Digital
to Analog Conversion
In order to move the cart and the ramp it is necessary to convert the
digital signal from the parallel port into two analog signals that can range
from 0 to 5V to drive the analog inputs of the plotter (D/A
Circuit Diagram). This is accomplished by using a Texas Instruments
TLC7528N dual 8-bit digital to analog converter chip. The D/A converter is
driven by the logic signals from the PC parallel port. These signals are
buffered using three 7407 TTL Hex Buffer chips.
The circuit design is fairly straightforward
but a reasonable amount of experience with digital logic is required to build
and debug the interface. As designed, it works properly with the software
written for the project but anyone attempting this project should recognize
that things don't always work out as expected in hardware/software interfacing!
The data sheets for the D/A chip are here.
The data sheet for the Photo-Darlington is here.
The data sheet for the solenoid drive
transistor is here.