My cable does not "output" any voltages!
The "tip" of our 2.5mm
is input to my cable, input to a FET without any pull up so you will
see no voltage there. Um, well, I take it back, I lied.
There is a 1 megaohm pull-up to DTR which is clamped to
5V via a zener. So if you have a very high input impeadance
volt meter, AND DTR in the DB9 is set HIGH, then you can see +5V on the tip.
The "ring" is our output signal but is open collector.
The collector of a 2N2222 so there is no voltage on it,
and this time I'm not lying. I rely on the target device
to "pull up" the line. This is how we manage to draw "ZERO"
power from the device. I get all the power needed to talk
to the PC from the PC.
To "see" our cable work without being plugged into a device, you
can connect tip to ring (loop back test) and pull them up
via a 5k or 10k resistor to 3 to 6 volts. The
software must make DTR(pin 4) "High" (plus volts)
and RTS(pin 7) "Low" (negative volts).
In a real pinch you can force RTS to ground and
see that it works. You can use a dumb terminal
program like hypertermal to send data and
see it echo back (loop back) by way of the tip-ring.
But wait, you should not have to be doing this kind of
testing! Because every cable is subjected to a
serious battery of tests prior to shipping. I designed
and built 6 automated cable test stations (at over $5,000 each)
in the last 10 years just to test these cables! And they have
tested over 850,000 of 'em! How about that?
Too bad I got only peanuts for royalties.
So buy one, or two, or a hundred or ten thousand from me
or one of my distributors today!
Back to Larry's Scanner cable deal.