Over the years, four types of parallel ports have evolved. Original, Type 1, Type 3, and Enhanced Parallel Port. Following is a description of each of these types.

Original Unidirectional. (One way communication) The original IBM PC did not have different types of parallel ports available. The only available port at the time was unidirectional or one way, for communicating with a device such as a printer. Devices such as scanners, PostScript printers (printers that communicate information back to the computer), zip drives, or CD-ROM drives are unable to use this type of connection. Many older computers (486 class and below) often use this type of parallel port.

Type 1 Bi-directional. (Two-way communication) In 1987 a new parallel port was introduced by IBM when the PS/2 computer was released. This allowed peripheral manufacturers to use the parallel port as an interface because this allowed true bi-directional communication.  

Type 3 DMA (ECP) Direct Memory Access. This port type featured greater throughput by using direct memory accessing techniques. The types of parallel ports previously discussed used the CPU to control data flow. With these ports, the CPU sends one byte to the I/O addresses used by the port, tests to see if it was sent or not, and then sends the next. This process continues until all the information to be sent has been processed. The Type 3 port, however, enables you to define a block of memory that you want transmitted (perhaps a file stored in memory), and then enables the port controller to pace the sending of the information. This frees the CPU to perform other tasks, allowing faster communication.

Enhanced Parallel Port (EPP) Specification. This is a newer specification from Intel, sometimes referred to as the Fast Mode Parallel Port. While the IBM Type 3 port is enables you to move large amounts of data without trying up the CPU, the EPP mode is designed to allow fast bi-directional communication with sophisticated peripheral devices like scanners. To do this, an Enhanced Parallel Port, when operating in that mode, redefines the parallel pins so they become bus control lines to augment the communication and control between your system and the external device.

If you are experiencing problems with your scanner the most likely solution is to change your parallel port mode to EPP. Today’s computers allow you to change the parallel port mode with a setup utility. To get to the setup utility, restart your computer and look for a message that says something like "Press <F1> to enter setup." It may say something other then <F1> as it differs for each computer. Press the specified key to enter setup.