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    How Parallel Ports Workby Jeff Tyson

    If you have a printer connected to your computer, there is a very good chance

    that it uses the parallel port. While USB is becoming increasingly popular, theparallel port is still the most used interface for printers.

    A typical parallel port on the back of your computer.Parallel ports are used to connect a host of popular computer peripherals:

    Printers Scanners CD-writers Externalhard drives Iomega Zip removable drives Network adapters Tape backup drives

    In this edition ofHow Stuff Works, you will learn why it is called the parallel port,what it does and just how it works.

    Parallel Port Basics

    Parallel ports were originally developed by IBM, as a way to connect a printer toyour PC. When IBM was in the process of designing the PC, the companywanted the computer to work with printersoffered by Centronics, a top printermanufacturer at the time. IBM decided notto use the same port interface on the

    computer that Centronics used on theprinter. Instead, IBM engineers coupled a 25-pin connector, DB-25, with a 36-pinCentronics connector to create a special cable to connect the printer to thecomputer. Other printer manufacturers ended up adopting the Centronicsinterface, 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 sendseightbits of data (one byte) at a time. These eight bits are transmitted parallel to(beside) each other, as opposed to the same eight bits being transmitted serially

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    (all in a single row) through a serial port. Thestandard parallel port is capable of sending 50to 100 kilobytes of data per second.Let's take a closer look at what each pin doeswhen used with a printer:

    Pin 1 carries the strobe signal. Itmaintains a level of between 2.8 and 5volts, but drops below .5 voltswhenever the computer sends a byteof data. This drop in voltage tells theprinter that data is being sent.

    Pins 2 through 9 are used to carrydata. To indicate that a bit has a valueof 1, a charge of 5 volts is sent throughthe correct pin. No charge on a pin

    indicates a value of 0. This is a simplebut highly effective way to transmitdigital information over an analog cablein real time.

    Pin 10 sends the acknowledge signalfrom the printer to the computer. LikePin 1, it maintains a charge and dropsthe voltage below .5 volts to let thecomputer know that the data wasreceived.

    If the printer is busy, it will charge Pin

    11. Then, it will drop the voltagebelow .5 volts to let the computer knowit is ready to receive more data.

    The printer lets the computer know if itis out of paper by sending a charge onPin 12.

    As long as the computer is receiving a charge on Pin 13, it knows that thedevice is online.

    The computer sends an auto feed signal to the printer through Pin 14using a 5 volt charge.

    If the printer has any problems, it drops the voltage to less than .5 volts onPin 15 to let the computer know that there is an error.

    Whenever a new print job is ready, the computer will drop the charge onPin 16 to initialize the printer.

    Pin 17 is used by the computer to remotely take the printeroffline. This is

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    accomplished by sending a charge to the printer and maintaining it as longas you want the printer offline.

    Pins 18-25 are grounds and are used as a reference signal for the low(below .5 volts) charge.

    Notice how the first 25 pins on the Centronics end match up with the pins of thefirst connector. With each byte the parallel port sends out, a handshaking signalis also sent so the printer can latch the byte.

    SPP/EPP/ECPThe original specification for parallel ports was unidirectional, meaning that dataonly traveled in one direction for each pin. With the introduction of the PS/2 in1987, IBM offered a new bidirectional parallel port design. This mode iscommonly known as Standard Parallel Port (SPP) and has completely replacedthe original design. Bidirectional communication allows each device to receivedata as well as transmit it. Many devices use the eight pins (2-9) originallydesignated for data. Using the same eight pins limits communication to half-

    duplex, meaning that information can only travel in one direction at a time. Butpins 18-25, originally just used as grounds, can be used as data pins also. Thisallows forfull-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, 500K to 2 MB, to be transferred each second. Itwas targeted specifically towards non-printer devices that would attach to theparallel port, particularly storage devices that needed the highest possibletransfer rate.

    Close on the heels of the introduction of EPP, Microsoft and Hewlett Packardjointly announced a specification called Extended Capabilities Port (ECP) in1992. While EPP was geared towards other devices, ECP was designed toprovide improved speed and functionality for printers.

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    In 1994, the IEEE 1284 standard was released. It included the two specificationsfor parallel port devices, EPP and ECP. In order for them to work, both theoperating system and the device must support the required specification. This isseldom a problem today since most computers sold support SPP, ECP and EPPand 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.

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