Alt-BEAM Archive
Message #04392
To: beam@sgiblab.sgi.com, davidperry@mail.geocities.com
From: Bruce Robinson Bruce_Robinson@bc.sympatico.ca
Date: Sat, 12 Jun 1999 22:04:21 -0700
Subject: [alt-beam] Re: bypassing a nv
davidperry@mail.geocities.com wrote:
>
>> Assuming you have a typical Nv with a resistor going to ground at the
>> input, all you have to do is use your switch to short the resistor.
>
> won't this act as a PNC?
>
I see what you're concerned about. The short answer is, sort of. It will
act as a PNC for just the one neuron, for a short period of time. But it
won't prevent the next neuron from firing.
In a typical 4 Nv microcore, what causes a neuron to fire is when the
input to the previous neuron switches from high to low. Normally the
drop in voltage is gradual as the capacitor discharges through the
resistor. All that happens when you short the resistor is the voltage
drops suddenly.
A PNC works by keeping the neuron input clamped to ground. This means
the input voltage never gets to go high, so it cannot subsequently
switch to low again. Thus the next neuron never fires.
The only time the shorted resistor could act as a true PNC is if your
walker's leg got stuck against the limit switch, and stayed that way
until the pulse circled all the way around the loop again. That would be
a pretty extreme condition, because part way around the loop, the pulse
would be attempting to reverse your motor.
By the way, Richard Piotter's suggestion to use a smaller resistor in
series with the switch is a little more complicated, but is a "gentler"
solution. His method is kind of like closing a door quickly -- mine is
more like slamming it (with resulting shock waves). If your Nv resistor
is 1 Meg, then a 10 k resistor in series with the switch will drain the
capacitor so fast you won't notice the delay. I use both methods,
depending on how much charge is likely to be in the cap, and also on
whether or not I have the proper resistor.
Anyway, shorting the Nv resistor should work fine. I've got the
arrangement breadboarded right now, as I wait for a replacement motor
for the one I stupidly trashed. Pressing the shorting switch does
exactly what it's supposed to -- stop the motor (LED for now) and fire
the next neuron.
Regards,
Bruce
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