Vector Imagery

[Tony Duell]

> 
> On 7/3/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
> > > 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.
> 
> Well... yes... I don't have enough analog theory in my head to come up with
> the value of R... 2*R is rather obvious once you have R.

Ah, OK... What you need to do is choose R so that it (or more accurately 
2*R) is large compared to the output impedance of the port lines (so that 
the 'extra' resistance added by the drivers isn't a cause of 
inaccuracies) and low compared to the load (IIRC, the output resistance 
of the R-2R ladder is simply R). 

Obviously you might well not be able to do this. In which case, you 
either have to buffer the port lines (to reduce the output impedance 
there) or add a buffer amplifier on the analogue output. Either solution 
involves active components that need power.

> > 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).
> 
> Two reasons come to mind - one, to make this a passive device off of
> the parallel port that doesn't require an external power wart, and

Can't you grab power from the computer? Obvious places would be either 
the keynoard connector or the joystick port. 

I have never forgivven IBM for not putting a +5V line on the parallel 
port. It would be so useful for homebrew add-ons. I have modified a few 
parallel cards (cut and jumper in the obvious way) so that pin 25 is a 
+5V output, the problem then is that any normal printer cable will short 
the 5V line to ground.

Incidnetally, the HP machine that currently occupies my bench has a 50 
pin Blue Ribbon serial port connector. On that are the +5V, +12V and -12V 
power supply lines :-)

> two, because I'm at the South Pole and I have to work with what I have
> on hand - there won't be another plane for nearly 4 months.

Ah, and you don't have any DACs in the junk box... But do you have 
suitable resistors? I've not done the calculations, but intuitively, you 
need the R's and 2*R's all to agree to better than 0.5% for an 8 bit 
converter. 0.5% resistors are not common.

-tony