Interfacing the Standard Parallel Port http://www.senet.com.au/~cpeacock
Interfacing the Standard Parallel Port Page 2
also backwards compatible with the Standard Parallel Port (SPP). Compatibility, Nibble & Byte
modes use just the standard hardware available on the original Parallel Port cards while EPP & ECP
modes require additional hardware which can run at faster speeds, while still being downwards
compatible with the Standard Parallel Port.
Compatibility mode or "Centronics Mode" as it is commonly known, can only send data in the
forward direction at a typical speed of 50 kbytes per second but can be as high as 150+ kbytes a
second. In order to receive data, you must change the mode to either Nibble or Byte mode. Nibble
mode can input a nibble (4 bits) in the reverse direction. E.g. from device to computer. Byte mode
uses the Parallel's bi-directional feature (found only on some cards) to input a byte (8 bits) of data in
the reverse direction.
Extended and Enhanced Parallel Ports use additional hardware to generate and manage
handshaking. To output a byte to a printer (or anything in that matter) using compatibility mode, the
software must.
1. Write the byte to the Data Port.
2. Check to see is the printer is busy. If the printer is busy, it will not accept any data, thus any
data which is written will be lost.
3. Take the Strobe (Pin 1) low. This tells the printer that there is the correct data on the data
lines. (Pins 2-9)
4. Put the strobe high again after waiting approximately 5 microseconds after putting the strobe
low. (Step 3)
This limits the speed at which the port can run at. The EPP & ECP ports get around this by
letting the hardware check to see if the printer is busy and generate a strobe and /or appropriate
handshaking. This means only one I/O instruction need to be performed, thus increasing the speed.
These ports can output at around 1-2 megabytes per second. The ECP port also has the advantage of
using DMA channels and FIFO buffers, thus data can be shifted around without using I/O
instructions.
Hardware Properties
On the next page is a table of the "Pin Outs" of the D-Type 25 Pin connector and the
Centronics 34 Pin connector. The D-Type 25 pin connector is the most common connector found on
the Parallel Port of the computer, while the Centronics Connector is commonly found on printers. The
IEEE 1284 standard however specifies 3 different connectors for use with the Parallel Port. The first
one, 1284 Type A is the D-Type 25 connector found on the back of most computers. The 2nd is the
1284 Type B which is the 36 pin Centronics Connector found on most printers.
IEEE 1284 Type C however, is a 36 conductor connector like the Centronics, but smaller. This
connector is claimed to have a better clip latch, better electrical properties and is easier to assemble. It
also contains two more pins for signals which can be used to see whether the other device connected,
(96 pages)
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