IEEE, to become what is formally known as the "IEEE 1284". However, the basic functionality still remains intact.

Plugs and numbering

Traditionally, the parallel port is utilised on a plain D-sub-25 female socket at the back of the computer. Consequently, all cables come with a male D-sub-25 plug on one end, with the other end varying between male or female D-sub-25 or male Centronics 36-pin.
The numbering is as follows:

Female socket, as seen on computers.
Female
Male socket, usually seen on cables.
Male

Notice that the pin numbering is reverse on the two sockets. This is very important when making our own cable. Conveniently enough, though, most spare plugs have the pin numbers embossed on the plastic insulator inside the plug itself, saving us the trouble of identifying the pins manually.

Signals

Each pin, naturally, corresponds to a discrete signal. Since the primary use of the parallel port was to communicate with printers, the signals were tailored to serve that functionality.
Below is a table which provides a primary analysis of the signals, along with pin numbers and signal names and logic. Note that a forward slash ( / ) in front of the signal name denotes negative logic.

  Pin # Signal Direction Description
1 /STR OUT Strobe - data transfer on
2 - 9 D0 - D7 OUT Data lines
10 /ACK IN Printer ready for next character
11 /BUSY IN Printer is busy
12 PE IN Printer out of paper
13 SLCT IN Printer on-line
14 /AF OUT Auto line feed (LF+CD)
15 /ERROR IN Error
16 INIT OUT Initiate printer reset
17 /SLCT_IN OUT Turn printer on-line
18 - 25 GND - Ground lines

Addresses and registers

The basic implementation of the parallel port organises these signals into three 8-bit registers. Each register has a unique IO address and a name. These three registers, which cover the basic parallel port functionality, occupy three sequential addresses starting from the port's base address.
Here is a bit-wise interpretation of those registers, with the corresponding port pin for each bit. Note that some signals are negative, which means that the function described is activated on "0". Do not confuse this negativity with the previous paragraph's negative signals, where the meaning is that writing a "1" on the register bit will output a "low" at the corresponding pin.

Data register
7 6 5 4 3 2 1 0
. . . . . . . . . Signal Pin # Function
D7 : D0 9 : 2 Data lines
Write only; address = baseaddr; LPT1 address: 0x378

Status register
7 6 5 4 3 2 1 0
. . . . . . . . . Signal Pin # Function
/ERROR 15 0: Error
SLCT 13 1: Printer on-line (selected)
PE 12 1: Printer out of paper
/ACK 10 0: Printer ready for next character
/BUSY 11 0: Printer is busy
Read only; address = baseaddr + 1; LPT1 address: 0x379

Control register
7 6 5 4 3 2 1 0
. . . . . . . . . Signal Pin # Function
/STROBE 1 0: Data transfer on
/AUTO FEED 14 1: LF plus CD
/INIT 16 0: Initiate printer reset
SLCT IN 17 1: Turn printer on-line
IRQ ENABLE - 1: Execute interrupt when /ACK=0
Write only; address = baseaddr + 2; LPT1 address: 0x37A

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