Xref: utzoo comp.realtime:315 comp.software-eng:2415
Path: utzoo!utgpu!watmath!att!rutgers!ucsd!usc!samsung!uunet!ficc!korenek
From: korenek@ficc.uu.net (Gary Korenek)
Newsgroups: comp.realtime,comp.software-eng
Subject: Re: Real-Time references wanted
Summary: my interpretation
Message-ID: <7012@ficc.uu.net>
Date: 16 Nov 89 19:30:44 GMT
References: <2925@zaphod.axion.bt.co.uk> <618@athen.sinix.UUCP> <7861@charlie.OZ>
Organization: Ferranti Int'l Controls
Lines: 82

In article <7861@charlie.OZ>, rad@aragorn.cm.deakin.oz.au (Robert Alan Dew) writes:
> In article <618@athen.sinix.UUCP> es@athen.UUCP (Dr. Sanio) writes:
> #In article <2925@zaphod.axion.bt.co.uk> rdoyle@zaphod.axion.bt.co.uk writes:
> #>I am currently involved in some work on real-time  [..]
> #>	1. What characteristics of a system make it "real-time"?
> #>	2. How many different classes of real-time systems are there,
> #>	   based on an assessment by characteristics?
> ...

Here's my interpretation of "real time" and "real-time computer system":

  Real-time:  Exactly that.  What's going on "as it occurs".  Broadcasters
     announce football games (for example) in real-time.  What the
     play-by-play announcers say has particular value because the game is
     happening *now*.  

  Real-time computer system:   A computer system that measures each event
     in a process as the events occur (in "real-time").  Possible processes
     where real-time measurement is desirable are:  pipe line, refinery,
     oilfield, combustion engine, aircraft, building enviroment (air
     conditioning/heating), electrical power generation (and distribution),
     manufacturing.  In general, the computer system acquires data from
     the process via instrumentation connected to certain physical parts
     of the process (these are called "points"...  it's a point that a
     measurement is taken).

  Two possible categories of real-time computer systems are open-loop and
     closed-loop.

  An example of an open-loop system:  the system obtains measurements from
     points in the process, converts the data from digital "instrument"
     format to "human usable" format, and communicates the data values
     to whoever is interested in seeing it (typically people who are
     responsible for the "quality" or "results" of the process).  These
     people can then take the data and either ignore it, or do something
     with it (go manually open or close a valve, as an example).  If the
     open-loop system includes a means to *control* (make chages to) the
     process, the responsible people may enter commands for the system to
     send out to the process (the system can open or close a valve, as an
     example).

     The main points to the open-loop system are:
        The system obtains data from the process, reports the data to
        responsible people, who then can elect to either to take action 
        (alter the process and the result of the process) or do nothing.
        People are "in control" of (and are responsible for) the process.
        The goal is to have the process produce the desired result.

  An example of an closed-loop system:  the system obtains measurements from
     points in the process.  Based on this data, the *system itself* decides
     if the process should be altered or left alone (the system itself is
     responsible for the desired result of the process).  If the system
     decides the process should be altered, it sends commands out to the
     process.  The system obtains more measurements from the process (which
     is now changing because of the commands previously sent out), again
     decides if the process should be altered, etc.

     The main points to the closed-loop system are:
        The system obtains data from the process, and based on this data
        decides either to take action or do nothing.  The system itself
        is "in control" of (and is responsible for) the process.  Again,
        the goal is to have the process produce the desired result.


  Note:  Some real-time systems are combinations of open-loop and closed-
     loop systems.  There may be one or more closed-loop systems that 
     control an individual part of a process.  The closed-loop controller(s)
     are connected to the open-loop system in such a way that when a 
     responsible person enters a command into the open-loop system (to alter
     the process), the open-loop system sends the command to the closed-
     loop controller, which then commands that part of the process to 
     change.  In this case, people *and* equipment share responsibility
     for the process to produce the desired result.


In summary, these are real-time systems because they deal with events that
occur within a process *as the events occur* (in real-time!).

-- 
Gary Korenek    (korenek@ficc.uu.net)    |          This space
Ferranti International Controls Corp.    |         intentionally 
Sugar Land, Texas       (713)274-5357    |          left blank