If you have a printer connected to
your computer, there is a good chance that it uses the parallel port.
While USB is
becoming increasingly popular, the parallel port is still a commonly used
interface for printers.
A typical parallel port on the back of your computer
Parallel ports can be used to connect a host of popular computer
In this edition of
HowStuffWorks, you will learn why it is called the parallel port,
what it does and exactly how it operates.
Parallel Port Basics
Parallel ports were originally developed by
IBM as a way to connect a printer to your
PC. When IBM was in
the process of designing the PC, the company wanted the computer to work
with printers offered by Centronics, a top printer manufacturer at
the time. IBM decided not to use the same port interface on the computer
that Centronics used on the printer.
Instead, IBM engineers coupled a 25-pin connector, DB-25, with a
36-pin Centronics connector to create a special cable to connect the printer
to the computer. Other printer manufacturers ended up adopting the
Centronics interface, making this strange hybrid cable an unlikely de facto
When a PC sends data to a printer or other device using a parallel port,
it sends 8 bits of
data (1 byte) at a time. These 8 bits are transmitted parallel to
each other, as opposed to the same eight bits being transmitted serially
(all in a single row) through a
The standard parallel port is capable of sending 50 to 100 kilobytes of data
Let's take a closer look at what each pin does when used with a printer:
- Pin 1 carries the strobe signal. It maintains a level of
between 2.8 and 5
drops below 0.5 volts whenever the computer sends a byte of data. This
drop in voltage tells the printer that data is being sent.
- Pins 2 through 9 are used to carry data. To indicate that a bit
has a value of 1, a charge of 5 volts is sent through the correct pin. No
charge on a pin indicates a value of 0. This is a simple but highly
effective way to transmit digital information over an analog cable in
- Pin 10 sends the acknowledge signal from the printer to the
computer. Like Pin 1, it maintains a charge and drops the voltage below
0.5 volts to let the computer know that the data was received.
- If the printer is busy, it will charge Pin 11. Then, it will
drop the voltage below 0.5 volts to let the computer know it is ready to
receive more data.
- The printer lets the computer know if it is out of paper by
sending a charge on Pin 12.
- As long as the computer is receiving a charge on Pin 13, it knows that
the device is online.
- The computer sends an auto feed signal to the printer through
Pin 14 using a 5-volt charge.
- If the printer has any problems, it drops the voltage to less
than 0.5 volts on Pin 15 to let the computer know that there is an error.
- Whenever a new print job is ready, the computer drops the charge on
Pin 16 to initialize the printer.
- Pin 17 is used by the computer to remotely take the printer offline.
This is accomplished by sending a charge to the printer and maintaining it
as long as you want the printer offline.
- Pins 18-25 are grounds and are used as a reference signal for
the low (below 0.5 volts) charge.
Notice how the first 25 pins on the Centronics end match up with the pins
of the first connector. With each byte the parallel port sends out, a
handshaking signal is also sent so that the printer can
latch the byte.
The original specification for parallel ports was unidirectional, meaning
that data only traveled in one direction for each pin. With the introduction
of the PS/2 in 1987, IBM offered a new bidirectional parallel port
design. This mode is commonly known as Standard Parallel Port (SPP)
and has completely replaced the original design. Bidirectional communication
allows each device to receive data as well as transmit it. Many devices use
the eight pins (2 through 9) originally designated for data. Using the same
eight pins limits communication to half-duplex, meaning that
information can only travel in one direction at a time. But pins 18 through
25, originally just used as grounds, can be used as data pins also. This
allows for full-duplex (both directions at the same time)
Enhanced Parallel Port (EPP) was created by
Zenith in 1991. EPP allows for much more data, 500 kilobytes to 2
megabytes, to be transferred each second. It was targeted specifically for
non-printer devices that would attach to the parallel port, particularly
devices that needed the highest possible transfer rate.
Close on the heels of the introduction of EPP,
Hewlett Packard jointly announced a specification called Extended
Capabilities Port (ECP) in 1992. While EPP was geared toward other
devices, ECP was designed to provide improved speed and functionality for
In 1994, the IEEE 1284
standard was released. It included the two specifications for parallel port
devices, EPP and ECP. In order for them to work, both the
system and the device must support the required specification. This is
seldom a problem today since most computers support SPP, ECP and EPP and
will detect which mode needs to be used, depending on the attached device.
If you need to manually select a mode, you can do so through the
BIOS on most