Arty computers

[Ethan Dicks]

On 2/1/07, Fred Cisin <cisin at xenosoft.com> wrote:
> > > The old GE machines with the "Thousands of Operations Per Second"
> > > analogue meters were kind of neat too.
>
> On Thu, 1 Feb 2007, woodelf wrote:
> > So just how does one construct such a meter?

An analog meter?  I hacked one onto a machine a couple of years ago...
it was a simple resistor ladder with a 3" panel meter attached to the
output.  It wasn't precise - the needle would dip when going from 0x1F
to 0x20 or from 0x7F to 0x80 since the values were approximage, but it
did the trick.  One thing I did do was to hack the internal resistor
in the meter to match the output levels that a modern desktop parallel
port could provide (ISTR the original meter scale was 0VDC - 10VDC and
I dropped it to approx 0V - 5V).  All one does at that point is to
write a tiny app that takes a number from 0-255 and emit it on a
parallel port, and the needle swings to that point.  You can't change
it rapidly due to physical inertia, but for slow changes, it works
great.

If you want a digital representation of load, you can either drive the
LEDs directly from the parallel port, or to get more than 8 lights
from an 8-pin port, try a bargraph driver like the LM3914...
http://www.switkin.com/software/geeklights/index.html

> Howzbout the same way that after-market ATs measured their speed for the
> front panel digital display?   (Jumpers to set two numbers, and "turbo"
> line to switch between them)

I recall looking at them at one point, but they are essentially a
forest of jumpers to light various segments of either a 2 or 2-1/2
digit LED - no drivers whatsoever.  One _could_ light them up under
computer control, but you'd have to roll your controls.  Long ago, I
did hang a pair of Radio Shack 7-segment LEDs off of a PET User Port
with some trivial logic to use the 8th bit to select digit A vs digit
B, then the lower 7 bits of the port went right to the segments.  A
small machine-language routine tapped into the 60Hz clock interrupt
refreshed the digits.  For a modern machine, I'd recommend some sort
of external latch and multiplexer at least, or perhaps a dedicated LED
driver like the ICL7218 (MAX7218?)

(here's one I made a while back with 16 digits from one parallel port
and two driver chips)

http://www.penguincentral.com/retrocomputing/retrogaming/scoreboard.html

-ethan