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From: heiby@mcdchg.chg.mcd.mot.com (Ron Heiby)
Newsgroups: comp.sys.m88k
Subject: Re: Request for help: Location Monitors on the Motorola MVME188 board
Message-ID: <54642@mcdchg.chg.mcd.mot.com>
Date: 19 Jan 91 00:14:38 GMT
References: <14423@june.cs.washington.edu> <1991Jan17.221444.17741@eagle.lerc.nasa.gov>
Organization: Motorola Computer Group, Schaumburg, IL
Lines: 28

fsset@bach.lerc.nasa.gov (Scott E. Townsend) writes:
>What is a little puzzling to me is that the manual makes the point that
>accesses to a location monitor address will NOT generate a DTACK.  They imply
>that the bus timeout mechanism will take care of the problem.

ANSI/IEEE Std 1014-1987 (VMEBus) says in 2.3.4 and its Figure 8 that
no DTACK is generated.  In the text, just before Rule 2.17, is the
text:
	"A third type of module, the location monitor, monitors the
data transfer and generates either, or both, of two on-board signals
whenever a byte location that it monitors is accessed.
...
	When the cycle takes too long, a fourth module, a bus timer,
intervenes by driver BERR* low, completing the data-transfer
handshake, and allowing the bus to resume operation."


I find the standard somewhat difficult to read and was not able to
find the other information I was looking for, the exact section that
declares it illegal to have multiple slaves set up to respond to a
single access.  Since I cannot find it, I suppose it might be allowed,
but I *really* doubt it.  Even if it were, since VME is an
asynchronous bus, it would be possible for a fast board to have
responded to such an access before other (slower) boards recognized
the access taking place.
-- 
Ron Heiby, heiby@chg.mcd.mot.com	Moderator: comp.newprod
"Give me voice mail or give me drugs!"/"Mandatory Drug Testing? Just Say NO!!!"