ethan.dicks at gmail.com
Thu Feb 1 21:11:24 CST 2007
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
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...
> 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)
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