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From: crds@ncoast.UUCP (Who Am I)
Newsgroups: comp.sys.m68k
Subject: Re: DTACK*
Message-ID: <4905@ncoast.UUCP>
Date: Sun, 18-Oct-87 23:40:56 EDT
Article-I.D.: ncoast.4905
Posted: Sun Oct 18 23:40:56 1987
Date-Received: Tue, 20-Oct-87 23:45:19 EDT
References: <4215@pyr.gatech.EDU> <1613@bloom-beacon.MIT.EDU>
Reply-To: crds@ncoast.UUCP (Glenn A. Emelko)
Distribution: comp
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 28
Summary: With respect to Wolfgang (I just read it)

Wolfgang (and Jim),

Well, I'd trust a logic analyzer before a data book anyday!  I also did a bit
more reading in my data book... I did misinterpret the #28 (trailing edge time
markers) signal, which is t[SHDAH] (/AS, /DS High to /DTACK high) -- it seems
that the minimum time is 0ns, with a MAXIMUM (as Wolfgang pointed out) of
120ns on the L8 part.  Searching further, I see mention of "negating /DTACK",
but I also see a note which says "... The term negate or negation is used to
indicate that a signal is inactive or false." Well, this leaves room for
discussion, what they mean by "INACTIVE" is unknown (hehehe).  Reading the
rest of my book produced only one other interesting fact... if you are using
a 68xx series peripheral (ie. 6845 CTR controller) you will be using a control
line called "/VPA" (Valid peripheral address).  In that section, it says:
"/DTACK should not be asserted while /VPA is asserted." and continues to
explain how the addresses are in a high impedance state, etc. etc.

I have not ever actually TRIED to pull /DTACK low to see if it does indeed
work on any system, but as I stated in the last, I would not recommend that
ANYONE try it!  If the system is capable of running at full speed on the bus,
then /DTACK will be asserted before the falling edge of S4 anyhow, and there
is nothing that you can do to make it go faster.  If not, so be it.  Clearly,
system reliability is much more important than system speed (who needs a fast
computer that doesn't always compute properly?).

'nuf said,
Glenn A. Emelko

"Yes, now I read the manual myself"