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From: wolfe@pdn.UUCP (Mike Wolfe)
Newsgroups: comp.sys.m68k
Subject: Re: DTACK*
Message-ID: <1585@pdn.UUCP>
Date: Mon, 19-Oct-87 12:37:58 EDT
Article-I.D.: pdn.1585
Posted: Mon Oct 19 12:37:58 1987
Date-Received: Thu, 22-Oct-87 00:05:01 EDT
References: <4215@pyr.gatech.EDU> <1613@bloom-beacon.MIT.EDU>
Organization: Paradyne Corporation, Largo, Florida
Lines: 28
Summary: DTACK grounded

In article <1613@bloom-beacon.MIT.EDU>, wolfgang@mgm.mit.edu (Wolfgang Rupprecht) writes:
> In article <4215@pyr.gatech.EDU> jkg@pyr.UUCP (Jim Greenlee) writes:
> >I have a basic fundamental (probably stupid) question about DTACK*. I was
> >told  (by a Motorola person) that it was possible to wire DTACK* directly
> >to ground and have a 68000 system run at full speed, provided the memory
> >access time was fast enough. 
> 
> This advice is in direct conflict with the timing diagram for the
> 68000.  In the 8 Mhz. part, you have only 120 ns to get /DTACK high
> after /AS goes high. (ref: moto data book for the 68000).
> 
> What I observered via a logic analyzer is this (and I find this
> fascinating!): The /DTACK somehow gates the 60000's external address
> bus latches. If The /DTACK stays low, the address bus stayes 'stuck'

I seem to remember back in ~= 1981 there was a news letter called
'DTACK GROUNDED', these were people that were interested in running a 68000
as fast as possible (pushing clock rate, fast ram, etc.). If I recall
they actually sold a board that ran with DTACK grounded. I'm not 68000
expert (In fact many people are still trying to figure out just what I am ;-)
but I am pretty sure that the 68000 did run DTACK grounded at one point. If
this has changed then its changed since '81.

-----------------------------------------------------------------------------
Mike Wolfe                                     Paradyne Corporation
{gatech,codas,ucf-cs}!usfvax2!pdn!wolfe        Mail stop LF-207
Phone: (813) 530-8566                          8550 Ulmerton Road, PO Box 2826
                                               Largo, FL  34649-2826