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RE: [SeattleRobotics] Why FSMs? Example! (was Loki)

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  • Randy M. Dumse
    Alan KM6VV said: Monday, December 31, 2007 9:30 PM ... That s the basic idea behind IsoMax(TM). We have a scheduled (periodic) interrupt that should run faster
    Message 1 of 212 , Dec 31, 2007
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      Alan KM6VV said: Monday, December 31, 2007 9:30 PM
      > Sometimes it's tough to avoid polling. Interrupts are one
      > solution. Or if periodic checking can service the device
      > within latency requirements.

      That's the basic idea behind IsoMax(TM). We have a scheduled
      (periodic) interrupt that should run faster than your fastest
      latency requirements. As I say, for mechanical systems, 100 Hz
      is pretty good. For electronic ones, you may want to run faster.

      This is the basic scan loop concept used in Relay Ladder Logic
      and PLCs.

      In our language however, we gather all the waste cycles (those
      not used by the polling interrupts to check transitions and do
      actions) to give to the foreground routine which will usually
      communicate with the operator, which is ultimately the final
      waste of time, particularly if there is no operator there. A
      task can be autostarted here as well, such as data logging and
      progress following if desired. But if there is an operator, he
      can query memory, dump values, see what's going on
      interactively, while the real real-time program just keeps
      runing in the background. The operator can even add more
      programming while real-timer operations go on in the background
      without consequence.

      We started with the lowest common denominator, the scheduled
      (periodic) scanning. We can also run in interrupts, where the
      interrupt itself calls the FSM directly. And we've also
      implemented other versions, to allow layers of latencies. But we
      haven't released those codes yet. The next step has one layer
      for mechanicals, and another layer which can be much faster for
      faster processes like PID, and electronic communications.

      > However there must still be a little overhead (depending on
      > the processor) for entering a new function, such as stack
      frames (PC).

      Yes, there's a bit of overhead, but the context switches we do
      are very fast. No major stack frames to mess with et. al.

      > Waits. "The rationale behind this method is a state is a
      > wait.". Why a wait? You give up a "tick" of the rate you
      > call your various state machines, but what's the wait?

      Well, follow the line of reasoning: A real-time program must
      wait for external synchronization. But rather than blocking, it
      can hand the wait off once it know the synchronizing event
      hasn't happened yet. (An interrupt event if the hardware
      supports this, or a timer event which can cause a periodic look
      at a rate higher than the latency requirements.) The wait is the
      state. State comes from Latin, Stare to stand. Nothing happens
      when you stand. And you "stand at ease" until there is something
      to do.

      > Even faster if it's a vector instead of a
      > switch/case, I see that.

      Much much faster. Why compute the state each time? You knew what
      the new waiting state was going to be last time a transition
      happened. So why re-look it up every time the machine is called.
      Better to just go to the test that determines whether to leave a
      state or not. Again, set up the next wait vector on the way out
      of the current state.

      Then we also play some tricks in how we link the active machines
      so they ripple through their time slice as quickly as possible.

    • PeterBalch
      Dr. Bruce. ... have ... OK, I looked it up. In Danaidae (monarchs), Satyridae (whites, graylings) and Nymphalidae (emperors) the two front legs are very short,
      Message 212 of 212 , Jan 14, 2008
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        Dr. Bruce.
        > Interesting question and explanation - except that butterflies actually
        > 6 legs.... 4 long and 2 short.

        OK, I looked it up. In Danaidae (monarchs), Satyridae (whites, graylings)
        and Nymphalidae (emperors) the two front legs are very short, have no claws
        and are useless for walking. I thought Papillionidae (swallowtails) were
        the same but they're not, they have 6 good legs.

        It gets wierder. In Riodinidae, the males have 4 walking legs but the
        females have 6.

        Lycaenidae (blues) and Hesperidae (skippers) all have 6 legs.

        What on earth can one learn from that?

        Anyway, Alan asked what "bugs" are quadrupeds. I said mantises, monarchs
        and swallowtails. I was wrong about swallowtails. I was taught all this
        stuff decades ago so forgive me if I'm a bit rusty. I even spent one summer
        as a student doing fieldwork in the west of Virginia catching swallowtails
        and monarchs so I certainly _ought_ to have remembered the details.

        I still can't think of any other arthropods with just 4 walking legs. I
        think some crustacean larvae have only 4 legs but they don't use them for

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