#Hello, Agnus?
11 messages in this thread
I have an A500 that shows 512K of chip RAM when I do an AVAIL from CLI. I
take it this means it has the 8370 Fat Agnus chip.
Here comes the confusion-I've been told that I can replace the 512K Fat
Agnus with a 1 meg Agnus and get 1 meg of chip RAM. I've also been told that
the 1 meg Agnus is not plug in compatible with 512K (8370) Fat Agnus-and to get
more chip RAM from a 512K Fat Agnus Amiga I'd need to use a DKB MegAchip
package (which replaces most of the custom chip set and uses a 2 meg Agnus).
I'd like to get 1 meg of chip RAM in the most painless way possible, and I
know some A500s came equiped with 1 meg Agnus', but I don't know if there were
other changes other than the one chip. Can I just use a 1 meg Agnus-and if so
what is it's number (8372A?).
Barry
Barry,
The 8372A is pin compatible with the 8370 and, if you have a 512K expander
in your trapdoor, wull give you 1meg of chip ram. However, any more than 512K
in your trapdoor will not be recognized. If you want more fast ram then get an
accelerator, supraRX ram expander, or a hard drive that has ram expansion
capability using the side door.
…Via Autopilot and Gforce 040/33 //
…Only Amiga makes it possible \\/
Kenton,
Thanks for the information. I have 4 megs of fast RAM, so it looks like
all I'll need is the new Agnus. I don't think I'll need more than 1 meg of chip
RAM, though.
Barry
Barry,
I believe that to enable the 1mb of chip RAM a trace must be cut on your
A500 mother board. You may already have the correct Agnus chip but not the
trace cut to enable it. I have an A500 with 512 chip and 1.5mb fast RAM. I
have been considering researching this trace cut further so that I can use
DPaintIV better. Ask your question again in all of the Amiga forums and I am
sure you will get the right answer. Good luck.
David
David,
Thanks for the information. I just took a look inside and found that it
does have the 8372A (instead of the 8370 specified in the manual).
As for cutting a trace to enable the other 512K, there are at least 2
different PCBs for the A500-I wonder if the traces are the same? The traces are
very small, and repairing an incorrectly broken trace would be difficult.
Barry
Ok, here's the deal…
The FAT Agnus (8370) contains no RAM, but addresses a 512K section of RAM
located on the motherboard. With most A500's this was the ONLY RAM the A500
shipped with. This RAM is called CHIP RAM because the Agnus CHIP can access
this RAM directly, and does not need to asks the CPU to feed it the info from
the RAM. The Agnus & CPU take turns accessing CHIP RAM, and share it.
The RAM plugged into the side of A500 (thru the 86-pin Expansion slot), is
connected only to the CPU, and only used by the CPU. The Agnus doesn't know of
this RAM, can't access this RAM, and doesn't care about went it is being
accessed. This RAM is called FAST RAM.
As is, the A501-Slot of the A500 allows upto 512K of FAST RAM to be added.
Normally, only the CPU uses this RAM, but there is an option were it can be
re-configured as CHIP RAM so that the Agnus can access & use it. However, once
re-configured, the A501 expander becomes a requirement, and the Amiga can't
boot without it.
The AmigaOS allows upto 2-megs (2048K) of CHIP RAM, but the A500 only has the
necessary hardware for 1-meg (1024K) of CHIP RAM — 512K on the motherboard
plus another 512K in the A501-slot.
The FAT AGNUS (8370) could access only 512K of RAM, so only the 512K of
motherboard RAM was needed. The ECS Agnus (8372A) could be toggled to access
1024K or 512K of RAM. This allows the ECS Agnus to be dropped into an
unexpanded A500.
If the ECS's 1024K option is used, the A501 must be re-configured into being
CHIP RAM (by cutting a solder jumper connected to the GARY). And the ECS Agnus
must also be re-configured by toggling a solder jumper near it from 512K mode
to 1024K mode.
Ron.
Ronald,
Thanks for the information. I opened up the machine and confirmed that it
does indeed contain the 8372A. I do have the A501 in one of my 500s, and an
equivilant in the other. I have no intention of removing them, so converting
them to address the one meg of chip RAM would be no problem.
Do you know exactly what trace needs to be cut and what jumper
re-configured? If not, do you know of a really good repair center that might be
able to tell me? It doesn't sound like a big job, but I'd have to be certain of
the trace to cut, since they're so small It'd be difficult to repair if the
wrong one were cut. The mother board PCB is Rev 5.
You seem to know a bit about Amiga. What is the difference between the
OCS, ECS and AGA chip sets? I know the AGA has greater graphics capability, but
I'm fuzzy on the OCS-ECS differences (I take it from your message, that one
difference is the 8370 vs 8372A).
To further test your knowledge, the mother board has some writing on it,
above the revision number, I take these to be initials and names of people who
worked on it, but the first thing is says is, "B52 / Rock Lobster". Any idea
what the B-52s and their Rock Lobster song have to do with Amiga? I guess they
could have been listening to the song when they were designing the thing, but
it seems like a strange thing to do.
Barry
BM <<have the A501 in one of my 500s, and an equivilant in the other. I have no
intention of removing them, so converting them to address the one meg of chip
RAM would be no problem.>>
Good! You have most of the work already done — the getting of an 8372A and an
A501 expansion. However, I would use the real A501, since many of the 3rd
party A501-clones lacked the option to be used as CHIP RAM.
The first thing to do is toggle the Agnus into 1-meg mode. Look to the right
of the 68000 CPU and just above the KickStart chip. You should see three
small, silver, square, solder pads. This is the JP-2 jumper. The middle pad
will have a thin solder trace connecting it to the bottom pad, this is the 512K
mode.
Cut the trace between the middle & bottom pads, and use a bit of solder to
create a trace between the top & middle pads. This flips the Agnus to 1024K
mode.
Now you must re-configure the A501. This is the hardest part, since the rev.5
A500 didn't have the JP-7 jumper used for re-configuring the A501. You will
have to find the trace that connects the EXRAM signal of the A501 to the GARY
chip. When connected, the A501 is FAST RAM, when it is cut the A501 is
configured as CHIP RAM.
I believe the trace to cut runs from pin #32 of the 56-pin A501 slot to pin #32
of the GARY chip. It is best to cut the trace at the spot just *ABOVE* a pad
(small silver disk) by the 56-pin connector. On the rev.5 motherboard this pad
is located by counting 8 positions down from the top of the connector (or 3/4th
inch). The pad will be the first of several dierctly the the left with a trace
going toward the Gary chip.
—
BM <<What is the difference between the OCS, ECS and AGA chip sets? I know the
AGA has greater graphics capability, but I'm fuzzy on the OCS-ECS differences
(I take it from your message, that one difference is the 8370 vs 8372A).>>
The original Agnus was the #8361, a thin DIP chip that looked just like the
Denise & Paula. This was the version used in the A1000.
The A500 & A2000 used a new 'FAT' chip design for the Agnus. This new square
chip internalized several support chips (like the genlock support), but added
no new features. There were two versions, the NTSC (#8370), and the PAL
(#8371) used in Europe. This two versions are part of the Original Chip Set
(OCS).
Commodore then developped the Enhanced Chip Set (ECS). This was a new FAT
Agnus & Denise. They could be dropped into most existing A500s & A2000s adding
several immediate enhancements, with added abilities with re-configuring of the
hardware.
This ECS Agnus (#8372a) combined with the ECS Denise (#8373) added:
– new memory limits (1024K for A500/2000, 2048K for A3000)
– new Blitter Range (32K by 32K pixels in size)
– new mode resolutions (Superhires, 1280 pixels horizonal pixels per
scanline on standard NTSC & PAL displays)
– new monitor scanrates (640×480 non-interlaced screens (VGA))
– new Genlock capabilities
– Chromakey (allows any color reg to control video overlay)
– BitPlaneKey (allows any bitplane to enable the video overlay)
– BorderBlank (creates a transparent "frame" surrounding the
active area.)
– BorderNotTransparent (makes an opaque "frame" surrounding the
active area.)
The #8372b ECS Agnus was the version for the A3000. This version removed the
genlock to external chips to make room for supporting more CHIP RAM. Using this
version in an A500/2000 requires a extra curcuit board for adding the missing
genlock and connecting the extra CHIP RAM.
The #8375 ECS Agnus was a version of the #8372b, with the pins re-arranged for
a cleaner motherboard design. It was used in the A500-PLUS, and some versions
of the DKB MegAchip expander for the A500/2000.
—
BM <<Any idea what the B-52s and their Rock Lobster song have to do with Amiga?
I guess they could have been listening to the song when they were designing the
thing, but it seems like a strange thing to do.>>
The designers were fans of the B-52s, and did alot of strange things with the
Amiga designs. Hidden messages in the Workbenches, hidden icon showing
signatures of the designers, the signatures of the designers in the A1000 case
(including a dog's paw).
Ron.
Ronald,
I was able to convert both of my A500's to address 1 meg of chip RAM.
Thanks for the information. Your instructions were good. It went fairly easily.
The only slight problem I had was when I mis-read your message about where to
cut the trace from the A501. I thought pin 32 of the expander would be the 8th
pin down and when it wasn't, I had to get a logic tester out and see which
things went where. I then re-read your instructions and found out you were just
giving the location of the pad, not where it connected.
When I opened my second A500 I found it had the Rev 6A mother board. It
was a bit easier because of the second well labeled jumper. Oh, the expander
(non 501) was usable as fast RAM (I tested it in the first 500, after making
the mods, before going on to the second A500). I'd like to mention it's name,
so others would know it can be used as chip RAM, but it dosn't have a name on
it and I don't remember who made it.
OK, you've known everything about Amiga so far-let me try one more thing.
Overscan-I notice that with "standard" overscan mode 480×352, many programs
showing images through the A520 leave a black bar about 1/2 an inch on the left
side of the image (on a 19" monitor). Only Photon Paint 2 and the first version
of a display program called "si" manage to fill the video screen with a 480×352
image. Is this a problem with the programs, the screen size, the A520 or what?
When using Mostra, if I increase image size to 480×362 (in Deluxe Photolab),
and do the same with the display size in Mostra, I get edge to edge coverage.
Interestingly- though I can set the display size to anything, I can't get edge
to edge coverage unless I increase the actual picture size to 362.
Thanks again for all your help and knowlegde. I didn't know the A1000 had
the signatures inside the case (and the dog print)- I thought that was the ill
fated Apple Lisa, not did I know about hidden icons. I did know about the 1.2
title bar messages- removed from 1.3 for obvious reasons. I'm sure Commodore
was less than pleased with the, "We made Amiga, they &%#!ed it up"
message-however true it is!
Barry
BM <<OK, you've known everything about Amiga so far-let me try one more thing.
Overscan-I notice that with "standard" overscan mode 480×352, many programs
showing images through the A520 leave a black bar about 1/2 an inch on the left
side of the image>>
The well just ran dry. I don't have any experience with A520 & overscan.
When you said 480×352, did you mean 352 pixels across & 480 scanlines down?
The standard overscans for NTSC/PAL under AmigaDOS 3.x, (and I assume for
AmigaDOS 2.x) are:
NTSC:HiRes – 724×241
NTSC:HiRes interlaced – 724×482
NTSC:LoRes – 362×241
NTSC:LoRes interlaced – 362×482
PAL:HiRes – 724×283
PAL:HiRes interlaced – 724×566
PAL:LoRes – 362×283
PAL:LoRes interlaced – 362×566
Under 2.x & 3.x, the 362 (or 724) picture size should be the one to offer edge
to edge coverage.
However, before AmigaDOS 2.x, there was no standard overscan size. Some older
programs offered their own 'standard' overscan, and some others also offer an
additional 'maximum' overscan.
Some of your software may be defaulting to the OS's overscan screens, while
other programs may be opening custom screens with simply more overscan than a
standard, non-overscan screen size has.
Ron.
Ronald,
I don't think the well ran dry-I think you hit the nail on the head yet
again. The paint and graphics programs I'm using are pre 2.x and 3.x, so their
"standard" overscan must be an arbitrary number-which would explain the trouble
I'm having. Thanks again. It looks like the well is still pumping 🙂
Barry
P.S. Yes, I did mean 352×480.