Vector Imagery

[Ethan Dicks]

On 7/3/06, Allison <ajp166 at bellatlantic.net> wrote:
> >
> >Subject: Re: Vector Imagery
> >   From: "Ethan Dicks" <ethan.dicks at gmail.com>
> >   Date: Sun, 02 Jul 2006 18:49:18 +1200
> >     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>

> >I've built that first simple ladder DAC (to hang a 1950s Triplett
> >meter off of a parallel port for just random analog fun)...

> First, the 1950s triplett meter is likely 20,000ohms/volt at will load
> any ladder using resistors larger than 1000ohms in an R2R ladder.
> The 5v range looks like a 100K resistor across the probes.

I've already cracked open the meter and tweaked the internal load
resistor.  The simple 8-resistor ladder I built _works_, with the
exception of a noticable dip in voltage between 2^5-1 and 2^5, and
2^6-1 and 2^6, and 2^7-1 and 2^7.

> Second, The output port is TTL? CMOS?  IF TTL the output swing is
> not to 5V to with even light loads.  IF CMOS output voltage will sag
> if the load is too great.

To be honest, I have no idea - it's an integrated port on a modern
machine.  If I had to guess, I'd say it's a CMOS port, but I don't
know that for a fact.

> For 8bit R2R ladders I use R=10k and 74HC244(or other high current
> output CMOS) as a buffer.  The load is usually very high impedence
> (VTVM, FETVM, DVM, OPamp voltage follower).

OK... that's a place to start.  I know I can make it work with an
8-bit buffer, but once I get a powered chip external to the PC in the
mix, I might as well wait for the next plane and use an 8-bit or a
12-bit DAC and OPamp for a "real" D-A circuit.

-ethan