How Do Parallel Ports Work??

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.
Parallel ports can be used to connect a host of popular computer peripherals:
• Printers
• Scanners
• CD burners
• External hard drives
• Iomega Zip removable drives
• Network adapters
• Tape backup drives
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 standard.
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 serial port. The
standard parallel port is capable of sending 50 to 100 kilobytes of data per second.

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 volts, but
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 real-time.
• 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.
SPP/EPP/ECP
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) communication.
Enhanced Parallel Port (EPP) was created by Intel, Xircom and 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
storage devices that needed the highest possible transfer rate.
Close on the heels of the introduction of EPP, Microsoft and 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 printers.
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 operating 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 computers.