Powering (or not) a parallel-port-attached DAC (was Re: Vector Imagery)

[Ethan Dicks]

On 7/4/06, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
> 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.

I know that the "right" way to do it involves active components...
given my 95% success with passive components, I was hoping to squeak
by.  If I could get power and data from the same source (see below), I
wouldn't be quite so picky.

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

Keyboard connector is a possibility.  This (server-class) computer
doesn't have a joystick port.  As I mentioned in another message, my
breadboard is extracting power for projects from the USB port (cable
came from a dead keyboard), but I really don't think the mechanical
arrangement is viable in a user environment.  We've lost three of
these machines in 12 months by static discharge when plugging in USB
devices - you can see the resultant hole in the epoxy of the gate
array that does the USB... the machine is totally fried after a
spark... I'd prefer to glue the cover shut to keep users out; I
certainly don't want to leave a device behind that encourages them to
fiddle with the front USB ports.  No... we don't have any powered USB
hubs here - the ports the vendor designs in are the ports we have to
work with.

I'd probably be best served by rigging up a back bracket with a custom
power cable - that way, at least, it'd be out of sight, out of mind
when installed.

> 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.

That's a nice hack, and one I might have done for myself back in the
ISA days, but as I mentioned in another message, an official +5V
output pin on the Amiga 1000 caused lots of people lots of grief and
C= relented with the A500 and A2000 and went with a totally
PC-printer-cable-compatible port after the A1000.

I've seen some goofy designs for getting power to an external device -
molded DB25s with integral coaxial power connector (for an early
powered mouse), and more than one design for a male/female keyboard
power connector (Xircom did one for their PE3 Pocket Ethernet
Adapters)... it would have been so much easier if there had been an
official way to tap power.

> > 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...

We have many, many parts in drawers (I probably have access to 20-25
parts bins), but DACs are not among them.  I might be able to scrounge
something off of a dead board (we have some Vaisala weathersonde
boards and some UPS boards in the scrap pile - I did manage to score
16 IRF510s from a dead UPS a couple of months ago, perfect for a
thermo-controlled fan ;-)

> 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.

Ah... we have quantities of 1% resistors, but no 0.5% resistors that I know of.

As for my simple 8-value DAC, I used 5% resistors and hand-sorted them
for a (nearly) consistent 1:2 ratio up the line.  We have a number of
the ancient Ohmite red drawers with a dozen values per tray (there are
lots of components here from the 1960s, including a bucket full of
tubes/valves).

-ethan