ARCHIVED: What are bidirectional parallel ports, and ECP and EPP?

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Parallel ports (also known as printer or Centronics ports) were traditionally used to connect printers. Data was required to be sent to the printer, and the only data required in return was an indication of the status of the printer (e.g., paused, out of paper, jammed).

With the advent of peripherals such as scanners, Ethernet adapters, and external disk and tape drives, the parallel port became a convenient point of attachment for such devices. However, with these devices came a need to transfer large amounts of data in the reverse direction.

All parallel ports are capable of bidirectional data flow. However, the original Centronics design, used on the first IBM PCs, was not designed for efficient data transfer back to the computer. To achieve data transfer in the reverse direction, it uses four lines that were meant to be used to indicate the printer's status. Only four bits at a time can be transferred (this is referred to as nybble mode data transfer).

With time, several new parallel port standards were defined, and additional reverse-direction capabilities were added to the original Centronics design. The four types of parallel port defined by the IEEE 1284 specifications are:

Centronics original port: Capable of nybble mode transfer, four bits at a time.
PS/2 port: Capable of byte mode transfer (eight bits at a time). Appeared first on IBM PS/2s.
EPP (Enhanced Parallel Port): Referred to as the bidirectional port. Originally developed by Intel, Zenith, and Xircom. This port is capable of 500KB to 2MB per second data transfer rates.
The ECP (Extended Capability Port): The most recent specification, implemented by Microsoft and Hewlett-Packard, includes the use of FIFO buffers and RLE (Run Length Encoding) compression in both directions which allows much higher throughput than normal printer ports. To take advantage of the added speed, the peripherals (in most cases, scanners and printers) must be ECP capable.

For more information, see:

  http://www.fapo.com/1284int.htm