Powering (or not) a parallel-port-attached DAC (was Re: Vector Imagery) - test-drb@ccmp.vtda.org

3 Jul 2006

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