Parallel Port Basics

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.

Parallel ports were originally developed by IBM as a way to connect a printer to yourPC. When IBM was in the process of designing the PC, the company wanted the computer to work with printers offered byCentronics, 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. After IBM launched its PC in 1981, 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 8bitsof data (1 byte) at a time. These 8 bits are transmittedparallelto each other, as opposed to the same eight bits being transmittedserially(in a single stream) through aserial port. The standard parallel port is capable of sending 150 kilobytes of data per second.

Let's take a closer look at what each pin does when used with a printer:

Pin 1 carries thestrobesignal. It maintains a level of between 2.8 and 5volts, 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传输数据. 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 theacknowledgesignal 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 isbusy, 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 isout of paperby sending a charge on Pin 12.
As long as the computer is receiving a charge on Pin 13, it knows that the device isonline.

The computer sends anauto feedsignal to the printer through Pin 14 using a 5-volt charge.
If the printer has anyproblems, 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 toinitializethe printer.
Pin 17 is used by the computer to remotely take the printeroffline. This is accomplished by sending a charge to the printer and maintaining it as long as you want the printer offline.
Pins 18-25 aregroundsand are used as a reference signal for the low (below 0.5 volts) charge.