Richard B. Johnson wrote:
> On Fri, 26 Feb 1999, Neil Conway wrote:
> [SNIPPED]
> >
> > But how can you promise the customer that your system will respond to
> > events while they are current if you DON'T control the latency?
> >
> > This is the crux.
>
> I have a CPU which does nothing but poll a port. I want to detect
> when the sun comes up which happens once a day. There is a photo
> controller relay connected to that port. I need to know this time
> within one second.
>
> Therefore I need to poll this port at least twice per second.
> Twice comes from Shannon since this has become a sampled system.
(Well that's not an appropriate application of sampling theory. If you
only need to know to within 1 second, then you only have to sample at <1
second intervals. So 0.9 second intervals would be just fine. But
that's fluff.)
> Do we care how much time it takes to poll this port as long as
> the specified conditions are met? This is the latency and, in
> fact we don't care as long as the required specifications are
> met. If the latency is 1/2 second, we can't guarantee that the
> port is polled twice per second. However, if the latency is
> one microfortnight less than 1/2 second we can.
Well, substitute (<1second) for every (1/2second) and I totally agree
with you. No argument whatever. In my thinking, you must simply bound
the latency of your system to this upper limit. End of story. I never
said anything to contradict what you just said, in fact I thought that's
exactly what _I_ said :-). I don't think we have any disagreement here
so we must just have misunderstood each other - if I'm at fault then I
wholeheartedly apologise for any confusion...
Neil
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