Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!ukc!reading!minster!ken
From: ken@minster.york.ac.uk
Newsgroups: comp.dcom.lans
Subject: Re: "real-time" over a lan: token ring vs ethernet vs ?
Message-ID: <650139711.11949@minster.york.ac.uk>
Date: 8 Aug 90 18:21:52 GMT
References: <19300@well.sf.ca.us> <64442@yarra.oz.au>
Reply-To: ken@SoftEng.UUCP (ken)
Distribution: comp
Organization: Department of Computer Science, University of York, England
Lines: 31

In article <64442@yarra.oz.au> chris@yarra.oz.au (Chris Jankowski) writes:
>In article <19300@well.sf.ca.us> berger@well.sf.ca.us (Robert J. Berger) writes:
>>
>> We are looking to make a special purpose dedicated lan for controlling
>> up to 128 devices. These devices will run a real time os such as PSOS or
>> VxWorks. There will be up to 16 master devices made up of unix workstations
>> running Unix System V.4. The workstations will be initiating most traffic.
>	  ^^^^^^^^^^^^^^^
>> The main time critical response we need is to have a guaranteed worst case of
>> the workstation sending a message to one or several of the slaves, where the
>> message must get to the slave within 5 milliseconds.  Most other traffic needs
>                                       ^^^^^^^^^^^^^^^
>My conclusion is that it looks as your 5ms may be insignificant 
>compared to variability of the time on workstations.
>
>Am I right or wrong or maybe I do not know something important?
>Anybody cares to comment? 

Well pointed out. I still laugh at people selling `Real-Time' Unix, with
claims like "We run real-time Unix, and you can use NFS, etc, etc". If you run
NFS then you ain't running in real-time. If you want guaranteed response times
don't use Unix, use a real-time operating system.

Ken

--
Ken Tindell             UUCP:     ..!mcsun!ukc!minster!ken
Computer Science Dept.  Internet: ken%minster.york.ac.uk@nsfnet-relay.ac.uk
York University,        Tel.:     +44-904-433244
YO1 5DD
UK