Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!deimos.cis.ksu.edu!uxc!csd4.milw.wisc.edu!jgreco
From: jgreco@csd4.milw.wisc.edu (Joe Greco)
Newsgroups: comp.sys.cbm
Subject: REU Hacking.
Message-ID: <2720@csd4.milw.wisc.edu>
Date: 1 Jun 89 05:36:11 GMT
References: <2451.247F6F5D@isishq.FIDONET.ORG> <7028@cbmvax.UUCP>
Sender: news@csd4.milw.wisc.edu
Reply-To: jgreco@csd4.milw.wisc.edu (Joe Greco)
Organization: UW-Milwaukee Home for Out-of-date 8 bit Hackers
Lines: 48

In comp.sys.cbm article <7028@cbmvax.UUCP>, daveh@cbmvax.UUCP (Dave Haynie) wrote:
]in article <2451.247F6F5D@isishq.FIDONET.ORG>, izot@f171.n221.z1.FIDONET.ORG (Geoffrey Welsh) says:
]>> From: daveh@cbmvax.UUCP (Dave Haynie)
]>> For home hacking, if it work, do it.  For a commercial product, you'd 
]>>better be sure it'll always work.
]
]>    Like the 1700/1750 REUs, that didn't work on some C64s, eh? <grin>
]
]The 1700/1750 units are C128 products.  They aren't intended for the C64,
]and in fact a 1750 has an excellent chance of turning your C64 power
]supply into a smoking pile of ash.  While they _may_ work on a C64, it's
]at your own risk....
]

Gee, my supply runs three 64's with no problems.... not to mention a
host of other hardware.  (Turn *THAT* into a pile of ash  :-)

A few months ago I had been discussing the feasibility of hooking up
two REU's to a single machine (64) by remapping one unit to another I/O
location (and not $DE00, since I need that as well).  I built the
needed decoder to decode the entire $Dxxx region into pages, and
hooked in a 1700 at $D500 (1750 at standard address).  It didn't work,
it strangled any data stored or fetched.  Not only that, but it was
nasty about it.

Well, I explored the interface to the 64 a little.  Tracing a few
lines revealed that the only "special" lines the REU appears to use
are BA (Bus Available), DMA (Direct Memory Access), and the I/O line
which I had already dealt with.  The I/O line hack definitely worked
(adding a second decoder chip to decode $D5-$D7 and $D1-$D3 instead of
letting phantom images reside there).  Which meant that there was an
"apparent" problem with either DMA or BA.  I decided to play with DMA,
since BA is an output signal and I could not imagine what I could try
to do to it.  I determined that the DMA outputs from the REU's needed
to be OR'ed together, since the DMA outputs did not seem to be very
tristated.  I did a quick OR gate hack with small signal diodes (Radio
Shack is closed at 2am), and everything seemed to work fine.

But it doesn't.  If you try anything "complex," it mangles it.  A
simple stash/fetch appears to work to either unit, but loading up
something like RAMDOS fails miserably.  It seems to be working ALMOST.

Ideas, anyone?
--
jgreco@csd4.milw.wisc.edu		Joe Greco at FidoNet 1:154/200
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