Nemesis hacking

From: Michael Grove <grovem_at_ridgecrest.ca.us>
Date: Mon, 14 Sep 1998 15:24:52 -0700

(Warning: The below ramblings could be those of a madman).

I spent about 25 hours of my weekend hacking around
with my Mighty Sonic and Nemesis setup, so someone
had to hear about it:).

The Mighty Sonic is very much like an Afterburner,
the later using more modern chip technology. However
they both seem to have about the same operating speed
limit.

My A/B is reliable at 40Mhz, and will operate at 42Mhz
if DMA for floppy or SCSI is not needed. At 44.5Mhz it
dies. No tricks with the installation, just what the
docs recommend. The A/B Falcon is a Rev G motherboard.
I installed a CPU socket (not for the faint at heart)
and replaced the 33Mhz RC040 with a 40Mhz RC040 (this
didn't make any noticeable difference).
Nemesis installation is by the book, working with the
supplied resistor. (Even with a NOVA installed, 42Mhz
still causes DMA problems. In VGA/RGB T/C resolutions,
DMA is not reliable, and, 40Mhz with the NOVA, any
color depth, DMA works perfect). I guess I could
tweak the buffer mod a little......

Here are some interesting points, some probably already
well known, that I realized while hacking. This was
all by the seat of the pants with the only tool being
a multimeter. No scopes at home (yet).

At one time, I had the Mighty Sonic and Nemesis installed
on the Rev G motherboard. The machine was very reliable
at 40Mhz, no DMA problems except when in T/C modes. I
remember trying different clocks, and even getting it
to run at 45Mhz for a short period before it crashed,
and marginally reliable at 42 (no DMA). I used the
Nemesis switch AB_HI.PRG with Videlity and a 50Mhz
Nemesis HI clock for 640*480 TC video with reliable
DMA. That machine was disassembled after about 8 months
of use, and the Mighty Sonic was replace with the above
mentioned A/B setup.

About six months ago, I reinstalled the Mighty Sonic in
a Rev K Falcon motherboard, without the Nemesis. It has
been a very reliable machine, but lacked the punch the
increased bus speed provides. This weekend I installed
the Nemesis in this machine, but I couldn't get the
reliability that the setup had with the Rev G motherboard.
I tried various techniques installing the Nemesis as
documented in the manual, and some of my own tricks
(inductors, shielded wires, ect). The machine would
run fairly reliable at 40Mhz, but because it was so
stable at 32Mhz, I decided to remove the Nemesis.
Before reversing the installation, I decided to see,
for the sake of speed (what does Kann say, "for hates
sake":), how fast I could clock the machine. I left
the FPU out of the game as it is a trial and error
quest all in itself. As before, NemLo at 40Mhz worked
and Nem HI at 42Mhz worked. I tried the 44.5Mhz and
once out of every 15 or 20 switches from Lo to Hi, I
got a Fuji. This made me think that maybe something
funny was going on during the clock change from Lo to
Hi. After a deep breath, I decided to desolder the 40Mhz
clock from the Nemesis and install a socket. It wasn't
to hard, but I have two Nemesis......
After replacing the 40Mhz Nemesis Lo clock with the
44.5Mhz clock, I could switch from 32Mhz to 44.5Mhz
without a crash. I thought this quite impressive as
the Mighty Sonic is not clocked like the A/B. First,
the Nemesis must be switched from 32Mhz to LO (44.5Mhz).
The M/S is then still getting it's clock from the
expansion bus (22.25Mhz). To switch to 44.5Mhz, the
M/S must switch from where the clock input is to an
input buffer on the card (this is where the A/B takes
a modern approach). Gembench showed ~265% at 42Mhz and
~285% at 44.5Mhz (I didn't write it down, I think too
this was in 16 color mode). A total benchmark using
Gembench was about ~540% at 44.5 Mhz (AFAIKRemeber).
The only side effect I noticed was the display was
unstable with lines running across the screen (this
machine has the sync fix installed). I thought that
strange as the videl works fine at 25Mhz bus, so the
display problem must come from the M/S. This card
also has 8-4mb 30 pin simms, and ran very hot. Run
time lasted about three minutes, or about six feet
of mouse travel:).
The other thing I noticed was the Nemesis will not
work with anything less than a 40Mhz clock in the
Lo socket. A 36 and 38Mhz just caused a baby shit
yellow screen display. Also, the Mighty Sonic
doesn't like the buffer cap or resistor installed,
just a straight line from the buffer chip.
Umm, it has a Wizztronics Falcon memory card too.

I didn't go any further (installing the NOVA),
but at least managed to run the 37.5Mhz rated
MACH chips and 30 pin 60ns simms at 44.5Mhz.

I also have an Apple 040 50Mhz accelerator card.
Even though we have come to the conclusion that
it is the MACH chips limiting the A/B speed, the
Apple accelerator card has the same type of MACH
chips on board.........

I wonder if removing the Nemesis, and using a
PowerUP 2 and playing with clock speeds on the
Afterburner might increase reliability by ~2 to
4Mhz?

I am also VERY interested in how Centek is running
at 50Mhz with the Centurbo II.

I've been very nice to the secretary at work (credit
card holder) and the AO too. It seems that I may have
an 040 to 060 adapter soon. Anyone care to write an
060 initialization boot sector creator for floppy???????

MG
Received on on. sep. 16 1998 - 11:28:00 CEST