How Parallel Ports Work

While most of you must be aware of the USB interface in your computer for connecting external devices, one of the most commonly used interface for connecting printers still happens to be a parallel port. Till date, parallel ports are being used to connect a lot of popular computer peripherals such as scanners, printers, external hard drives, tape backup drives, CD burners, Iomega Zip removable drives, network adapters etc.


Originally developed in order to connect a printer to the PC by IBM, parallel ports slowly made their way into other devices as well. The IBM decided to add a 25 pin connector, DB-25 to a 36-pin connector (created by Centronics) in a bid to develop a special cable to connect the printer to the computer.

Now the transfer of data using a parallel port from PC to the device takes place at a rate of 8 bits or 1 byte at one time. This 8 bits transfer takes place in a parallel fashion, instead of being transmitted serially (as in USB). At any given time, the standard parallel port can deliver up to 50 to 100 kilobytes of data per second.


To understand its working, you will need to understand the function of the various pins involved in making of the parallel port:
• Pin 1 is responsible for carrying strobe signal. While it maintains 2.8 to 5 volts, it drops below 0.5 every time a byte is sent by the computer. This drop helps the printer identify that data is being sent.
• Pin 2 and 9 are the carriers of data. To indicate data value 1, a charge of 5 volts is sent by the concerned pin. No volt indicates 0 values. This data transfer system helps in effectively transmitting digital information over analog cable.
• Pin 10 is responsible for sending the acknowledgment signal from the printer to the computer. It works on the same lines as Pin 1. In cases when the printer is already busy, it will charge pin 11. Once the printer is free again, it will drop the voltage below 0.5 volts to inform the computer that it is ready to take data. If the printer is out of paper, it sends commands to the computer using a charge on Pin 12.
• Charge on Pin 13 lets the computer know that the device is online.
• A charge of 5 volts on Pin 14 lets the printer know of an auto feed signal.
• In cases when printer is facing problems, the voltage on Pin 15 will take a drop to less than 0.5 volts, hence letting the computer know f its error.
• Once a new printing job is required and ready, the computer will drop the charge on Pin 16. This will initialize the printer.
• The computer makes use of Pin 17 to take the printer offline remotely. This happens as the computer maintains certain charge on pin 17 until the time it wants the printer back to online mode.
• Lastly, pins 18 through 25 are used in order to provide reference signal for low voltage or charge.

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