Path: utzoo!mnetor!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: hardware question on DTACK'
Message-ID: <3290@cbmvax.UUCP>
Date: 10 Feb 88 05:34:31 GMT
References: <1107@louie.udel.EDU>
Organization: Commodore Technology, West Chester, PA
Lines: 37

in article <1107@louie.udel.EDU>, rminnich@udel.EDU (Ron Minnich) says:
> 
> I am trying to add some memory to an A1000. For several reasons
> the hardware is not asserting DTACK. This brings up a question:
> something else is! Is there a watchdog timer in the custom chips that
> automatically asserts DTACK after x cycles? I guess there would have to
> be, else if you addressed non-existent memory the thing would hang. 
> Can anyone fill me in? The hardware manual is not helpful in this case.
> I am hoping to hear that DTACK is not just asserted always by the hardware 
> as this memory cycles in 675 ns (i.e. slowly) so i really need DTACK
> to work right.

> ron (rminnich@udel.edu)

This may have been answered already (though GRR's been as busy as I have), but
ordinarily, /DTACK is driven by the system.  An expansion board has three
options concerning /DTACK.  The simplest is just to obey the /DTACK the system
gives you, but that's not always possible.  The second option is to delay the
system generated /DTACK by asserting XRDY when your board is selected.  This
will hold off /DTACK, but it must be issued very quickly, like 50ns-60ns into 
the cycle.  Your third option is to assert /OVR when your board is selected.
This will cause the Amiga (any variety) to tri-state /DTACK so you can drive
it yourself.

BTW, the hardware manual isn't helpful on any expansion issues.  There are
two books published by Commodore-Amiga (available only from CATS) that
detail hardware design.  The first is "Schematics and Expansion Specifications",
which covers the basic expansion design, A1000 schematics, casework, etc.  This
is about $20.00.  The second is "A500/A2000 Technical Reference Manual", which
contains everything in the A1000 book except A1000 specific stuff, plus notes
specific to A500 and A2000 machines (mechanicals, schematics, design 
guidelines).  This one is around $40.00.

-- 
Dave Haynie  "The B2000 Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"