This circuit uses a R-2R Ladder Resistor Network to convert digital data from PC Printer Port to Analog. This can be used as a Millivolt Source or Programmable Power Supply.
This circuit is a R-2R, Digital to Analog Converter. You can replace the R2R with a D-A Chip for better performance. This circuit is just to understand the Concept, for learning and Experimenting.
The MFR resistors are 1%, hence the precision of this Conversion is not very good. It is ok for servo applications or closed loop conversion along with the ICL7135. That can be tried with software.
A 4053 Analog Switch controlled by PC software will give you a negative or positive output. R41 trimpot is to calibrate or scale output. If you modify this circuit, you could output data at a fast rate, then you have a staircase generator. You can write software for a waveform generator or function generator too.
You can design a dual tracking power supply over this. You can loop it with the DVM circuit shown earlier and make a 12 bit accuracy millivolt or milliamp source. It will depend more on the software and little addons to circuit.