nuckolls.bob(at)cox.net
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 Posted: Fri Sep 19, 2008 6:02 am Post subject: Brad's reply on P-lead switch functionality |
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At 07:44 PM 9/18/2008 -0700, you wrote:
| Quote: |
Robert L. Nuckolls, III wrote:
>
><nuckolls.bob(at)cox.net>
>
>Received a reply from Brad on functionality of the p-lead switch
>in Emag products:
>
>"Grounding the p-lead 1) sends a status signal to the processor (telling it
>to stop firing), and 2) disables the driver chips (so they can't fire)."
Umm, that ~sounds~ good - but what does it mean? In the 30+
years that I've been working with computer hardware, I've
never run across that expression for stopping the processor.
In situations were a runaway process could result in damage,
the normal method for halting a processor isn't via a status
semaphore, but by halting the CPU clock pulse stream -
either via grounding the output of the clock, or removing
power from the clock crystal. With no clock pulses, the CPU
cannot execute instructions, period. It's the only way to be
completely certain that the CPU will in fact stop.
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Brad's description of the p-lead signal functionality
may be interpreted as follows:
First, it sets a discrete input to the processor that
causes the software routines to stop triggering the
coil for the purpose of generating a spark. The
processor doesn't "halt in place" it's expected to
recognize an operational command and to honor that
command until it goes away.
Second, the p-lead is tied to the 'drivers' between
logic level (processor) and power level (spark coil)
such that no communication between them is possible.
I.e., even if the processor has wandered off into the
weeds, the physical connection between logic and
output is broken.
I do this in all of my processor or logic based
smart actuator designs. There's a "logic world" that
runs at 5 volts in itty-bitty chunks (read fragile)
of silicon and provide the "smart" side of the
actuator's design. Then there's the power side . . .
usually a brushless DC motor with ratings from 0.1
to several horsepower . . . it runs on 28VDC at
lots of amps.
To get "smarts" to communicate with "power" you need
a combination of level shifters and drivers that
translate from the 5 volt milliamps world to the
28 volt amps world. I always bring the operating
power for these drivers or level shifters out to
interface with the ship's flight management systems.
Since these systems are already certified with level
B or level A software, I let THEM do failure monitoring
on my product while providing with a brick-wall-shutdown
for causing my operation to cease.
This feature relieves my software and hardware from
both the rigors of high-risk software certification
AND the need to do failure monitoring. I can push
those tasks off onto hardware and software that is
already taking on that responsibility for other
systems . . . so adding my system to it is not
burdensome. But it works only if you have this
brick-wall-shutdown feature not unlike that which
Emag has incorporated into their product.
Bob . . .
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