This is one of the simplest circuit for a PC parallel port based DLA. The Circuit is mainly used to protect the port which is fabricated on the motherboard itself.
The IC 74LS04 is an inverter and acts as a buffer. The Zener is used for undervoltage and overvoltage protection. If a voltage > 5V appears on the input, The buffer will not transmit this voltage to the PC. In the case of a voltage < 0V, The Zener will break down and 0.7V will appear at the input, hence again protecting the PC.
The software consists of two parts a graphical interface which shows five digital waveforms,.Various features of the waveforms can be controlled by the software. There is also a text interface in which the circuit is used as a 4-Channel DLA. The software is in pascal , an executable version is also avaliable.
Source: Electronics Lab