Mike Feldman (feldman%mycroft@gswd-vms.ARPA)
Fri, 10 Apr 87 07:47:18 CST
> The answer to your question is that the MAP people do not fully understand
> all the sources of probabilistic delay that you catalog. ...
> In their defense however, they have always been concerned about the situation
> which might arise when a failure in a manufacturing process causes 500
> sensors to all start sending emergency warning messages at the same instant.
> This can lead to lockup on an Ethernet in a way which should not occur on a
> token bus.
I was envolved in designing an early token bus process control system for
a control system manufacturer in '69 - '70, when I first saw the Ethernet spec
and articles describing it. I was a junior software engineer at the time,
and I suggested to the powers-that-be that Ethernet was much simpler to
implement, and that we were already doing it for the token recovery sequence
(silence on the wire).
It's my opinion that it was dogma among communications engineers even then
that you needed deterministic bus access time and guaranteed throughput for
control system muxes. There were two cases where the system had to perform
under Ethernet-busting loads. One was during customer accecptance testing,
and the other was when the reactor pressure relief valve failed open.
I got angry reactions when I suggested Ethernet would be cheaper and easier
to build and would have better performance almost all of the time. They
were even more irritated when I suggested the if determinism and known
throughput were so important that they use time division multiplexing.
These days, there is a lot of support among control system suppliers for MAP.
There hope is that the semiconductor industry will see a large market and
implement all the hard, expensive lower layers in silicon, because so far it's
been too expensive for each system house to develop one that's a comercial and
technical success. It might happen, but they still won't get the deterministic
round trip message delay needed to implement stable fast feedback loops.
These opinions are my own.
Gould Computer Systems Division
1101 East University Avenue
Urbana, IL 61801
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