Little Board/P5x Technical Manual
2–27
Please refer to the IEEE-1284 standard fo r the complete list of require ments for a co mpliant cable
assembly, including recommended connectors

Latch-Up Protection

The parallel port incorporates chip protection circuitry on some inputs, designed to minimize the
possibility of CMOS “latch up” due to a printer or other peripheral being pow ered up while the
Little Board/P5x is turned off.
Parallel Port Registers
The low-le vel so ftware interface to the parallel p ort consists o f eig ht addre ssable reg isters. The
address map of these registers is shown in Table 2– 21.
Table 2– 21. Parallel Port Register Map
Register Name

Address

Data Port Base address
Status Port Base address + 1
Control Port Bas e address + 2
EPP Address Port Base address + 3
EPP Data Port 0 Base address + 4
EPP Data Port 1 Base address + 5
EPP Data Port 2 Base address + 6
EPP Data Port 3 Base address + 7
Note: EPP registers are only accessible when in
EPP mode
Standard and Bi-Directional Operation
You can use the parallel p ort as a standard outpu t-only printe r port o r as a PS/2-co mpatible bi-
directional data port with up to 12 output lines and 17 input lines. The bi-directional mode can be
very valuable in custom applications. For example, you might use it to control an LCD display , scan
keyboards, sense switches, or interface with optically isolated I/O modules. All data and interface
control signals are TTL-compatible.
Set the parallel port’s default mode using Setup.

Using the Parallel Port in Bi-Directional Mode

To use the p ort as a bi- direc tional data o r dig ital con trol po rt you must se t the de fault m ode to bi-
directional in Setup or put it in bi-directional mode with a BIOS call. The following code examp le
shows how to set the parallel port mode to bi-direction al.