[33m
THE OBESE AGNUS,THE NEW 500 BOARD AND OTHER RECENT EXPERIENCES.
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[3m By Andrew Wilson 23/07/89.
WARNING.. ANY MODIFICATIONS MENTIONED IN THIS ARTICLE SHOULD ONLY BE DONE
BY TECHNICALLY COMPETENT PERSONNEL: TECHNICIANS ETC.(not
engineers) AS REPAIRS TO THESE MACHINES ARE COSTLY AND SERIOUS
DAMAGE EASILY DONE.THE REMOVAL OF PLCC CHIPS (A500 AGNUS) FROM
THEIR SOCKETS WITHOUT THE PROPER TOOLS IS PARTICULARLY
HAZARDOUS. MOST OF THESE MODIFICATIONS REQUIRE GOOD SOLDERING
SKILLS ETC. ALL WORK SHOULD BE CARRIED OUT AT AN ANTISTATIC
WORK STATION. STATIC DOES EXIST, IT IS NOT A FAIRY STORY PUT
OUT BY 3M CORP. TO SELL THEIR ANTISTATIC GEAR. SEE
ANTISTATIC WARNING ELSEWHERE IN THIS SECTION. ALL POSSIBLE
CARE HAS BEEN TAKEN IN PREPARING THESE NOTES BUT ANY
MODIFICATIONS ARE DONE ENTIRELY AT YOUR OWN RISK.
All modifications stated as fact have been done by me, mods
put up as possibilities are my thoughts on a subject and have
not been tried.
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[Ed: Andrew is the brains behind the original Proton Ram boards,
which are still valued very highly by their owners. He is now
running his own Amiga-specific hardware manufacturing business
(on a human scale, with human scale prices, too) and it may not
be an exaggeration to say that there is little he doesn't know
about the Amiga in general, and hardware specifically. He
provides numerous products for all models of the Amiga,
including 8-meg Ram boards, hard disks, digitisers, real-time
clocks, and many other products. You'll see below that he
really knows what he's talking about, and he's always been
available to me when I have a hardware problem. He can be
contacted at:
2 Meredyth St,
Millswood 5034.
With the expert help of Mario Nicotra, this mod was done to my
early (Rev. 5A) A500, and it worked fine - the business of the
switch from NTSC to PAL screens was done by bending the socket
of pin 41 of the Agnes chip, perhaps the easiest of the alter-
natives mentioned by Andrew. Note also that the full details
required are here: all the docs from the US and even from
Commodore here tend to forget to show you how to make the change
from NTSC to PAL...
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IN THIS INSTALLMENT
------------------
The Obese Agnus Chip
Obese Agnus Software Compatibility
Fitting the Obese Agnus to the REV5 A500 (Original 500)
The New REV6A A500 Board
The Different Types of A500s and Identifying Them
[ED: 2000 Modification follows at bottom]
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[33m
The Obese Agnus Chip.
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The Obese Agnus Chip (8372A or 318069-02) is the new Agnus chip capable
of addressing 1 MegaByte of Chip Ram on the A500 or B2000. This allows
you to have more than TWICE as many windows and screens open than is
possible with a standard Fat Agnus equipped machine. If opening twice
as many screens and windows isn't your thing then it also means that
the Hi-res modes of such programs as Digiview, DeluxePaint and others
which refuse to run until all extra drives, windows, screens are closed
will now open and also allow you to run extra flip screens etc. Games
such as Boulderdash/Emerald Mines which refuse to load unless all
external drives are disconnected are now much happier as well.
Commodore is now shipping the Obese Agnus in most of the current A500s
(without telling anybody) although Commodore in the USA will admit to
it when questioned. These A500s are set up in Fat Agnus Mode so that
even if an A501 board is plugged into them no improvement is obtained
without further internal modification.
[33m
Obese Agnus Software Compatibility.
-----------------------------------[0m
Already 1 animation package (VideoFX 3D I believe) has been found which
will not run with the 8372A in either Fat or Obese mode. This is a
problem caused by the programmer reading or writing to registers in the
Agnus chip in an illegal (non-Commodore approved) manner and may
(should) be fixed in later releases of the package. CHECK that any
software that you HAVE to use works with ONE Meg of Chip Ram before
committing to this upgrade. All other programs tried so far, about
150, have been completely compatible.
[33m
Fitting the Obese Agnus to Early A500s.(REV5 Boards)[0m
---------------------------------------
Contrary to a message appearing on many local Bulletin boards it is not
necessary to have the new Rev 6A board to fit the Obese Agnus to an
A500. The 8372A can be fitted to the older Rev 5 boards simply by
doing the following :
1 Remove the Fat Agnus 8371 from its socket (see warning above).
Check orientation of pin 1 to facilitate installation of 8372A. Pin
41 of the Agnus socket which is earthed on all REV5 boards controls
whether the A500 starts up in NTSC or PAL mode. When it is earthed
the Obese Agnus initialises in NTSC Mode which means that all USA
owners need to do is plug in the chip and they are in the correct
mode. We on the other hand have to de-solder and remove the socket
from the board to isolate pin 41 which is connected to earth on both
sides of the board. This chip was designed in the USA!! Removing
the socket is a difficult and laborious task so other methods of
isolating pin 41 are possible. I DO NOT recommend trying to drill
through the board as the two pins 41 and 42 are very close together,
the trace between them is thick and there are thin tracks either side
of it under the socket on the top of the board. Bending the contact
on the socket is a possibility but make sure you get it right or
you'll be removing the socket anyway. Placing a thin piece of hard
plastic, eg mylar, between the pin and the contact is another
possibility. By putting a SPST toggleswitch between pin 41 and earth
it is possible to switch between NTSC and PAL on resetting the
machine. This would give good compatibility with American software
and for this reason and others DO NOT just break or cut pin 41 off
the chip. Pin 41 is taken to a jumper pad on REV6A boards.
2 Insert the Obese Agnus 8372A into the socket paying particular
attention to the orientation. Pin #1 is located in the centre of the
side immediately clockwise from the beveled corner of the chip and is
marked by a small dimple. The 8372A should be installed with pin 1
to the front of the socket. This is marked by a large 1 on the
silkscreening on the circuit board just above the words "FAT AGNUS".
I overstress this point because this is an expensive chip and a
wrongly inserted chip which has had power applied to it probably has
a value of 0000 dollars. The socket on the REV6A boards has been
rotated 90 degrees anticlockwise. BE WARNED!!
3 Locate the jumper JP2 on the circuit board to the right of the 68000
chip. Cut the trace between the centre and lower pads carefully with
a sharp knife. Join the centre and upper pads with a small piece of
light wire. This jumper changes the address of the A501 board or
equivalent from C00000H to 080000H to make it contiguous with the
first 512K of memory to give a 1MEG block of chip ram. The three
pads of JP2 can be wired to a SPDT toggleswitch to allow changing
from one memory location to the other if there is any software
incompatibility with the new setup.
4 Pin 32 of the GARY chip (located above right of the Agnus socket)
must be open circuit for this upgrade to work. This pin is the
*EXRAM signal which is normally earthed by the A501 board when it is
plugged into the A500. Several methods of achieving this are
available as follows:
a. If you have installed one of my A501 memory disable switches
or other similar modification which open circuits pin 32 of the
A501 memory expansion connector then this will serve the
purpose. Leave it in the OFF position and disregard the rest of
section 4.
b. Locate the GARY chip and remove from its socket. Locate pin 32
and CAREFULLY bend it slightly outwards so that when GARY is put
back in its socket pin 32 does not go into its socket. Return
the GARY chip to the socket making sure that it is correctly
positioned ie. pin 1 and notch to the right hand side. Pin 32
can be connected through a SPST toggleswitch to earth to
preserve compatability with the Fat Agnus.
******* ALL DUAL INLINE PACKAGE INTEGRATED CIRCUITS ARE NUMBERED IN
THE SAME MANNER, ONLY THE NUMBER OF PINS VARIES.
5 Check all work done in all previous stages, and if everything is
correct connect it to power supply, mouse and monitor and turn on the
power. If it does not boot up as normal power off immediately and
recheck. The longer the power is switched on with a fault the
greater the damage. Check for bent or folded pins on GARY if you
have had him out. If you have the disk drive disconnected or wrongly
connected it will take a lot longer before the hand and disk appear
on the screen.
6 Once you have booted successfully run PERFMON from the TOOLS drawer
of the EXTRAS Disk [Ed: or PopInfo from MD11]. It should show 1MEG
of Chip Ram and no Fast Ram on a 1MEG A500. Now try loading a
picture, The Tiger on Megadisc 5 is most suitable, repeatedly. It
should be possible to load more than twice as many as before (about 8
as against 3 I think from memory).
[33m
The NEW REV6A 500 Board.
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This board has been shipped by Commodore for approx. 2 to 3 months.
The main reason for the change has been to allow Commodore to use the
newer 1 Megabit chips and to allow for 1 megabyte of memory on the main
board. This means that the A501 board is no longer needed if the extra
512K is installed on the main board. Current cost of this upgrade is
approximately $160.00 . If the Real Time Clock is needed then clock
boards plugging into the A501 slot are available for $50-60. There is
provision for several resistor packs on the board although they are not
fitted and their use is not clear. The change has little to do with
the Obese Agnus upgrade although it is a little easier to fit to a
REV6A board. The board also contains several new jumper pads, details
of which are given below.
REV5 BOARD REV6A BOARD APPARENT USE
JP1 Yes Yes Connects 555 reset timer to main reset
JP2 yes yes Maps A501 @ C00000 or 080000
JP3 yes yes Swaps *CAS of 512k banks of RAM
JP4 no yes PAL or NTSC via pin 41 of Obese AGNUS
JP5 no yes 28MHz CLK, XCLK and XCLKEN to AGNUS
JP6 no yes 7MHz clock on pin 7 of 86way Exp Connector
JP7A no yes Controls *EXRAM to GARY
JP7B no yes CIA TICK from VSYNC or pin 32 of A501
JP8 no yes LPEN from FIRE0 or FIRE1
JP9 no no?
J910A no yes Connects AUDIN to pin 18 of RS232 Port
JP10B no yes Connects AUDOUT to pin 11 of RS232 Port
JP11 no yes Connects CSYNC to BCSYNC or Video Hybrid
(A Week Later)
As can be seen from the above table most of these new jumpers are for
convenience and do not represent radical improvements over the previous
board.
Conversion of a REV6A board to work with Obese Agnus is fairly simply
and will be covered in a later article which will also cover fitting
the extra 512K of RAM to the main board.
[33m
The Different Types of A500s and Identifying Them.
---------------------------------------------------[0m
This section does not attempt to identify the different types of disk
drives, keyboards etc. which, particularly in the case of the A500,
seem to change almost weekly.
The A500.
The base model of the range which, contrary to what B2000
salesman say, is expandable to Approx. 15 Meg of RAM, Hard
Drives, etc. Somewhat limited power supply so a larger one is
needed for hard drives.
There are two basic types :
A. The original REV5 board supplied from the beginning up until
early 1989. Requires the A501 Board or equivalent for
expansion to 1 Meg and use of Obese Agnus.
B. The new REV6A board supplied from April/May 1989 on. The way to
recognise it is to check the expansion bus connector on the
left hand side of the computer. You'll notice that the early
model A500 has a "fat finger" metal strip at the back end of
the connector - the new A500 doesn't, instead it has two thin
"fingers", ie, 2 metal strips in the corresponding place, of
the same width as the rest of the strips of the connector.
More on this and other related matters in the next issue.
2000 MODIFICATION
Ed: The modification for the A2000 is pretty similar, and has been
done by Mario (mentioned above) on a very souped-up system. Keeping
in mind all the warnings and hints above, it goes like this:
A. Remove computer cover, floppy cable (noting position of the red
wire on the cable), power plug from connectror CN400, and power
supply and drive assembly from the main unit.
B. Remove the old slimmer Agnes as above, preferably using a chip
extractor. Insert the new chip with the right alignment.
C. Find Jumper J101 located on the lower right side of the power
connector CN400, and consists of 3 posts - 2 of the posts are
shorted together using a shorting block. Remove the shorting block
and move it to the left, towards the power connector. For further
info, see Page 13 of the A2000 system schematics - this jumper
will enable address line 19 from the 68000 which controls the 1
meg address range.
D. Locate and cut Jumper pad J500 (to the lower left of the 8520 at
U301). This is a 2-pad jumper with a trace connecting the two
pads. See Page 9 of the A2000 system schematics - cutting this
trace keeps the _EXRAM signal from being tied to ground.
E. Finally, to enable PAL rather than NTSC - locate and cut J102,
NOT J201 as suggested in some docs. This is the equivalent of
bending pin 41 of Agnes in the A500 mod.
The mod is now finished, so re-assemble the unit, power up and
hope for the best! And don't attempt it unless, as mentioned
above, you're very adept at electronics hacking. Show these docs
to such a person, however, and there should be little difficulty.
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[43m[32m SAVING DF0: - BOOT UP FROM DF1: [0m
by[3m Les Ayling[0m
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Several of my friends who are Amiga owners have had problems with the
internal drive. This modification will overcome that problem by
allowing Amiga 500 owners to boot from df1:.
This mod is also useful for people who own the Commodore 5 1/4 inch
drives and wish to boot from them. More importantly, people who have
added an 80trk 5 1/4 inch drive (such as myself) as described in the
file ADD_5.25_DRIVE can boot from it. As 5 1/4 inch disks can be
bought for as low as $5.95 for 10 in some places, it is well worth
considering as a cheap backup library option OR indeed as an
alternative primary storage medium.
You will need the following tools and parts:
* Low wattage soldering iron, suitable for PCB's
* DPDT switch mini toggle variety is fine
* 1 foot of ribbon cable
* solder
* small snippers
* hand drill
* tool to unscrew the @#$%^& non-standard case screws
* Exacto knife or similar
Start by unscrewing the 6 hex style screws around the edge of the
case (Goodbye warranty!). Do not remove the three phillips screws as
they are holding the drive in place. Turn the machine back the right
way up and remove the lid by pressing around the edge to ease off the
catches. Remove the keyboard making careful note of the orientation of
the connector on the main board!!!!! Now remove the further 4 hex
screws which are holding the top rf shield in place and remove the
shield. Unplug the disk drive POWER cable from the drive end only.
Remove the 34-way disk drive patch lead taking note of its orientation
(This saves so much time when you have forgotten later on!) Unscrew
the Disk drive unit and remove it.
The main PC board should now come free with the lower RF shield.
Unscrew the hexagonal socket retainers on the I/O connectors at the
rear of the board and this shield should come free.
You are now ready to start!
Locate connector 11 (CN11) on the component side, this is the socket
that you just unplugged the disk drive patch lead from. In front of it
should be U8 which is an 8520 chip.
CN11 is 2 rows of 34 pins.Count along the front row from the right
hand side until you see the 5th and 6th pins (pins 9 and 11 of the
connector respectively). There should be two circuit traces that lead
from these pins and disappear under U8. Using the exacto knife cut
both of these traces leaving no stray copper. A 1mm cut should be
sufficient.
Turn the board over to the solder side.
Leave the board for a moment and get the DPDT switch.
For ease I will label its pins as such:
A B
C D
E F
Cut a piece of ribbon cable about 1' long with 4 conductors.
Strip and tin the wires then connect them to A, B, C and D.
Cut two small pieces of wire and connect A-F and E-B.
Tin and strip the other ends of the ribbon cable.Using the snippers
trim these tinned ends until there is about 1 1/2 to 2mm showing. Any
longer is unnecessary and can lead to solder blobs etc.
Grab the Amiga pc board again. On the solder side locate pins 9 and
11 of CN11 as described earlier. Solder the lead from A to pin 9 and
the lead from B to pin 11.
Locate pins 13 and 14 of U8.
If you are unsure of pin numbering on IC's refer to one of the data
sections of DSE's or ROD Irving catalogues. Otherwise looking at the
chip there is a notch at the right hand end of the board. Pin 1 is
behind this notch (relative to the pc board) and the numbers increase
in an anti-clockwise fashion around the chip. Solder the lead from C
to pin 13 of U8 and the lead from D to pin 14 of U8. That completes
the soldering side of things.
All that remains is to drill a hole for the switch and put the rest
back together. There is a convenient spot to the right of the joystick
socket that will house 3 or 4 switches comfortably.
PLEASE PLEASE PLEASE be careful of the orientation of the connectors
when assembling your amiga.
To use the mod:
1. Remove ALL disks from ALL drives.
2. Flick the switch to the NORMAL position.
3. Turn the machine ON or reset if already ON.
4. If you wish to boot from drive DF1: now is the time to
flick the switch to the NON-standard position.
5. Insert the appropriate disks in the appropriate drives.
HOW it works.
The mod works by switching the drive select lines SEL_0 and SEL_1.
SIMPLE!
[33m
IMPORTANT:!
