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From: bon@JPL-VLSI.ARPA
Newsgroups: mod.computers.vax
Subject: Clock to be accessible by two microVAXes
Message-ID: <8602050857.AA07629@ucbvax.berkeley.edu>
Date: Mon, 3-Feb-86 16:42:18 EST
Article-I.D.: ucbvax.8602050857.AA07629
Posted: Mon Feb  3 16:42:18 1986
Date-Received: Fri, 7-Feb-86 21:06:26 EST
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Approved: info-vax@sri-kl.arpa

This is a request for information on the availability of a clock which can
be read from 2 microVAXes for a real-time application.

We are implementing a robotic task which will have one microVAX II doing
vision and another commanding robot arms using visual feedback.  My problem
is conceptually very simple -- I need a precise time base which is consistent
for both machines to about 0.1 millisecond.  Ideally, I would like to have
a single millisecond time-of-day clock which takes care of its own register
incrementing, etc., but which can be read simply by doing a load from memory
to memory-mapped I/O locations.  I don't care about interrupts or anything
more sophisticated.  Another possibility would be to have a clock in each
VAX and start them based on an externally-generated pulse.  The consistency
achieved by this method depends on both the delays between receipt of the
pulse and activating the clocks, and on the accuracy of crystals in each
clock to maintain synchronization -- I'm not crazy about having to rely
on these.


If anyone knows of vendors which sell devices which might help us with our
time-keeping problem, I would appreciate hearing from you with their names
and addresses/phone numbers.  Also, if anyone has solved similar problems,
I would be interested in hearing how.  Thanks in advance for any help you
can give.

					Bruce Bon
					Bon@JPL-VLSI.ARPA