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:
- 5
modes of operation for data transfer
- A
method for the host and peripheral to determine the supported
modes and to negotiate to the requested
mode.
- Defines
the physical interface
- 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.
