Vector Imagery

[Tony Duell]

> What I really would like to find is a formula for calculating R2R
> ladder values so that I could get a 0V-5V output from an 8-bit
> parallel port.  Constructing it is easy, once one knows what resistors
> to pick.

Eh? The resistors in the ladder are R and 2*R, for some suitable value of 
R. 

The (maximum) output voltage depends on 2 things. Firstly the output 
voltage from the parallel port (and note, if those 8 output signals don't 
switch between the same voltages, you will get non-monotonicity as you 
observed). And secondly, the load you connect to the output.

I would also think that the output impedance of the parallel port needs 
to be taken into account. And that may well be non-linear (which may make 
things difficult).

Another cause of non-monotinicity is, of course, errors in the individual 
resistor values.

Just out of curiousity, why do you want to use an R-2R ladder, rather 
than a single-chip DAC? Commercial vector displays that I've come across 
either used a DAC chip (Vectrex), a DAC module (DEC GT40), or a rather 
complicated circuit with tweakers for the top 6 bits (HP 1350). 

-tony