Introduction to the IEEE 1284-1994 Standard

This section is implemented as a multilevel document. This page serves as an executive summary of the 1284 standard. By clicking on the various highlighted points, you may explore each concept in greater detail.


The recently released standard, "IEEE Std.1284-1994 Standard Signaling Method for a Bi-directional Parallel Peripheral Interface for Personal Computers", is for the parallel port what the Pentium processor is to the 286. The standard provides for high speed bi-directional communication between the PC and an external peripheral that can communicate 50 to 100 times faster than the original parallel port. It can do this and still be fully backward compatible with all existing parallel port peripherals and printers.


Click here for a history and background of the parallel port.

The 1284 standard defines 5 modes of data transfer. Each mode provides a method of transferring data in either the forward direction (PC to peripheral), reverse direction (peripheral to PC) or bi-directional data transfer (half duplex). The defined modes are:

  • Forward direction only
  • Compatibility Mode
    "Centronics" or standard mode
  • Reverse direction only
  • Nibble Mode
    4 bits at a time using status lines for data.
    Hewlett Packard Bi-tronics
    Byte Mode
    8 bits at a time using data lines, sometimes referred to as a "bi-directional" port.
  • Bi-directional
  • EPP
    Enhanced Parallel Port- used primarily by non-printer peripherals, CD ROM, tape, hard drive, network adapters, etc....
    ECP
    Extended Capability Port- used primarily by new generation of printers and scanners

All parallel ports can implement a bi-directional link by using the Compatible and Nibble modes for data transfer. Byte mode can be utilized by about 25% of the installed base of parallel ports. All three of these modes utilize software only to transfer the data. The driver has to write the data, check the handshake lines (i.e.: BUSY), assert the appropriate control signals (i.e.: STROBE) and then go on to the next byte. This is very software intensive and limits the effective data transfer rate to 50 to 100 Kbytes per second.

In addition to the previous 3 modes, EPP and ECP are being implemented on the latest I/O controllers by most of the Super I/O chip manufacturers. These modes use hardware to assist in the data transfer. For example, in EPP mode, a byte of data can be transferred to the peripheral by a simple OUT instruction. The I/O controller handles all the handshaking and data transfer to the peripheral.

Overall, the 1284 standard provides the following:

  1. 5 modes of operation for data transfer
  2. A method for the host and peripheral to determine the supported modes and to negotiate to the requested mode.
  3. Defines the physical interface
  4. Defines the electrical interface
    • Drivers/Receivers
    • Termination
    • Impedance

In summary, the 1284 parallel port provides an easy to use, high performance interface for portable products and printers.

 

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